Showing posts with label evolution. Show all posts
Showing posts with label evolution. Show all posts

Thursday, January 19, 2012

Time to Death - Boas Monitor Prey's Heartbeat During Constriction

A threatening Boa constrictor. JCM
The 19th century literature on constriction by snakes often describes the prey as being crushed. And, for many years it was thought that constricting snakes killed their prey by preventing the prey from breathing. A coil from the snake's body was tightened each time the prey exhaled, gases returning the the prey's lungs would have to be exhaled, so when the prey exhaled the snake tightened its hold, making it impossible for the prey to inhale. There was another hypothesis that was overlooked. In 1912 Frank Wall proposed snakes induce asphyxia by essentially stopping blood flow to the heart.

 A new study by Scott Boback and colleagues suggests that constriction may be more sophisticated than previously thought. Killing prey by constriction is both energy expensive and potentially dangerous to the snake, constriction requires and significant increase in aerobic respiration and the prey may retaliate and injure the snake. The authors tested constricting boa constrictors to see if they adjusted their hold on prey. They developed a method of isolating a rat's heartbeat as a potential cue, by implanting a simulated heart in a dead rat that replicated the size, rate and stroke volume of a rodent heart. They then tested how the constriction effort varied as snakes constricted rats with: a simulated heartbeat throughout constriction; a simulated heartbeat for the first half of constriction and then shut off, and no heartbeat. The results suggest tightness and duration of a constricting snake’s coils are timed to perfection, matching the heartbeat and weakening state of the snake’s prey.

Snakes constricting dead prey with a simulated heart beat constricted for a much longer time than in previous studies (averages of 12 vs 23 minutes). The authors propose that longer constriction times may have been required prior to the evolution of endotherms (birds and mammals) because ectotherms have slower metabolisms and can survive for longer periods of time with reduced amounts of oxygen.

Citation
S. M. Boback, A. E. Hall, K. J. McCann, A. W. Hayes, J. S. Forrester and C. F. Zwemer. 2012. Snake modulates constriction in response to prey's heartbeat. Biology Letters doi: 10.1098/rsbl.2011.1105.

Tuesday, November 22, 2011

The Arms Race in Dendrobatid Frogs

Shape dough into frogs, that's normal in a day care. At university, it is more rare. However, the biologist Mathieu Chouteau in 3600 he made a carefully painted as part of his Ph.D. project in the Department of Biological Sciences at the University of Montreal. A month of work to which his wife had worked closely.

He then divided this bestiary in sites previously identified in the middle of the Amazon rainforest. "The hardest part was carrying my models without arousing suspicion at the airport and customs controls," says Mathieu Chouteau in an interview from Peru, where he pursued a fellowship in collaboration with the National Museum of Natural History of Paris.

In an article to be published in December in Natural History that has already been the subject of a new site ScienceNOW (Helen Fields, "Why Are There So Many Colors of Poisonous Frogs", November 4, 2011), he concludes that the Predation is a natural selection factor in the development of multiple motifs in the species Ranitomeya imitator, a frog under a centimeter in various colors. The models on clay he has documented fact in the evolution of colors and patterns of dendrobates: the role of predators.

As we learned from our first visit to the Montreal Biodome, brightly colored frogs which can be seen in the rain forests launched a clear message to predators: do not approach me because I am poisoned! But why is it some green with a pattern of leopard and other yellow lines while in the same species? Presumably because predators taste the local people they do not associate with frogs in the area, hoping that these frogs are edible. "When predators are they are dealing with a different species, they attack. In the long term, this would explain the unity of the patterns and colors, "says Bernard Angers, who led the research of Mr. Chouteau.

The methodology is rather original. Three times for three days at two sites, Mathieu Chouteau noted his false attacks on frogs - mostly pecking. The least attacked were those most closely resembles the native species, while those who were most distant showed signs of aggression. He had more than 300 fake frogs per site.

Seeing that no food, some predators attacked plasticine frog.

Why play dough to observe predation? The doc had this idea by browsing the scientific literature. "We have successfully used models in clay for snakes, salamanders and also dendrobates," said Mathieu Chouteau. It was in the Peruvian jungle terrain ideal for testing the hypothesis, as two radically different colonies of frogs have been studied: one attending the plain is dotted with green and the other on top of a mountain is yellow line ... The two settlements are separated by about 10 km. False frogs were deposited in two sites in specific locations. As for colors and patterns, they differed in several combinations.

What was most surprising in this research is "the very small spatial scale at which evolutionary processes occur." In the case of Dendrobates, a distance of 10 km is sufficient to demonstrate an adaptation clearly different. "A second surprise has been the learning capacity of the communities of predators, but also the speed with which this process begins when a new aposematic signal is massively introduced exotic," said the biologist.

"We have proved empirically that avian predators can recognize and avoid aposematic signals in various sites," he wrote in conclusion of his article.

This process could be the cause of the wide variety of color patterns observed not only in frogs but also among many species of butterflies and bees and other animals. The purpose of the postdoctoral fellowship is also to explain the polymorphism in the butterfly genus Heliconius. "Given that such a project request to be on the ground regularly, I settled in the small town of Tarapoto , where I am responsible for implementing a research center to facilitate studies of mimetic neotropical butterflies, "he says via email.

Bernard Angers admiration for his student, especially as this research is only one of four components of his doctorate. Patience, creativity and discipline were to converge to implement such a protocol. It also highlights the qualities of photographer Mathieu Chouteau (including readers of Forum can get an idea on page 1).

If they are of great beauty, dendrobates are among the most toxic animals on the planet. The student has experienced when, after a hard day's work Cainarachi, it has not taken the time to wash their hands before snack. "Error! Approximately five minutes after starting to eat, I had the lips and throat stung and it ended with a serious food poisoning that lasted two days, "he recounts.

