Showing posts with label arboreal snakes. Show all posts
Showing posts with label arboreal snakes. Show all posts

Sunday, October 16, 2011

Rodents, Snake Evolution, and Dates

South America was isolated during most of the Cenozoic and evolved a terrestrial vertebrate fauna that included many mammals, including caviomorph rodent. Antoine and colleagues (2011) have now describe South America's oldest known rodents, based on a new diverse caviomorph assemblage from the late Middle Eocene, about 41 million years ago (MYA) of Peru, including five small rodents with three stem caviomorphs. This means rodent dispersal is not linked to the Eocene/Oligocene global cooling and drying episode (about 34 MYA), as previously thought, instead rodents arrived in South America during the much warmer and wetter conditions of the Mid-Eocene Climatic Optimum. Thus, rodents evolved in China about 55 MYA (early Eocene), reached India, Southeast Asia, and Africa by about 46 MYA, and were in South America by 43 MYA. The authors phylogenetic results reaffirm the African origin of South American rodents and support a trans-Atlantic dispersal of these mammals during Middle Eocene times. This discovery further extends the gap of 15 million years between first appearances of rodents and primates in South America. But perhaps of more interest to people, who read this blog, is what impact did it have on snakes? Click on the table to enlarge it.

Rodents are snake food - many species feed on rodents today - and it has been long thought that snakes evolved their macrostomate lineage (snakes with the ability to gape their mouths to swallow excessively large prey) to feed on mammals. Could this prey have been rodents? Rodríguez-Robles et al. (1999) thought rodents were the reason the erycine boas evolved a large gape. Recently Pyron and Burbrink (2011) published a revised list of dates for the appearance of the different lineages of snakes based upon the DNA clock, these dates are shown in the attached table, and the snake clades are shown in their order of appearance. A quick look at the table shows the first mammal eating snakes alive today were the pythons which appeared a mere 40 MYA. Given that pythons probably evolved in Australasia and rodents were not present in Gondwanan it seem probably that pythons evolved there large gape to eat something else - marsupial mammals seem more likely. The earliest snakes with the macro-gape that appear in the list are the acrochordids, completely aquatic snakes, snakes that feed on fish - they were around 84.66 MYA according to this data. Given that boines were in South America 45 MYA, and rodents did not arrive until 43 MYA, it is unlikely they evolved their huge gape to consume the mammals they do today- see video. Therefore, it appears macro-gape snakes may have first evolved their big, elastic mouths to eat big fish. 

Pierre-Olivier Antoine, Laurent Marivaux, Darin A. Croft, Guillaume Billet, Morgan Ganerød, Carlos Jaramillo, Thomas Martin, Maëva J. Orliac, Julia Tejada, Ali J. Altamirano, Francis Duranthon, Grégory Fanjat, Sonia Rousse, and Rodolfo Salas Gismondi. 2011. Middle Eocene rodents from Peruvian Amazonia reveal the pattern and timing of caviomorph origins and biogeography. Proceedings of the Royal Society B published online before print October 12, 2011, doi:10.1098/rspb.2011.1732. 

Pyron, R. A. and Burbrink, F. T. (2011), EXTINCTION, ECOLOGICAL OPPORTUNITY, AND THE ORIGINS OF GLOBAL SNAKE DIVERSITY. Evolution. doi: 10.1111/j.1558-5646.2011.01437.x 

Rodríguez-Robles, J. A., Bell, C. J. and Greene, H. W. (1999), Gape size and evolution of diet in snakes: feeding ecology of erycine boas. Journal of Zoology, 248: 49–58. doi: 10.1111/j.1469-7998.1999.tb01021.x

Monday, May 16, 2011

How Many Species of Snakes Are There?

The number of species of snakes described each year since 1758.  The text in black notes the authors who contributed a significant number of species in various years where the number of species that were described spiked.
Ask this question on Goggle and most of the top dozen websites provide answers that range from "more than 2000" to "2950." The exception is the Reptile Database website that reports 3149 species as of February, 2008, and 3,315 species as of January 2011. Since this list undoubtedly receives the most detailed attention and updating by herpetologists it is likely to be the most authoritative answer for the moment. But really - how many species of snakes are there?

Using the Reptile Database I graphed the number of currently recognized snake species described each year from 1758 to 2010, divide the 3315 species by the number of months and you get about 1.09 snake species have been described each month for the past 253 years. However, there is reason to believe that there are many species that remain to be described.

Passo and Lynch (2010) revised the cryptozoic snakes in the genus Atractus from the middle and upper Magdalena drainage of Colombia. Prior to the publication of this paper five species were known from the region, they added three new ones, an increase of 60%. Of the 135 currently recognized species of Atractus, 42 (31%) species have been described since 2000. The blind snakes of the genus Typhlops also currently number 135 species, of these 14.8% have been described since 2000. Another species rich genus is Oligodon, the Asian Kukri Snakes. Currently 68 species are recognized, six (8.8%) have been described since 2000. Thus, many snake species clearly remain to be described.

The reasons for this situation appear to be the tendencies of Albert Günther and George Boulenger, both world renowned 19th century herpetologists, to lump species together. They decided that many of the species described between 1758 and 1860 were in fact species that had been named more than once. As they tried to organize the reptiles in the collection at the British Museum of Natural History they described some new species, but they also synonymized many species previously described under names they thought represented the earliest description of a particular species. This could have been a highly useful service to science, but unfortunately they did this many times without actually examining type specimens.

