Thursday, November 28, 2013

Sub-lethal skin sections in the wrinkled frog effective against snake predation

Frogs and toads are packets of proteins and calories to many predators. In response anurans have evolved a plethora of defenses against their predators, many of these defenses are chemical. Dendrobatid frogs may kill a potential predators with its defensive toxins, but many species of anurans seem to produce toxins that are sub-lethal. Anuran skin secretions may also have other functions, such as antibacterial protection for their skin. But defensive molecules are abundant in a more or less defenseless group of amphibians.
The Japanese striped snake, Elaphe quadrivirgata,, and the prey it usually avoids, Glandirana (Rana) rugosa.
Photo credit: Alpsdake, and Open Cage.
Some molecules produced by frogs alter the predator's behavior. African clawed frogs  induce yawning and gaping movements in the northern water snake Nerodia sipedon as well as two other natural predators, the African aquatic snakes Lycodonomorphus rufulus and L. laevissimus.

In a new study Yoshimura and Kasuya (2013) examine the impact of the molecules produced by the adult wrinkled frog (Glandirana (Rana) rugosa) which has warty skin with a secretion that has a strong and unique odor. The wrinkled frog is rarely found in the natural diet of the Japanese striped snake (Elaphe quadrivirgata), which is considered a general predator of amphibians, mammals, birds, and reptiles. In a previous study, newborn Japanese striped snake with no prey experience ate few wrinkled frogs. When adult Japanese striped snakes  were forced to swallow wrinkled frogs, all the snakes spat out the frogs and opened and closed their mouths in a gaping behavior. The snake did not change its movements or other behaviors and they did not die shortly after contact with the wrinkle frog. These observations suggested that wrinkled frogs are not highly toxic but that they escape from predation by snakes.

Yoshimura and Kasuya (2013) conducted two experiments to examine whether the skin secretion of adult wrinkled frogs is effective for the evasion of predation by snakes. In the first experiment , they compared the proportion of snakes that bit and swallowed wrinkled frogs with the proportion that bit and swallowed the cricket frog, Fejervarya limnocharis, which resembles wrinkled frogs in size and appearance. In the second experiment they coated the natural prey organisms of the snakes with secretions from the wrinkled frog or the cricket frog to examine the effects of the secretions.

They found the wrinkle frog was less frequently bitten or swallowed by snakes. The snakes that bit wrinkled frog spat out the frogs and showed mouth opening (gaping) behavior, while the snakes that bit the cricket frogs did not show gaping behavior. They also compared the responses of the snakes to wrinkled frogs and F. limnocharis secretions. They coated palatable Rana japonica with secretions from wrinkled frogs or cricket frogs. The frogs coated by wrinkled frog's secretion were less frequently bitten or swallowed than those coated by F. limnocharis secretion. The authors concluded that compared to different frog species of similar sizes, the adult wrinkled frog was less frequently preyed upon by, and that its skin secretion was effective in avoiding predation by snakes.

Yoshimura Y, Kasuya E (2013) Odorous and Non-Fatal Skin Secretion of Adult Wrinkled Frog (Rana rugosa) Is Effective in Avoiding Predation by Snakes. PLoS ONE 8(11): e81280. doi:10.1371/journal.pone.0081280

Wednesday, November 27, 2013

A second look at the Tethyan limbed snakes

Artist's reconstruction of Pachyrhachis problematicus.
Three fossil marine snakes with hind-limbs Pachyrhachis problematicus, Eupodophis descouensi, and Haasiophis terrasanctus. are known from the upper Cretaceous (Cenomanian) of the Middle East. All of them were collected from the region that formed the ancient Tethys Sea. The first species was described in 1979 and a reduced limb lizard,but later recognized as a snake, the others were described in 2000. The phylogenetic status of these snakes has been controversial. In a new paper Palci et al. (2013) have re-evaluated the fossil anatomy of these three species and have drawn some interesting conclusions.  They found no evidence of a laterosphenoid in Haasiophis and Eupodophis; Pachyrhachis and Eupodophis retain a jugal; Haasiophis and Eupodophis, have  chevron bones in the tail; Haasiophis has a large number of unfused intercentra along the anterior portion of the body's vertebral column; and Pachyrhachis has numerous mental foramina on the dentary, as well as at least one sacral vertebra with unfused sacral ribs.