Mathieu-Robert Sauvé

Sunday, September 25, 2011

Novel Squamates

In the 1970's I remember several conversations with zoo curators and reptile keepers who swore female snakes would give birth to offspring without ever being in the presence of a male or if they had been with a male it had been years before the offspring/eggs developed. The latter situation was often attributed to sperm storage. At the time, I was skeptical, but parthenogenesis was known in lizards since Ilya Darevsky discovered all female rock lizard, Lacerta saxicola, populations in 1958 in southern Russia. L. saxicola (Eversmann, 1834) has now been removed from Lacerta and placed in the genus Darevskia, with about 25 other species of rock lizards, the generic name derived from the name of the Russian herpetologist. But, four decades later parthenogenesis has been documented in many squamates, and it has apparently evolved at least 40 times independently. 
Squamate reptiles (lizards + snakes) are both numerous and diverse with 61 families and more than 9000 species. With the diversity come novel traits that have evolved multiple times - like parthenogenesis. In a forthcoming paper Sites et al. (2011) summarize many of these novel traits in squamates - traits like parthenogenetic, viviparity, limb-reduction and limb loss, herbivory, and venom. The authors note that squamates are the only vertebrate group with true parthenogenesis; the clade has more origins of viviparity than any other group of vertebrates; and squamates have undergone dramatic changes in body form (lizard-like to snake-like) dozens of times.

They also note that new phylogenetic hypotheses are challenge our ideas about squamate biology and are emerging at all taxonomic levels. Phylogeny based research is revealing much about ecological aspects of of parthenogenesis as well as finer details about the origins of several forms of viviparity.

Citation
Sites, J. W., Jr, T. W. Reeder, J. J. Wiens. 2011. Phylogenetic Insights on Evolutionary Novelties in Lizards and Snakes: Sex, Birth, Bodies, Food, and Venom. Annual Review of Ecology, Evolution, and Systematics, DOI: 10.1146/annurev-ecolsys-102710-145051

Monday, September 19, 2011

The Moluccan Short-tailed Snakes - The Most Basal Homalopsids

The Moluccan Short-tailed Snakes of the genus Brachyorrhos are poorly-known, terrestrial–fossorial snakes that eat worms and are endemic to eastern Indonesia. During the past 200 years the snakes in this genus have been assigned to at least six different families. In 1987 Samuel McDowell suggested that they may be homalopsids. This seemed unlikely- homalopsids are aquatic, they eat fish, crustaceans, and frogs; homalopsids also have enlarged grooved fangs on the rear of the maxillary bone, and none of the known species have a rostral scale that separates the nasal scales. However, in my 2007 book, Homalopsid Snakes, Evolution in the Mud, I listed the genus as being of uncertain status. In a recent paper, Murphy et al. (2011) recovered the first molecular sequences for Brachyorrhos and tested the position of the genus within snake phylogeny. Bayesian and Maximum Likelihood analyses of three mitochondrial and one nuclear gene strongly resolve Brachyorrhos within the rear-fanged semi-aquatic family Homalopsidae, as the sister to all other genera and sampled species. Thus, Brachyorrhos makes an excellent model for a possible ancestor of the homalopsids, an ancestor that was terrestrial-fossorial and fang-less and produced decedents that evolved aquatic lifestyles, rear-fangs, and fish-crustacean diets.

Murphy, J. C., Mumpuni, K. L. Sanders. 2011 (in press). First molecular evidence for the phylogenetic placement of the enigmatic snake genus Brachyorrhos (Serpentes: Caenophidia)  Molecular Phylogenetics and Evolution, doi:10.1016/j.ympev.2011.08.013.

Thursday, September 15, 2011

Global Snake Diversity

Pit vipers, represented by this Ridge-nose Rattlesnake were once thought to 
have evolved quite recently. Now, based on the DNA clock they are estimated 
to have first evolved about 35.6 million years ago. Today there are about 200 
species of pit vipers. But the colubrines evolved about the same time and have 
more than 600 species. In a forthcoming paper Pyron and Burbrink investigate 
why some clades of snakes are species rich, and others are species poor. JCM
Examine the numbers of amphibians and reptiles in various clades and a great disparity is readily apparent. The Lepidosauria (tuataras + (snakes+lizard)) is an excellent example, there are two species of living tuataras, but their sister group the Squamata (lizards+snakes) contains about 9000 species. This is not only true for snakes, salamander species number about 600, while frog species number more than 6000. Ideas to explain these kinds patterns have not been lacking. Species richness has usually been attributed to either the age of the clade (older clades have fewer species due to extinctions) or the rate of diversification (evolution rate of a particular clade) has been constrained by ecological factors, like the number of niches available.

In a forthcoming paper R. Alexander Pyron and Frank Burbrink examine global snake diversity and find it varies by two orders of magnitude in living lineages. Many older lineages contain only one or two species while a few younger clades may contain more than 700 species. They suggest that the patterns cannot be explained by background rates of speciation and extinction. Instead most of the diversity appears to derive from a radiation within the superfamily Colubroidea. After the colubroids evolved they invaded new geography and they evolution advanced venom-delivery systems. Pyron and Burbrink also found negative relationships between clade age, clade size, and the diversification rate suggesting the potential for possible bias in estimated diversification rates. This has been interpreted by some authors as support for ecologically mediated limits on diversity. However, evidence from the fossil record suggests that numerous clades were much more diverse in the past, and that extinction has been an important factor on the diversity patterns of living snakes. Thus, failure to adequately account for extinction appears to prevent both rate- and diversity-limited models from fully characterizing richness dynamics in snakes. The authors suggest that clade-level extinction may provide a key mechanism for explaining negative or hump-shaped relationships between clade age and diversity, and the prevalence of ancient, species-poor lineages in numerous groups.

Citation
Pyron, R. A. and Burbrink, F. T. (2011), Extinction, Ecological Opportunity, and the Orgins of Global Snake Diversity. Evolution doi: 10.1111/j.1558-5646.2011.01437.x

Sunday, July 10, 2011

The Rise and Rise of the Flying Reptiles

Extremes in pterosaur morphology.
The giant and probably flightless
 Quetzalcoatlus from the Late
Cretaceous of Texas was as tall a
s a giraffe. The small insectivorous
Anurognathus from the Late Jurassic
of Germany is seen flying above
 the artist’s head. Drawings by
Mark Witton.

University of Bristol Press release issued 6 July 2011
Pterosaurs, flying reptiles from the time of the dinosaurs, were not driven to extinction by the birds, but in fact they continued to diversify and innovate for millions of years afterwards.

