Anyone who’s anyone has heard of the Anaconda. But in fact ‘the’ Anaconda is the Green anaconda Eunectes murinus. Most zoologically-informed people know that there’s a lesser-known, smaller relative of this large species, namely the Yellow or Paraguayan anaconda E. notaeus. Usually only reaching 3-4 m in length (as opposed to 5-9 m for the Green anaconda), the Yellow anaconda [photo here by Dave Hone] typically has a yellowish ground colour, and black spots, patches and streaks decorate its length.
Far less well known is that there are actually a few other anacondas as well. The Dark-spotted or de Schauensee’s anaconda E. deschauenseei (wow, vowels) was originally described from Marajo Island in the mouth of the Amazon (Dunn & Conant 1936). Its status as a valid species has sometimes been questioned, but recent work supports its distinction, and it seems closely related to the Yellow anaconda (Dirksen & Böhme 2005). This is interesting, as the two are widely separated (by about 1000 km). It’s therefore been suggested that their current distributions are relictual, reflecting the former fragmentation of the Amazonian forest by extensive savannahs (O’Shea cited in Shuker 2002). However, whether Amazonia ever was fragmented by savannah habitat is a controversial subject – in fact some of the literature on this area concerns the distribution of grassland-dwelling snakes, and I’ve been promising to cover this subject since 2006 [Dark-spotted anaconda below from wikipedia. UPDATE (added Sept' 2010): apparently the photo does not depict a Dark-spotted anaconda at all, see comment 30].
Even less well known is that a fourth species is currently recognised: E. beniensis Dirksen, 2002 from Beni Province, Bolivia. Originally identified as the result of hybridisation between Green and Yellow anacondas, E. beniensis was shown to be distinct by Dirksen & Böhme (2002), and as the sister-taxon to the E. deschauenseei + E. notaeus clade. It has a brownish ground colour, possesses five stripes on its head, and is patterned with dark, solid blotches. A few other anaconda species have been named over the years (including E. scytale Stull, 1935 and E. barbouri Dunn & Conant, 1936), but none are currently regarded as valid.
All anacondas are semi-aquatic boas with small, dorsally positioned eyes and relatively narrow heads. They predominantly rely on ambush, catching and eating amphibious and aquatic reptiles, mammals and birds as well as fish. Large individuals sometimes eat large caiman and mammals as big as capybaras and tapirs. A young Yellow anaconda specimen had swallowed Limpkin eggs (the Limpkin Aramus guarauna is a rail-like wading bird of the American tropics) [another captive Yellow anaconda below. I had to crop the image strangely thanks to the flash on the glass].
A brief diversion on snake phylogeny that many of you will find boring and may well not want to read
Phylogenetic work shows that anacondas are part of a New World boa clade that also includes Boa constrictor (the only snake commonly referred to by its scientific name: its ‘real’ common name is Common boa), Corallus (the tropical American tree boas) and Epicrates (the Greater Antillean or rainbow boas). Eunectes seems to be the sister taxon to Epicrates, though Burbrink (2005) found that Epicrates cenchria – the Rainbow boa – was the sister-taxon to the Eunectes + ‘rest of Epicrates‘ clade. If this is right, it means that all those other Greater Antillean boas currently included in Epicrates will need a new generic name, as E. cenchria is the type species for the genus Epicrates Wagler, 1830. Several names are available; I think the oldest is Chilabothrus Duméril & Bibron, 1844 (but I can’t find out what the type species of Chilabothrus is).
Incidentally, B. constrictor has always been a bit of a weird taxon from a phylogenetic perspective, as several studies (most notably Kluge 1991) have found it to be more closely related to the Madagascan boas Acrantophis and Sanzinia than to other Neotropical taxa (Kluge (1991) actually regarded all three taxa as congeneric). Needless to say, this inferred relationship (which recalls the distribution of certain iguanian lizard clades) resulted in various different biogeographical scenarios. However, Vences et al. (2001) and Burbrink (2005) later found B. constrictor to be the most basal member of a New World boine clade that also includes Corallus, Eunectes and Epicrates [Common boa below from wikipedia].