 The authors ran three phylogenetic analyses using alternative out groups (varanoids; iguanians; and skinks+ amphisbaenids, + dibamids) to polarize the character transformations. The in-group consisted of all well-preserved fossil snakes from the Cretaceous, the madtsoiids (mostly Gondwanan snakes with a fossil record extending from the Upper Cretaceous to late Pleistocene from South America, Africa, India, Australia and Southern Europe), and taxa that are representative of all major groups of living snakes. The analyses suggested Pachyrhachis, Eupodophis, and Haasiophis are either a series of stem taxa at the base of the radiation of snakes, or they are members of a clade of fossil snakes that are the sister group to all living alethinopidians (all living snakes minus the scolecophidians).

The authors also found  free intercentra located at the base of the anterior pre-cloacal vertebrae of Haasiophis terrasanctus. If these intercentra are homologous with the cervical intercentra of limbed squamates, this would suggest snakes experienced a considerable amount of axial elongation that involved not only the dorsal but also the cervical region, a point that is supported by the posterior extension of some cervical muscles in snakes. Of interest, a similar pattern of axial elongation as been observed  in dolichosaurs, adriosaurs, and pontosaurs—a group of lizards that have been considered close relatives of snakes since the 19th century. The authors comment that they do not consider these lizards ancestral to snakes, but they may have a close phylogenetic relationship with them.

Citation
Alessandro Palci , Michael W. Caldwell and Randall L. Nydam (2013) Reevaluation of the anatomy of the Cenomanian (Upper Cretaceous) hind-limbed marine fossil snakes Pachyrhachis, Haasiophis, and Eupodophis. Journal of Vertebrate Paleontology, 33:6, 1328-1342.


Monday, November 25, 2013

New species of Neotropical Treerunners in the genus Plica

Treerunners are diurnal, medium sized lizards that sit in the open on vertical surfaces, and are often in small colonies that include adults of both sexes and juveniles. The sounds they make scurrying on the bark of trees or rock outcrops draws attention to their presence and thus, they are common in museum collections.

The tropidurid lizard genus Plica (treerunners) currently contains four species restricted to South America east of the Andes. Two of these are relatively widespread (Plica plica and P. umbra). The other two species are associated with Pantepuis. Plica lumaria is known only from southern Venezuela’s Guaiquinima Tepui, and P. pansticta  from the Yutajé–Corocoro massif of Amazonas, Venezuela.

 Etheridge (1970) restricted the type locality for Lacerta plica Linnaeus to the vicinity of Paramaribo, Suriname, designating NRM.112 as the lectotype. Hoogmoed (1973) further restricted the locality to the confluence of the Cottica River and Perica Creek, Suriname. However, the collared treerunner, Plica plica, is known from the countries of Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, Suriname, and Venezuela, as well as the islands of Trinidad and Tobago, and including the Bocas Island group Additionally, two specimens collected in the 19th century in the British Museum with the locality data “Grenada.”

Of the four species of Plica currently recognized only P. umbra lacks tufts of spines on the neck; and it has 43–69 scales around mid-body. Plica lumaria is black, the superciliaries are directed laterally, it lacks clusters of spines on the fold below the auditory meatus, and it has 141–156 rows of scales around mid-body and 27–33 lamellae under the fourth toe. Plica pansticta has 143–164 scales around mid-body and 31–39 lamellae under the fourth toe. However, P. plica (sensu Etheridge 1970) has 92–202 scales around mid-body; 21–35 lamellae on the fourth finger and 28–45 lamellae on the fourth toe. The polytypic P. plica has been the subject of ecological, morphological, and phylogenetic studies.

Murphy and Jowers (2013) took another look at Plica plica and uncovered multiple species that have been overlooked. While they focused their work on noethern South America, they did examine specimens from across the distyribution of Plica plica and estimate it contains at least 10 species, four of which are described in the new paper published in ZooKeys.