A new study by Katy Prentice, done as part of her undergraduate degree (MSci in Paleontology and Evolution) at the University of Bristol, shows that the pterosaurs evolved in a most unusual way, becoming more and more specialized through their 160 million years on Earth. The work is published today in the Journal of Systematic Paleontology.
‘Usually, when a new group of animals or plants evolves, they quickly try out all the options.  When we did this study, we thought pterosaurs would be the same,’ said Katy.  ‘Pterosaurs were the first flying animals – they appeared on Earth 50 million years before Archaeopteryx, the first bird – and they were good at what they did.  But the amazing thing is that they didn’t really begin to evolve until after the birds had appeared.’
Katy’s study was done in conjunction with her supervisors, Dr Marcello Ruta and Professor Michael Benton. They looked at 50 different pterosaurs that ranged in size from a blackbird to the largest of all, Quetzalcoatlus, with a wingspan of 12 metres, four times the size of the largest flying bird today, the albatross. They tracked how all the pterosaur groups came and went through their history and recorded in detail their body shapes and adaptations.

The new work shows that pterosaurs remained conservative for 70 million years, and then started to experiment with all kinds of new modes of life. After birds emerged and became successful, the pterosaurs were not pushed to extinction, as had been suggested. It seems they responded to the new flyers by becoming larger and trying out new lifestyles. Many of the new lifestyle adaptations were seen in the pterosaurs skulls, as they adapted to feed on different food sources; some were seed-eaters, many ate fish, and later ones even lost their teeth. The rest of the body also showed a surprising amount of variation between different groups, when considering that the body forms have to retain many features to allow flight.

‘Pterosaurs were at the height of their success about 125 million years ago, just as the birds became really diverse too,’ said Dr Marcello Ruta. ‘Our new numerical studies of all their physical features show they became three times as diverse in adaptations in the Early Cretaceous than they had been in the Jurassic, before Archaeopteryx and the birds appeared.’
Pterosaurs dwindled and disappeared 65 million years during the mass extinction that killed the dinosaurs.  In their day they had been a fair match for the birds, and the two groups divided up aerial ecospace between them, so avoiding conflict.

‘We’re delighted to see a student mastering some tough mathematical techniques, and coming up with such a clear-cut result,’ said Professor Michael Benton. ‘Paleontologists have often speculated about the coming and going of different groups of animals through time, but the new study provides a set of objective measurements of the relative success and breadth of adaptation of pterosaurs through their long time on the Earth.’
Further information can be found on the Palaeobiology and Biodiversity Research Group's website: The rise and rise of the flying reptiles.

The drawings on this page are by paleontologist and freelance palaeoartist, Dr Mark Paul Witton.  For more information about his work visit: MarkWitton.com

Paper
‘Evolution of morphological disparity in pterosaurs’ by Katherine C. Prentice, Marcello Ruta and Michael J. Benton in Journal of Systematic Palaeontology

Saturday, May 21, 2011

A "Missing Link" Fossil Between Laceretids and Amphisbaenians

The nearly complete fossil of Cryptolacerta 
hassiaca (Photo Credit:University of Toronto)

Until a recent discovery, theories about the origins and evolutionary relationships of snakes barely had a leg to stand on.

Genetic studies suggest that snakes are related to monitor lizards and iguanas, while their anatomy points to amphisbaenians ("worm lizards"), a group of burrowing lizards with snake-like bodies. The debate has been unresolved--until now. The recent discovery by researchers from the University of Toronto Mississauga and the Museum für Naturkunde Berlin, Germany of a tiny, 47 million-year-old fossil of a lizard called Cryptolacerta hassiaca provides the first anatomical evidence that the body shapes of snakes and limbless lizards evolved independently.

"This fossil refutes the theory that snakes and other burrowing reptiles share a common ancestry and reveals that their body shapes evolved independently," says lead author Professor Johannes Müller of Humboldt-Universität, Berlin.

The fossil reveals that amphisbaenians are not closely related to snakes, but instead are related to lacertids, a group of limbed lizards from Europe, Africa and Asia. "This is the sort of study that shows the unique contributions of fossils in understanding evolutionary relationships," says Professor Robert Reisz from the University of Toronto Mississauga, the senior author of the study. "It is particularly exciting to see that tiny fossil skeletons can answer some really important questions in vertebrate evolution".

The German research team, led by Müller and American graduate student Christy Hipsley, used X-ray computed tomography to reveal the detailed anatomy of the lizard's skull and combined the anatomy of Cryptolacerta and other lizards with DNA from living lizards and snakes to analyze relationships. Their results showed that Cryptolacerta shared a thickened, reinforced skull with worm lizards and that both were most closely related to lacertids, while snakes were related to monitor lizards like the living Komodo dragons.

Even though snakes and amphisbaeans separately evolved their elongate, limbless bodies, the discovery of Cryptolacerta reveals the early stages in the evolution of burrowing in lizards. By comparing Cryptolactera to living lizards with known lifestyles, co-author and U of T Mississauga paleontologist Jason Head determined that the animal likely inhabited leaf-litter environments and was an opportunistic burrower.

"Cryptolacerta shows us the early ecology of one of the most unique and specialized lizard groups, and also reveals the sequence of anatomical adaptations leading to amphisbaenians and their burrowing lifestyle," says Head. "Based on this discovery, it appears worm-lizards evolved head first."

Citation
Johannes Müller, Christy A. Hipsley, Jason J. Head, Nikolay Kardjilov, André Hilger, Michael Wuttke, Robert R. Reisz. Eocene lizard from Germany reveals amphisbaenian origins. Nature, 2011; 473 (7347): 364 DOI: 


Sunday, March 27, 2011

Dinosaur Petroglyphs, A Return to Reality

Not long ago I found a movie on Netflix titled Dragons or Dinosaurs, remembering a Nature episode, The Dragon Chronicles, done by Rom Whitaker that looked at how ancient myths of flying, fire-breathing dragons originated, I put it on my list. A few minutes into the movie and I realized it was a very slick piece of creationist propaganda, in fact it was a masterpiece. Confusing fantasy with reality is what creationsim has come to do best. And I have to give them credit, they have done a spectacular job of undermining science education in the USA. The movie's website carries the following teaser.

"Dragon images, legends and lore exist all over the world in many different cultures. But what if dragons were actually dinosaurs? Dinosaurs are often used to discredit the Bible, so what if their existence actually helps prove its veracity?"

A visit to the website reveals that they have even created a study guide to accompany the movie.

The movie contained a twist of reality I had not previously seen - and as a high school biology teacher I saw many. Rock art depicting dinosaurs was used in an attemp to confirm the notion that humans and dinosaurs lived together. Apparently, the dino rock art has been used by creationsists since the late 1990's.