Boas – the seven genera grouped together as Boinae – have traditionally been allied with the sand boas and rubber boas (Erycinae) in a group called Boidae. Several other groups, including the bolyeriines (Round Island boas), tropidophiines (or tropidopheines, the Caribbean dwarf boas or wood snakes), ungaliophiines (or ungaliopheines, the Neotropical banana boas or Neotropical dwarf boas) and pythonines (pythons) have also been included within Boidae ‘traditionally’. Or, at least, this is the impression I always got from the popular literature: in the technical literature, a lot of work has shown that some of these ‘boa-like’ snakes are not really so boa-like (e.g., McDowell 1979, Kluge 1991) [the bolyeriine Casarea dussumieri shown below, from wikipedia].
While it is widely agreed that all boa-like snakes are basal to Caenophidia (the acrochordid + colubroid clade), it remains controversial whether all boa- and python-like snakes form a clade (the correct name for which is probably Booidea*), or whether they form a series of outgroups to Caenophidia. Lee & Scanlon (2002) found boines, pythonines and erycines to form a clade (Booidea), while ungaliophiines, tropidophiines and bolyeriines were basal members of an ‘Advanced snake’ clade that also included Caenophidia. Vidal & Hedges (2002), employing data from nuclear and mitochondrial genes, also found tropidophiines and bolyeriines to be closer to Caenophidia than to boines, pythonines and erycines. They later found tropidophiines to group with Anilius, the South American pipe snake, in a position more basal than the boa-python + Caenophidia clade, and they also recovered some other weirdness: uropeltids (shield-tailed snakes) were up close to caenophidians, for example, and xenopeltids (sunbeam snakes) and Loxocemus (the Mexican burrowing snake) were found to be part of the boine-pythonine-erycine clade (Vidal & Hedges 2002). At the risk of elaborating further, I’ll stop there. You may note that many details of snake phylogeny are contentious, and that the potential for a lot of interesting research remains. Dammit, so much for a text-lite ‘picture of the day’ post… DAMMIT!
* Does anyone know of any literature that provides phylogenetic definitions for snake clades? I have yet to see any.
PS – if you’ve come here hoping to read about the new, sauropod-mimicking dacentrurine stegosaur Miragaia, please take my advice and go here instead.
Refs – –
Burbrink, F. T. 2005. Inferring the phylogenetic position of Boa constrictor. Molecular Phylogenetics and Evolution 34, 167-180.
Dirksen, L. & Böhme, W. 2005. Studies on anacondas III. A reappraisal of Eunectes beniensis Dirksen, 2002, from Bolivia, and a key to the species of the genus Eunectes Wagler, 1830 (Serpentes: Boidae). Russian Journal of Herpetology 12, 223-229.
Dunn, E. R. & Conant, R. 1936. Notes on anacondas, with descriptions of two new species. Proceedings of the Academy of Natural Science, Philadelphia 88, 503-506.
Kluge, A. G. 1991. Boine snake phylogeny and research cycles. Miscellaneous Publications, Museum of Zoology, University of Michigan 178, 1-58.
Lee, M. S. Y. & Scanlon, J. D. 2002. Snake phylogeny based on osteology, soft anatomy and ecology. Biological Reviews 77, 333-401.
McDowell, S. B. 1987. Systematics. In Seigel, R. A., Collins, J. T. & Novak, S. S. (eds) Snakes: Ecology & Evolutionary Biology. Macmillan (New York), pp. 3-49.
Shuker, K. P. N. 2002. The New Zoo. House of Stratus, Thirsk, North Yorkshire.
Vences, M., Glaw, F., Kosuch, J., BÃ¶hme, W. & Veith, M. 2001. Phylogeny of South American and Malagasy boine snakes: molecular evidence for the validity of Sanzinia and Acrantophis and biogeographic implications. Copeia 2001, 1151-1154.
Vidal N, & Hedges SB (2002). Higher-level relationships of snakes inferred from four nuclear and mitochondrial genes. Comptes rendus biologies, 325 (9), 977-85 PMID: 12487103
– . & Hedges, S. B. 2004. Molecular evidence for a terrestrial origin of snakes. Proceedings of the Royal Society of London B (Suppl.) 271, S226-S229.