Caption. (a) Plica caribena sp. n. named for its Caribbean coastal distribution. Photo JCM; (b) Plica rayi named in honor of Ray Pawley for his life long work and interest in herpetology. Photo Zelimir Cernelic; (c) Plica plica. Photo Cesar Barrio Amoros; (d) Plica medemi named after Colombian herpetologist and collector of the specimen Fredico Medem. Photo JCM (e) Plica kathleenae named in honor of Kathleen Kelly, Division of Reptiles, Field Museum. Photo JCM.
The allopatric species described in the paper are associated with northern South American geography. Plica plica is associated with the Guiana Shield (Suriname, Guyana and Venezuela). A second species, P. caribeana sp. n. is associated with the Caribbean Coastal Range of Venezuela including Trinidad and Tobago. A third, very distinctive species, P. rayi sp. n. is associated with the Middle Orinoco at the eastern edge of the Guiana Shield.  Two other species, P. kathleenae sp. n. and P. medemi sp. n., are each based upon a single specimen, one from the Sierra Acarai Mountains of Guyana, and the other from southern Meta, Colombia. In addition to the morphological analyses, they sequenced 12S and 16S rDNA gene fragments from one Plica plica from Trinidad to assess its relationship and taxonomy to other mainland Plica. The results suggest Plica caribeana sp. n. likely diverged prior to the separation of Trinidad from Northern Venezuela. Isolation in the Caribbean Coastal Range during its rapid uplift in the late Miocene, combined with a marine incursion into northern Venezuela may have contributed to their genetic divergence from other populations.

Citation:
Murphy JC, Jowers M. 2013. Treerunners, cryptic lizards of the Plica plica group (Squamata, Sauria, Tropiduridae) of northern South America. ZooKeys

Saturday, November 23, 2013

New study reports US frogs have relatively few abnormalities

A 10-year study shows some good news for frogs and toads on national wildlife refuges. The rate of abnormalities such as shortened or missing legs was less than 2 percent overall — indicating that the malformations first reported in the mid-1990s were rarer than feared. But much higher rates were found in local "hotspots," suggesting that where these problems occur they have local causes. The results were published Nov. 18 in the journal PLOS ONE.

"We now know what the baseline is and the 2 percent level is relatively good news, but some regions need a deeper look," said Marcel Holyoak, professor of environmental science and policy at the University of California, Davis, and a co-author on the study. Hotspot regions included the Mississippi River Valley, California and south-central and eastern Alaska.



Mari Reeves, a graduate student working with Holyoak, led the data analysis and is corresponding author on the paper. Reeves now works at the U.S. Fish and Wildlife Service in Alaska.

Fieldwork for the study was carried out by the Fish and Wildlife Service at 152 refuges across the country between 2000 and 2009. Researchers collected more than 68,000 frogs and toads for the study. The complete dataset is available to researchers and the public online.

The aim of the study was to understand where and when these abnormalities occur — are they widespread, or localized? Are they persistent, or do they appear and fade away? — rather than to identify specific causes, Holyoak said. Understanding the patterns of these hotspots in space and time can help researchers home in on likely causes, he said.

The results show that abnormality hotspots occur in specific places, but within these hotspots the rate of malformations can change over time, Holyoak said.

"We see them at an elevated frequency one year or for a few years, and then they recover," he said.

The most common problems observed were missing or shortened toes or legs, and skin cysts. Only 12 cases of frogs with extra legs were found.

Many different potential causes have been put forward for the abnormalities, including pollution from industry or agriculture, parasites, ultraviolet exposure and naturally occurring heavy metals leaching into water bodies. The exact cause may vary from place to place, Holyoak noted.

The study comes against a background of a general decline in amphibian populations both in the U.S. and worldwide. For example, the California red-legged frog celebrated by Mark Twain's story is now listed as threatened. Frogs and toads may be especially sensitive to changes in climate and air or water quality. It's not clear whether hotspots of malformations contribute to this general decline, Holyoak said, but the new dataset will help researchers explore the problem.