Now, Phil Senter and Sally Cole have provided the first scientific examination of the dinosaur petroglyps and of course find that they are infact the usual creationist distortion of facts. The sauropod art work is at Kachina Bridge in Natural Bridges National Monument, Utah. Kachina Bridge is a massive sandstone formation resembling an archway over 60 m high and wide, formed by the undercutting of a rock wall by flowing water. The images are rock paintings and petroglyphs formed by pecking, abrading, incising, and scratching. Other, earlier examples are associated with hunter-gatherers that occupied the study area prior to 1000 B.C and atrributed to Ancestral Pueblo farming societies dating from approximately 200 to 1300 years before present. Some of the art work may have been made by more recent protohistoric or historic Paiute, Ute, or Navajo groups. Among the images made by prehistoric people on the walls of Kachina Bridge is what appears to be an unambiguous depiction of a sauropod dinosaur, Senter and Cole call this Dinosaur 1. And, they test hypothesis that a given petroglyph depicts a dinosaur predicts that the image is not a composite; depicts an animal; has features that cannot be reconciled with non-dinosaurian local fauna; has features of a specific, identifiable dinosaur; and is entirely human-made. They tested the predictions for Dinosaur 1 and three other alleged dinosaur petroglyphs at Kachina Bridge by on-site visual examination under varying light conditions. Their examination revealed that the “neck” and “back” of Dinosaur 1 are a composite of two separate petroglyphs, and its “legs” are a natural mud or mineral stain. A second alleged sauropod petroglyph is only a mud stain. The other two alleged dinosaur petroglyphs are human-made, but neither depicts an animal. Senter and Cole conclude that the four Kachina Bridge “dinosaurs” are in fact illusions produced by pareidolia. None of them support the predictions of the hypothesis that a dinosaur is depicted. Therefore, the dinosaur rock art fantasy joins the pile of discredited evidence from the creation movement.

Unfortunately, many state and local science curiculums have become so rigid that discussion of this kind of controversy in middle or high school classes has become difficult. In fact, having students read this propaganda and carefully examining it with the scientific critiques is an excellent way to deflate much of the creationist non-sense and enhance critical thinking across the curriculum.

Citation
Senter, P. and S. J. Cole. 2011. "Dinosaur pteroglyphs at Kachina Bridge site, Natural Bridges National Monument, southeastern Utah: not dinosaurs after all. Palaentologia Electronica 14(1):2A:5p;

Monday, March 21, 2011

Brighter Coloration Does Not Always Mean Greater Toxicity


A red morph of the Granulated
Dart Frog, Oophaga granulifera
Photo credit: Patrick Gijsbers.
Hugh Cott, noted the relationship between warning coloration and toxins in 1940 when he wrote, "Perhaps the most specialized and effective of all methods of defense is the use of poisons. Toxic properties have been developed in a wide range of animal life, and are frequently associated with warning coloration." A new study now suggests that at least one brightly colored morph of a dendrobatid frog may be less toxic than morphs with more subdued coloration. Ian Wang (2011) notes that the prevailing theory, following Cott, suggests that aposematic coloration evolves with toxicity so that increased toxicity will accompany greater conspicuousness. Dart frogs in the Dendrobates (=Oophaga) histrionicus group (D. histrionicus, D. pumilio, and D. granuliferus) have populations with unique color morphs spanning the visual spectrum, and while some of the morphs would seem likely to to be aposematic others are cryptic to the human eye. How these polymorphisms are mainatined is porrly understood. Wang measured spectral reflectance, toxicity, and did a phylogenetic reconstruction on nine populations of  O. granuliferus (Taylor, 1958) on the Pacific coast of Costa Rica. and found  that the less conspicuous color morphs are actually significantly more toxic than the brightest, most conspicuous phenotypes and that the more toxic, less-conspicuous form evolved from a less toxic, more conspicuous ancestor. Through gas chromatography—mass spectrometry analysis of toxin profiles, Wang traced the increase in toxicity in the less-conspicuous populations to an acquisition of specific alkaloids, some of which are convulsants. Wang's results challenge the idea that increased conspicuousness always evolves with increased toxicity and support the idea that once aposematism has been established in a species, phenotypic variation may evolve from brightness and toxicity becoming decoupled.

Literature
Cott, H. B. 1940. Adaptive coloration in Animals. London: Methuen & Co. Ltd. (Quote from page 253.)


Wang, I. J. 2011. Inversely related aposematic traits: reduced conspicuousness evloves with increased toxicity in a polymorphic poison-dart frog. Evolution, doi: 10.1111/j.1558-5646.2011.01257.x

Thursday, March 3, 2011

Turtle Size & The Environment


Hatchling Chrysemys picta. JCM
Biologists from the UCLA Division of Life Sciences have reported the first quantitative evidence for an evolutionary link between habitat and body size in turtles and tortoises.

The study, whose lead author is a high school student volunteer in the laboratory of UCLA evolutionary biologist Michael Alfaro, is currently available online in Biology Letters, a journal of the Royal Society. It will appear in a print edition later this year.

Turtles and tortoises, also called chelonians, represent a diverse group of reptiles that have been present on Earth for more than 200 million years. The 330 species of present-day chelonians can be found dwelling on remote islands, traveling across vast expanses of ocean, and living in desert and freshwater habitats on every major continent.

Even more surprising than the wide variety of places animals call home is the vast disparity in their body sizes. The largest chelonians weigh over 1,000 pounds and are more than 6 feet in length, while the smallest weigh just a few ounces and would easily fit in the palm of your hand.

Combining statistical computer modeling with genetic data and the fossil record, Alfaro, an associate professor of ecology and evolutionary biology, and his colleagues demonstrated that different environments have specific optimal body sizes for their chelonian inhabitants.

These researchers act as "evolutionary detectives," piecing together how the tremendous diversity in living chelonians today evolved from a common ancestor that lived millions of years ago. DNA sequences from modern chelonians provide important clues for determining the evolutionary path followed by their progenitors, said co-author Graham Slater, a National Science Foundation–funded UCLA postdoctoral scholar in ecology and evolutionary biology.

The results show a surprisingly strong statistical correlation between habitat change and significant adjustments in body size. Chelonians living in marine or island habitats have an optimal body size several times larger than their cousins on the mainland, said first author Alexander Jaffe, a high school student at Harvard–Westlake School in North Hollywood, Calif. Marine turtles have the largest optimal shell length (about 4.5 feet), followed by island tortoises (approximately 2.5 feet), while freshwater and mainland chelonians are several times smaller (roughly 1 foot).