However, other factors are also involved. Species concepts have also changed. Concepts based strictly on scale counts and other morphology have been replaced with concepts based on isolation of gene pools and populations that are on their own evolutionary pathway.

As I worked my way through the homalopsid specimens in the 1990’s, it became clear that specimens labeled Enhydris jagorii represented at least three different species. Most of the specimens labeled jagorii had a mid-ventral stripe, a relatively high ventral count, there was another species with an exceptionally low ventral count, and yet a third species with an intermediate ventral count. Each had a distinctive pattern, but similar dorsal scale row counts, and head scale arrangements. Just laying one of each next to each other, it was clear they were distinct. Thus, three species were considered to be one. This situation is not uncommon, thus it would appear that the actually number of snake species could be conservatively estimated to increase by 1.5 to 2 times.

Murphy, J. C. 2007. Homalopsid Snakes, Evolution in the Mud. Krieger Publishing, Malabar, Florida.

Passos P. and J. D. Lynch. 2010. Revision of Atractus (Serpentes: Dipsadidae) from Middle and Upper Magdalena Drainage of Colombia. Herpetological Monographs, 24:149-173.

Wednesday, January 19, 2011

A Displaced Snake Story From Australia

 The flooding in Australia has produced numerous stories about displaced snakes. Here is just one from the Gympie Times (a Queensland News outlet).

Snakes Look For new Food Supply
Carly Morrissey,  20th January 2011

For Gympie snake catcher John Keady all the recent flooding rain means snakes, snakes and more snakes.
“I’ve been flat out like a lizard drinking,” Mr Keady said this week. “There’s snakes coming out of our ears, they’re very, very active.”Flooding in Gympie meant snakes headed for higher ground and now that waters have receded there are a lot of displaced snakes in unfamiliar territory. The worst part, Mr Keady said, is the flood has moved or killed the snakes’ normal food supply. He said rats and mice have been drowned or moved to higher ground and in the next few days there will be a “pile” of snakes all out looking for food, following their scent. Passionate about snakes and all animals Mr Keady is just thankful that people call him to relocate the snakes. “At least we can be there to save them,” Mr Keady said. He has been around the region with his son catching and relocating snakes to the bush during the last week. Recently the pair has caught snakes in Gympie, on the Southside, in the Forestry complex, at Cedar Pocket, Tewantin and Noosa.
Yesterday Mr Keady was called to catch a carpet snake at Nambour. He had also caught a green tree snake, red belly black snake and a brown snake. “I’ve had 12 calls this morning (Wednesday).” He said the majority of calls were for snakes that had found their way into homes to try and stay dry during the rain and flood.
Some of their natural homes would have been flooded and other food sources would have been killed. This has led to some close calls with people and a busy time for Mr Keady. But the snake trainer said it wasn’t just snakes that have been displaced due to flooding. He had been receiving calls about possums taking up residence in people’s homes and sheds.

Monday, December 20, 2010

Snake on a Rope

Boa constrictor climbing up a rope.
How does a snake climb a vertical
surface without slipping?
(Credit: Bruce C. Jayne)
In a unique study involving young boa constrictors, University of Cincinnati researchers put snakes to work on varying diameters and flexibility of vertical rope to examine how they might move around on branches and vines to gather food and escape enemies in their natural habitat.

The findings by Greg Byrnes, a University of Cincinnati postdoctoral fellow in the department of biological sciences, and Bruce C. Jayne, a UC professor of biology, are published in the December issue of The Journal of Experimental Biology.

The UC researchers sent the snakes climbing up varying widths and tensions of ropes as they explored snake movement in relation to their musculoskeletal design and variation in their environment.

They found that regardless of diameter or flexibility of the rope, the snakes alternated curving between left and right as they climbed the ropes. On the thicker ropes, they were able to move greater portions of their bodies forward as they climbed. As the ropes became thinner and more flimsy, the snakes used more of their bodies -- including their back, sides and belly -- to manipulate the rope for climbing. On larger diameter ropes, snakes applied opposing forces at the same location to grip the rope. While on smaller diameter ropes forces were applied in opposite directions at different locations along the rope, resulting in increased rope deformation. Although energy is likely to be lost during deformation, snakes might use increased surface deformation as a strategy to enhance their ability to grip.

"Despite the likely physical and energetic challenges, the benefits of the ability to move on narrow and compliant substrates might have large ecological implications for animals," write the authors. "Arboreal organisms must often feed or hunt in the terminal branch niche, which requires the ability to move safely on narrow and compliant substrates."

Jayne points out that although the large muscles of boa constrictors make them fairly stocky and heavy compared to other snakes, this anatomy probably increases their strength. All of the snakes gripped the ropes using a concertina mode of locomotion, which is defined by some regions of the body periodically stopping while other regions of the body extend forward. "It turns out boa constrictors are strong enough so that they can support their weight with a modest number of gripping regions," adds Jayne.

The researchers say their findings are the first study that has explicitly examined the combined effects of diameter and compliance on how an animal gets around. Future research is underway to compare differing muscular anatomies in snakes and relate it to their function in terms of their behavior and their environment.

Byrnes, G. and  B. C. Jayne. Substrate diameter and compliance affect the gripping strategies and locomotor mode of climbing boa constrictors. Journal of Experimental Biology, 2010; 213 (24): 4249 DOI: 10.1242/jeb.047225