Mari K. Reeves, Kimberly A. Medley, Alfred E. Pinkney, Marcel Holyoak, Pieter T. J. Johnson, Michael J. Lannoo. Localized Hotspots Drive Continental Geography of Abnormal Amphibians on U.S. Wildlife Refuges. PLoS ONE, 2013; 8 (11): e77467 DOI: 10.1371/journal.pone.0077467

Friday, November 22, 2013

Rhinoderma & Bd

Rhinoderma darwinnii. (Photo Credit:
Copyright Claudio Soto-Azat)
Deadly amphibian disease chytridiomycosis has caused the extinction of Darwin's frogs, believe scientists from the Zoological Society of London (ZSL) and Universidad Andrés Bello (UNAB), Chile.
Although habitat disturbance is recognised as the main threat to the two existing species of Darwin's frogs (the northern Rhinoderma rufum endemic to Chile, and the southern Rhinoderma darwinii from Chile and Argentina), this cannot account for the plummeting population and disappearance from most of their habitat.
Conservation scientists found evidence of amphibian chytridiomycosis causing mortality in wild Darwin's frogs and linked this with both the population decline of the southern Darwin's frog, including from undisturbed ecosystems and the presumable extinction of the Northern Darwin's frog.

The findings were published 20 Nov in the journal PLOS ONE.

Professor Andrew Cunningham, from ZSL's Institute of Zoology says: "Only a few examples of the "extinction by infection" phenomenon exist. Although not entirely conclusive, the possibility of chytridiomycosis being associated with the extinction of the northern Darwin's frog gains further support with this study."

Hundreds of specimens of Darwin's frogs and other amphibians from similar habitats collected between 1835 and 1989 were tested in order to find DNA pieces of Batrachochytrium dendrobatidis (Bd), a fungus that causes the disease chytridiomycosis. In addition, 26 populations of Darwin's frogs were surveyed in Chile and Argentina between 2008 and 2012 for the presence of Bd.

Darwin's frogs were named after Charles Darwin who first discovered R. darwinii in 1834 in south Chile during his famous voyage around the globe. The species have a distinct appearance, having evolved to look like a leaf, with a pointy nose. Research leader Dr. Claudio Soto-Azat, from UNAB and former ZSL PhD student says: "Amphibians have inhabited the earth for 365 million years, far longer than mammals. We may have already lost one species, the Northern Darwin's frog, but we cannot risk losing the other one. There is still time to protect this incredible species," Dr Soto-Azat added.

Amphibians provide an important ecosystem service by maintaining balance in the environment. Without them insect plagues and their subsequent effect on agriculture and public health would be more frequent. ZSL scientists are working to further understand the reasons behind the extinction of Darwin's frogs, and ensure the long-term survival of the species.

Citation
Claudio Soto-Azat, Andrés Valenzuela-Sánchez, Barry T. Clarke, Klaus Busse, Juan Carlos Ortiz, Carlos Barrientos, Andrew A. Cunningham. Is Chytridiomycosis Driving Darwin’s Frogs to Extinction? PLoS ONE, 2013; 8 (11): e79862 DOI: 10.1371/journal.pone.0079862


Monday, November 18, 2013

Stretching the lower jaw

The ability of skin and organs to stretch is important in determining the size of prey a snake can swallow. Snake skin consists of a keratinized epidermis divided into thick scale regions and thinner, folded interscale regions that are underlain by a dermis containing a complex array of fibrous connective tissues. In a new study Close and Cundall (2013)  examine the skin of the lower jaw of the northern watersnake, Nerodia sipedon, to determine how skin morphology changes when it is highly stretched during ingestion of large prey.

Biomechanical properties of connective tissue have been relatively well studied and collagen is responsible for much of the skin's ability to resist mechanical failure. In mammalian skin, at low levels of strain (up to about 20%) there is little change in the tissue's mechanical properties because collagen undergoes reorientation prior to stretching and slippage. At increased loads collagen account for skin's viscoelastic properties. However, under prolonged periods of strain, collagen alone is incapable of full recovery from high levels of extension due to irreversible creep. Elastin in the deep dermis has been suggested as being responsible  for skin's elastic behavior, but this has not been supported by in vivo or in vitro studies of mammalian skin. Given the known structure and function of elastin it is likely that elastin in the dermis of snakes is responsible for the refolding of interscale skin and subsequently for returning the skin to its resting condition after prolonged periods of stretch.