Evolutionary biologist have long assumed there is a connection between habitat and body size in chelonians, but it was not possible until recently to show quantitative evidence for the relationship, Alfaro said.

Chelonians have had a special place in the history of evolutionary biology due to the attention given them in the writings of Charles Darwin, Alfaro said.

Giant island tortoises found in the Galapagos and Seychelles provide a classic example of "island gigantism," a well-observed phenomenon in which an island-dwelling species evolves to be much larger than its mainland counterparts. Because they provide uniquely isolated habitats, islands are regarded as natural experiments in evolutionary biology, according to Alfaro.

"Our study was focused on testing whether there was any evolutionary signal in support of the idea that being on islands allowed the tortoises to evolve large size," he said.

While it is clear that habitat is an important signal in the chelonian evolutionary tree, the specific ecological conditions that trigger the change in body size are more difficult to determine, Alfaro said.

One of the oldest groups of reptiles, marine chelonians such as early sea turtles might have fallen prey to giant seafaring Mesozoic reptiles, a situation which would make larger size a distinct advantage, Jaffe said. Larger size also plays a key role in maintaining body temperature and allowing for migration across considerable distances.

In the case of the giant tortoises, a larger body size gives them the ability to survive long periods without food, which may be necessary due to prolonged droughts that can occur in island habitats. Large body size also may allow giant tortoises to "raft" across vast expanses of ocean while going weeks without food, a feat documented through observations of giant tortoises with barnacle growth found on the mainland, Alfaro said.

"What is exceptional about chelonians is that they are one of the most distinctive groups of vertebrates, arose early in the history of terrestrial vertebrates, and persisted for a long time," Alfaro said. "Chelonians are good examples of evolutionary survivors."

The main goals of Alfaro's research group include studying the evolution of vertebrates and their subsequent diversity in shape, size and structure. This involves developing methods to identify time periods and locations on the tree of vertebrate life in which unusual amounts of species diversification have occurred, Alfaro said.

An 'incredible opportunity'

Jaffe, a senior at Harvard–Westlake School, started volunteering in Alfaro's laboratory when he was 16, after e-mailing Alfaro about his interest in conducting research. Jaffe spent almost 30 hours a week in the lab for two full summers and was able to turn his results into a first-authored paper — a feat rarely accomplished by high school students.

"Being part of this research group has been an incredible opportunity for me," Jaffe said. "I can't say how grateful I am. Not only did I learn the tools of the trade, especially in the lab, but also what it is like to start off with an abstract question and address it through data collection and interpretation."

Jaffe hopes to study biological sciences and pursue further research in college.

"Alexander was ready to take intellectual ownership of a project," Alfaro said. "In addition to being a very conscientious young scientist, Alexander really showed an interest in the questions that we are asking and in getting the data to answer those questions."

This research was federally funded by the National Science Foundation.

For more on Alfaro's research, visit his website at http://pandorasboxfish.squarespace.com.

Jaffe, A. J., G. J. Slater,  and M. E. Alfaro. 2011.  The evolution of island gigantism and body size variation in tortoises and turtles. Biology Letters doi: 10.1098/rsbl.2010.1084

Wednesday, February 16, 2011

Ichthyosaurs Are Diapsid Reptiles

The oceans of the Mesozoic contained species that we know today only from their fossil remains, species which looked quite unlike anything we are familiar with today. Ichthyosaurs may have looked slightly familiar, because despite the fact they were reptiles, they had dolphin- or fish-shaped bodies. The “fish lizards” first appear in the fossil record about 225 million years ago (MYA) and disappear about 90 MYA. While most species were in the 2 to 4 meter range, some grew larger, and a few of the earliest forms were less than 2 m long. Some specimens have been exceptionally well preserved and provide information on soft tissue, diet and reproduction. Their relationship to other tetrapods has been controversial and there has been much speculation on their origin but a consensus has been building that ichthyosaurs are indeed diapsid reptiles. The absence of a lower temporal region has been one of the sticking points to accepting them as diapsids. Liu et al. (2011) have now described the cranial skeleton of a new mixosaurid ichthyosaur specimen with a well-preserved lower temporal region from the Anisian Guanling Formation of easternYunnan. It is has the most primitive lower temporal region known in ichthyosaurs, and it was well preserved. The specimen provides definite direct evidence for the diapsid origin of ichthyosaurs. It also gives strong support to the hypothesis that the lower temporal fenestra in ichthyosaurs is lost due to the reduction of the jugal and the quadratojugal that comprise the primitive lower temporal arcade in diapsids. Given that these marine reptiles are in fact diapsids, the question remains what clade did they arise from? They may have shared an ancestor with the lizards.

Liu, J., J. C. Aitchison, Y.-Y. Sun, Q-Y Zhang, C.-Y. Zhou, and T. Lv, 2011. New mixosaurid Ichthyosaur specimen from the middle Triassic of SW China: further evidence for the diapsid origin of ichthyosaurs. Journal of Paleontology, 85(1):32-36. 2011

Saturday, February 5, 2011

Re-evolution of Teeth in a Frog


Gastrotheca guentheri
Louis Dollo, a French born, Belgian paleontologist is perhaps best known for supervising the reconstruction of Iguanodon fossils discovered in an underground mine in 1878; and for an idea which says, evolution is not reversible. The idea became known as Dollo’s Law and suggests that once organisms loose a structure during the course of evolution it will not re-evolve. Like most such “laws,” it was doomed to failure due to exceptions. John Wiens, Stony Brook University, has now documented the re-evolution of mandibular teeth in the marsupial frog, Gastrotheca guentheri. It has been long known that frogs do not have teeth on the mandible – with the exception of some frogs of the family Hemiphractidae. Weins used a time-calibrated amphibian phylogeny that demonstrates frogs lost their mandibular teeth 230 million years ago (MYA), only to have Gastrotheca gunetheri (or an ancestor) re-evolve mandibular teeth 5 to 17 MYA. George Kingsley Nobel recognized that Gastrotheca had true teeth, he wrote “The most remarkable osteological feature of the Hemiphractinae is the redevelopment of true teeth on the dentary….Such teeth do not occur in any other Sallientia…”

Weins’ results provide an exceptionally well documented example of re-evolution, demonstrating that mandibular teeth were lost in the ancestor of all living frogs and then re-evolved in the hemiphractid species G. guentheri. Weins points out that the re-evolution of mandibular teeth may not surprise herpetologists, but notes this example has been ignored in the recent literature on Dollo's law. The time involved from the loss of teeth to their re-evolution in Gastrotheca – an absence of at least 225 million years (and probably longer) is remarkable.

