Showing posts with label number of species. Show all posts
Showing posts with label number of species. Show all posts

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.