The skin and intermandibular soft tissues determine the lower jaw's extensibility and the upper jaws of snakes are limited in the degree to which they can move laterally. Therefore, suspensorial length and mobility, mandible length and lower jaw extensibility are the major determinants of gape size in snakes. Within the context of macrostomy - snakes being able to swallow exceptionally large prey - Cose and Cundall (2013)  examine how snake skin between the two sides of the lower jaw behaves during swallowing and how its function is related to structure.

Video records of skin behavior in the lower jaw of watersnakes feeding on fish or anesthetized watersnakes being stretched on an Instron machine showed that most skin extension involves the interscale skin. The largest intermandibular separation recorded during feeding was 7.7x the resting distance, but intermandibular separation reached 10x without tissue failure during mechanical testing. Histological and anatomical analyses of lower jaws fixed in resting, moderately or highly stretched conditions showed that stretching had little effect on scale regions of the epidermis. However, stretching flattened folds of interscale regions at both gross and cellular levels and imposed changes in epidermal cell shape. Stretching of the dermis is primarily limited to realignment of collagen and stretching of elastin in the deep dermis. The configuration of dermal elastin suggests a model for passive recovery of epidermal folding following release of tension.

Citation
Close M, Cundall D. 2013. Snake lower jaw skin: Extension and recovery of a hyperextensible keratinized integument. J. Exp. Zool. 9999A:1–20.

Tuesday, November 5, 2013

Snakes control blood flow to the spectacle to improve vision

Instead of eyelids, snakes have a clear scale 
called a spectacle. It works like a window, covering 
and protecting their eyes. When presented with a 
threat, the fight-or-flight response changes the spectacle’s
 blood flow pattern, reducing blood flow for longer 
periods than at rest, up to several minutes.
Nov. 4, 2013 — A new study from the University of Waterloo shows that snakes can optimize their vision by controlling the blood flow in their eyes when they perceive a threat.

Kevin van Doorn, PhD, and Professor Jacob Sivak, from the Faculty of Science, discovered that the coachwhip snake's visual blood flow patterns change depending on what's in its environment. The findings appear in the most recent issue of the Journal of Experimental Biology.

"Each species' perception of the world is unique due to differences in sensory systems," said van Doorn, from the School of Optometry & Vision Science.

Instead of eyelids, snakes have a clear scale called a spectacle. It works like a window, covering and protecting their eyes. Spectacles are the result of eyelids that fuse together and become transparent during embryonic development.

When van Doorn was examining a different part of the eye, the illumination from his instrument detected something unusual.

Surprisingly, these spectacles contained a network of blood vessels, much like a blind on a window. To see if this feature obscured the snake's vision, van Doorn examined if the pattern of blood flow changed under different conditions.

When the snake was resting, the blood vessels in the spectacle constricted and dilated in a regular cycle. This rhythmic pattern repeated several times over the span of several minutes.

But when researchers presented the snake with stimuli it perceived as threatening, the fight-or-flight response changed the spectacle's blood flow pattern. The blood vessel constricted, reducing blood flow for longer periods than at rest, up to several minutes. The absence of blood cells within the vasculature guarantees the best possible visual capacity in times of greatest need.

"This work shows that the blood flow pattern in the snake spectacle is not static but rather dynamic," said van Doorn.

Next, the research team examined the blood flow pattern of the snake spectacle when the snake shed its skin. They found a third pattern. During this time, the vessels remained dilated and the blood flow stayed strong and continuous, unlike the cyclical pattern seen during resting.

Together, these experiments show the relationship between environmental stimuli and vision, as well as highlight the interesting and complex effect blood flow patterns have on visual clarity. Future research will investigate the mechanism underlying this relationship.

"This research is the perfect example of how a fortuitous discovery can redefine our understanding of the world around us," said van Doorn.

Citation
K. van Doorn, J. G. Sivak. Blood flow dynamics in the snake spectacle. Journal of Experimental Biology, 2013; 216 (22): 4190 DOI: 10.1242/jeb.093658