For a news story on this article follow this link.

Literature
Noble, G. K. 1931The Biology of the Amphibia. McGraw-Hill, New York .

Wiens, J. J. 2011. Re-evolution of lost mandibular teeth ion frogs after more than 200 million years, and re-evaluating Dollo’s law. Evolution, doi: 10.1111/j.1558-5646.2011.01221.x

Tuesday, January 18, 2011

Convergence of Infrared Vision in 3 Snake Clades


Three families of snakes use infrared waves to detect prey and differences in environmental temperatures. The mechanism involved in this has only been recently discovered to involve the transient receptor potential (TRP) ion channels. TRPs are involved in various biological processes, including calcium and magnesium homeostasis, neuronal growth, temperature sensation, and pain sensation. The sensations caused by the pungent agents of wasabi and other mustard plants are generated by our transient receptor potential ankyrin 1 (TRPA1) channel. Recently, it has been discovered that the orthologous receptors (receptors sharing a common ancestral gene) of the western diamondback rattlesnake (Crotalus atrox), ball python (Python regius), and garden tree boa (Corallus hortulanus) detect infrared radiation, while those the Texas rat snake (Pantherophis obsoletus lindheimeri) does not. The genetic mechanism of infrared sensitivity of these snake-specific TRPA1 proteins is unknown. Yokoyama et al. (2011) have now identified the amino acid changes that are responsible for the dramatic functional changes in the three groups of snakes. They suggest three parallel amino acid changes (L330M, Q391H, and S434T) are responsible for the development of infrared vision in the three groups of snakes. Protein modeling shows that the three amino acid changes alter the structures of the central region of their ankyrin repeats. The article can be found on-line.

Citation:
Yokoyama, S., A. Altun, and D. F. DeNardo. 2011. Molecular convergence of infrared vision in snakes. Molecular Biology and Evolution  28(1): 45-48. doi:10.1093/molbev/msq267

Saturday, January 15, 2011

The Viviparous Lizard – Clues to Understanding the Transition


Watching a female squamate giving birth or lay eggs is similar in many ways, peristaltic waves move the embryo or the egg to the cloaca and the opening expands to allow the next generation to enter the world. Egg laying is an ancestral trait in squamates - but live birth has evolved numerous times in many different clades. Over the years I have had occasion to incubate Bullsnake, Fox Snake, and Green Snake eggs. In the first two species incubation time was at least eight weeks, while the Green Snake eggs, much to my surprise, hatched within about two weeks of being laid. Female Green Snakes hold their eggs for longer periods of time, the embryo is at a more advanced state of development, before the female deposits them in a nest – a trait thought to be one step in the transition between being oviparous and viviparous.

Transitioning between laying eggs and giving live birth has been considered the result of gradual changes, but it is well known that populations of some egg-laying species have live-birth so gradual may not be the correct adjective. The Viviparous Lizard is such a species. It is widely distributed in Eurasian, from Spain, Italy, Serbia, Bulgaria and Macedonia in the south it ranges northward into the Arctic Circle. In the Alps it can be found as high as 3000 m above sea level. The more southerly populations lay eggs, while populations at higher latitudes have live birth.

Male common lizard basking.
© James Lindsey/Wikimedia
In two recent papers Tania Rodríguez-Díaz of the Universidad de Oviedo, Spain and colleagues have examined the impact of incubation temperature on the young of the Viviparous Lizard, Zootoca vivipara, and the impact of egg retention on the timing of egg laying.

Rodríguez-Díaz et al. (2010) studied variations in the temperature selected by gravid females compared with those selected by males and non-gravid females of Zootoca vivipara of Northern Spain as well as the impact of incubation temperature on the hatchlings. They found cloacal temperatures of gravid females active in the field were lower than those of males and non-gravid females, as well as the temperatures selected in a thermal gradient created in the laboratory (32°C for gravid females; 34°C for males and non-gravid females). Effects of temperature were assessed by incubating eggs at five constant temperatures between 21 and 34°C. The incubation temperatures affected the hatchlings’ morphology and survival rates. Hatchlings incubated at 34 °C had shorter heads than those from other temperatures; and they had lower survival (58%), significantly lower than at the other temperature treatments (mean 93%). Gravid females select lower body temperatures, as might be expected based on the predictions made by the maternal manipulation hypothesis. The shift in preferred temperature by pregnant females would result in only a very short delay, if any, of hatching time and, because the temperature selected by pregnant females is much higher than average temperatures recorded in natural nests. Retaining eggs shortens incubation time, according to predictions of the cold-climate hypothesis and the authors’ found their experiments with the Viviparous Lizard were in agreement with both the maternal manipulation hypothesis and the cold climate hypothesis.

In a second paper, Rodríguez-Díaz and Braña (2011) investigated the Viviparous Lizard’s ability to retain eggs. Female Z. vivipara were forced to retain their eggs by keeping them on dry substrates. They then assessed the effects on embryonic development, hatching success, offspring phenotype, and locomotor performance. Forced egg retention for the additional week affected the developmental stage of embryos at egg laying, as well as hatchling robustness, and locomotor performance. Embryos from forced clutch retention treatment reached one level of development beyond control embryos at oviposition time. Embryos from control eggs were more developed than embryos from experimental eggs after approximately the same period of external incubation, showing that embryonic development is retarded during the period of extended egg retention, despite the high temperature inside the mother's body. The experimental group with forced egg retention had lower hatching success (21.1%) than in the control group (95.4%).

The results suggest retaining eggs interferes with development, and that simply retaining eggs for longer periods of time does not represent a clear advantage to offspring (or their mothers). However, females that can retain eggs during unfavorable climatic conditions can be successful and doing so may be an important step in the transition to live birth.

Literature
Rodríguez-Díaz, T., F. González, X. Ji and F. Braña. 2010. Effects of incubation temperature on hatchling phenotypes in an oviparous lizard with prolonged egg retention: are the two main hypotheses on the evolution of viviparity compatible? Zoology 113:233-38.
                                              
Rodríguez-Díaz, T., F. and F. Braña. 2011. Plasticity and limitations of extended egg retention in oviparous Zootoca vivipara (Reptilia: Lacertidae). Biological Journal of the Linnean Society 102:75–82.


Thursday, December 23, 2010

Horned Lizard Research and Observations


Phrynosoma modestum, JCM
The Phrynosomatidae, is a group of iguanian lizards that range from Canada to Panama and contains some of the most familiar saurians in North America, including: the spiny lizards (Sceloporus), horned lizards (Phrynosoma), and side-blotched lizards (Uta). The family contains nine genera (ten genera if the genus Sator is recognized separately from Sceloporus) and more than 136 species. Weins et al. (2010) used molecular techniques to show that phrynosomatids are divided into two major clades the Phrynosomatinae and Sceloporinae. Phrynosomatinae contains the horned lizard clade Phrynosoma and the sand lizard clade (Callisaurus, Cophosaurus, Holbrookia, and Uma).

Skull of a Phrynosoma with spiny processes.JCM
Horned Lizards of the genus Phrynosoma are remarkably cryptic. Their dorso-laterally flattened bodies, tan and brown coloration and spine covered heads and bodies make them exceptionally difficult to find. The spine covered heads and bodies of these lizards undoubtedly serve as a deterrent to predators, but horned lizards also eat ants and store the noxious formic acid from the ants in their blood. Yet some predators are able to deal with the spines and chemicals. O’Connor, et al. (2010) report finding an adult female Great Basin Nightsnake (Hypsiglena chlorophaea deserticola) in Kittitas County, Washington that regurgitated a half-digested adult male Pygmy Short-horned Lizard (Phrynosoma douglasii) estimated to be about 41 mm SVL and 6 grams. The snake's mass was estimated to be just over 12 g. so the predator-prey mass ratio was about 0.50.

Lahti, et al. (2010) found Phrynosoma (Tapaja) douglasii inhabiting 3 distinct microhabitat types (lithosol, loamy, and ecotone) within the shrub-steppe of central Washington’s Quilomene Wildlife Area. The study site had been used for grazing until 1979, and fires were minimal in the last 30 years. June and July were the peak activity months for lizards. Most lizards were encountered in lithosol (61%), a habitat with sparse vegetative cover and weathered fragments of rock; followed by ecotone (31%); and loamy (8%) microhabitats. Lizards, particularly those inhabiting lithosol microhabitats, did not usually retreat to shrub cover until approached within 1 m. While horned lizards are considered low-density species relatively high population densities have been reported for Phrynosoma (Tapaja) douglasii (14.3 to 14.6 lizards/ha in eastern Idaho). However, the authors report a density of about 2 lizards per hectare at their study site, a density that is more characteristic of that reported for other Phrynosoma species. Neonates were almost always encountered closest to Thymeleaf Buckwheat and would often retreat toward the plant when approached. Thymeleaf Buckwheat has the smallest and most compact growth form of any perennial plant at Quilomene Wildlife Area. In contrast, adults would usually retreat to either Stiff Sage or Rock Buckwheat, both of which are larger. The authors conclude that Phrynosoma (Tapaja) douglasii occurs at low densities in the shrub-steppe of Washington where females are larger and more abundant than males, neonates are rare, and reproductive output is low. Younger lizards maintain activity into hotter periods and remain active later in the activity season than do adults, a trait likely related to the importance of garnering sufficient energy to emerge in good condition after a long winter. While this species is most commonly encountered in shrub-steppe habitats, it shows considerable spatial and seasonal variation in the use of microhabitats.

Hellgren et al. (2010) describe the effects of rotational livestock grazing and prescribed winter burning on the resources and survival of the Texas Horned Lizard (Phrynosoma cornutum) in southern Texas. Winter burning provided an increase in food resources and led to increased survival rate in the second growing season after fire. However, grazing-induced changes in vegetation cover reduced survival, probably by increasing lizard vulnerability to predation. Fire and grazing reduced litter, increasing open ground and forb cover but did not alter woody vegetation. Ant activity was greater in burned sites and varied with grazing level, season, and year. Higher survival observed on burned sites in the second year after burning. Survival rates were ordered from highest in un-grazed sites to lowest in heavily grazed sites.

In three papers Cooper and Sherbrooke (2010a,b,c) investigated anti-predator behavior in the Round-tailed Horned Lizard (Phrynosoma modestum) and the Texas horned lizard (Phrynosoma cornutum). They (Cooper and Sherbrooke, 2010a) investigated the effects of repeated attacks by a predator on the Texas Horned Lizard, P. cornutum, and the opportunity cost of fleeing during a social encounter in P. modestum. The results suggest flight initiation distance was greater the second time a predator approached and probability of fleeing decreased as the distance between the predator and prey increased, but was greater when the predator turned toward than away from a lizard. The flight initiation distance was shorter during social encounters than when lizards were solitary. It appears that risk assessment by horned lizards conforms to the predictions of escape theory and is similar to that in other prey despite their specialized defenses. The results suggest that escape theory based on costs and benefits applies very generally, even to highly cryptic prey with specialized defense mechanisms.

In a second paper, Cooper and Sherbrooke (2010b) note that Phrynosoma modestum is eucryptic in that it resembles small stones and the authors predicted that flight initiation distance by P. modestum is shorter among stones than on uniform sand and that flight initiation distance is greater after movement and when standing than when still and lying on the ground.  Movement and upright posture disrupt crypsis in this lizard. The authors measured running speed and flight initiation distance to determine relationships among body temperature, speed, and escape decisions. Running speed and flight initiation distance were reduced at lower body temperature, suggesting that crypsis reinforced by immobility is more advantageous than longer flight initiation distance for cool, slow lizards. Thus, the Round-tailed Horned lizard adjusts its escape decisions to the current effectiveness of crypsis and escape ability.

Cooper and Sherbrooke (2010c) found that Texas horned lizards (Phrynosoma cornutum) would take flight sooner when approached rapidly rather than slowly and when approached directly rather than indirectly. They also found  P. cornutum were much more likely to move and jump when a model predatory bird passed overhead and cast a direct shadow on them as opposed to casting a shadow near the lizard. They suggest P. cornutum assess themselves as being in immediate peril when suddenly covered by a shadow. So, while the Texas Horned Lizard relies heavily on crypsis, they make escape decisions based on the degree of predation risk.

Literature
Cooper, W. E. and W. C. Sherbrooke. 2010a. Plesiomorphic Escape Decisions in Cryptic Horned Lizards (Phrynosoma) Having Highly Derived Antipredatory Defenses. Ethology, 116: 920–928. doi: 10.1111/j.1439-0310.2010.01805.x
Cooper, W. E. and W. C. Sherbrooke. 2010b. Crypsis influences escape decisions in the Round-tailed Horned Lizard (Phrynosoma modestum). Canadian Journal of Zoology, 88:1003-1010.
Cooper, W. E. and W. C. Sherbrooke. 2010c. Initiation of Escape Behavior by the Texas Horned Lizard (Phrynosoma cornutum). Herpetologica 66:23-30.
Hellgren, E. C., A. L. Burrow, R. T. Kazmaier, and D. C. Ruthven. 2010. The Effects of Winter Burning and Grazing on Resources and Survival of Texas Horned Lizards in a Thornscrub Ecosystem. Journal of Wildlife Management 74(2):300-309.
Lahti, M. E., D. D. Beck, and T. R. Cottrell. 2010. Ecology of the Pygmy Short-Horned Lizard [Phrynosoma (Tapaja) douglasii] in Washington. Northwestern Naturalist 91(2):134-144.
Montanucci, R.R. 2004. Geographic variation in Phrynosoma coronatum (Lacertilia, Phrynosomatidae): further evidence for a peninsular archipelago. Herpetologica 60 (1): 117-139
O'Connor, A. P., J. L. Wallace, R. E. Weaver, and M. P. Hayes. 2010. Pygmy Short-Horned Lizard (Phrynosoma douglasii): Unrecorded Prey for the Great Basin Nightsnake (Hypsiglena chlorophaea deserticola).  Northwestern Naturalist 91(1):79-81.
Wiens , J. A., C. A. Kuczynski, S. Arif , and T. W. Reeder. 2010. Phylogenetic relationships of phrynosomatid lizards based on nuclear and mitochondrial data, and a revised phylogeny for Sceloporus. Molecular Phylogenetics and Evolution 54:150–161

Sunday, December 19, 2010

Sex Determination in Gekkota


The Leopard Gecko, Eublepharis maculatus, is
one of the best studied geckos in terms of its
sex-determination mechanism. JCM
The known number of sex determining mechanisms and variations on them seem to be ever increasing in reptiles as we learn more about them. There are species with male and female heterogamety - sex chromosomes; species with temperature-dependent sex determination, and species with both systems. Within each of these there seems to be many variations. Geckos (Gekkota) are the second most specious lineage of lizards (skinks are the first) with more than 1300 species placed in six different families. The diversity of geckos and their sex determination mechanisms make them excellent candidates for studying the evolution of these mechanisms and current knowledge suggests that geckos have transitioned from one mechanism to another many times during their evolutionary history. Yet, of the 1300 species, relatively few (about 46) have been examined for the mechanisms they use. Tony Gamble of the University of Minnesota has recently summarized the sex determination mechanisms used by geckos in various lineages and discovered that at least 8 or 9 transitions have occurred within the last 150 million years, despite the low number of species that have been examined to date. The Carphodactylidae has not been studies at in this regard, and the Diplodactylidae, Phyllodactylidae, and Sphaerodactylidae are poorly known in terms of how they determination the sex of their offspring. Gamble’s work suggests the ancestral gecko used male heterogamety as the determining mechanism with temperature-dependent sex determination evolving 5 times independently from a genetic sex determination ancestor.

Citation

Friday, December 17, 2010

Crocodile Skinks of the South Pacific


Tribolonotus gracilis. JCM
Skinks come in a plethora of shapes and sizes, with about 1500 species they have invaded a huge variety of habitats and evolved a great diversity of life styles. They are found in almost all landscapes that support squamates and are perhaps the most successful lineage of living reptiles – if the measure is by the number of species. Among the most bizarre skinks are the South Pacific Crocodile Skinks of the genus Tribolonotus. Currently 8 species are recognized and they inhabit northern New Guinea and the Admiralty, Bismarck and Solomon Archipelagoes. They are semi-fossorial lizards, often found under vegetation and in the vicinity of water. At least two species are known to vocalize, and they demonstrate parental care. Hartdegen et al. (2001) described defensive vocalizations and parental care in captive specimens. They observed females curled around their egg and when eggs were gently handled by the observer the female exhibited defensive open-mouth lunges. When eggs were left uncovered by human observers they were reburied by the female. Hatchlings stayed near their mother (within 2 cm) and on occasion they were observed resting on the female's dorsum for two weeks after hatching. Their overall appearance is distinctive; they have unusually heavily keeled or spinose scales and two unique characters, abdominal glands and volar pores (pores on the plantar and palmer surfaces).

Recently, Austin et al. (2010) used molecular techniques and found evidence that Tribolonotus originated on either Greater Bougainville Island or in New Guinea, and subsequently dispersed to surrounding islands multiple times. Maximum body size ranged from 40 mm in T. blanchardi (and T. schmidti was a close second at 41 mm) to 125 mm in T. ponceleti. The authors did not find a phylogenetic explanation for differences in body size, and suggest that it evolved as the result of character displacement and ecological factors.

Exactly what the sister of Tribolonotus is remains a point of contention. They have been considered lygosomine skinks, allied with the genera Sphenomorphus, Mabuya, and Egernia by various authors. However,  Donnellan (1991) found Tribolonotus gracilis, has 32 chromosomes, a similar karyotype, to Egerina but there were differences that did not allowed a firm conclusion.

Literature
Austin, C. C., E. N. Rittmeyer, S. J. Richards and G. R. Zug. 2010. Phylogeny, historical biogeography and body size evolution in Pacific Island Crocodile skinks Tribolonotus (Squamata; Scincidae). Molecular Phylogenetics and Evolution, 57:227-236.

Donnellan, S. C. 1991. Chromosomes of Australian lygosomine skinks (Lacertilia: Scincidae).Genetica 83:207-222.

Hartdegen, R. W., M. J. Russell, B. Young, and R. D. Reams. 2001. Vocalization of the Crocodile Skink, Tribolonotus gracilis (De Rooy, 1909), and evidence of parental care. Current Herpetology 2001(2). On-line.

McCoy, M., 2006. Reptiles of the Solomon Islands. Pensoft Publishing, Sofia-Moscow.