Biology updates: snake legs, cat rolling, frog teeth and more

There hasn’t been a lot of terribly exciting evolution news lately, but there are a few items of interest (references to papers are at the bottom).

Snake legs:  It’s clear from various lines of evidence, including molecular data and paleontology, that snakes evolved from lizards who lost their legs.  But fossils of the transition are very, very rare, for snakes have fragile bones and vestigial limbs are hard to see in fossils.   A new paper in the Journal of Vertebrate Paleontology by Alexandra Houssaye et al., however, analyzes an amazing specimen: a half-meter long aquatic snake, 95 million years old, from Lebanon.  It’s a marine snake, and there’s controversy about whether these marine reptiles were the ancestors of all modern snakes or simply an offshoot of a terrestrial ancestor.  Likewise, we don’t know if snakes evolved from terrestrial lizards or, perhaps, aquatic ones (there’s at least one aquatic lizard: the Galápagos marine iguana).  Reagardless of these debates, the Houssaye et al. paper is nice for its remarkable analysis, via a noninvasive method (“synchrotron-radiation computed laminography”, or SRCL, whatever that is) of the vestigial legs of this early snake.

Here’s the specimen; it’s broken, so the tail segment, containing the tiny legs, is at the top right, next to the snake’s head.

And here’s the image from the SRCL scan.  Images B and D show the tiny leg, enlarged from the boxed segment in image A.  “F” is the femur, “T” the tibia, and “Fi” the fibula, all clearly homologous to the leg bones of lizards and other vertebrates.  Note that the scale bar is 1 mm long, so the leg is about 7 mm long (roughly a third of an inch).

The evolutionary origin of snakes is still a bit unclear, but what is clear is that they evolved from reptiles with legs.  The depiction of the vestigial legs here (surely on the way out, since they impeded swimming by causing drag) is a nice piece of evidence for evolution.

A description of the discovery from LiveScience is here (link seems to be broken, maybe temporarily).

Why do cats roll? Mark Abraham of the Guardian highlights an old paper (1994) by Hillary Feldman, who wanted to answer the age-old question, “Why do cats roll?”  (Citation below, the paper seems to be free.) Observing cats in a large enclosure, Feldman found that they roll for two reasons.  Females roll in oestrus as a form of sexual invitation to males, and males roll before other males as a form of submissive behavior.  These may not be surprising to cat owners, although the “roll” before a human may represent not submission, but a request for a tummy rub (viz. Baihu).  At any rate, before this study it wasn’t known whether cats showed the same kind of submissive rolling seen in social canids.

Sperm competition. Tim Birkhead, a professor at Sheffield in England, has done pathbreaking work on sperm competition in birds.  Over at The Browser, he’s interviewed about this cool phenomenon, and he recommends five relevant books, one by William Eberhard which I like a great deal.  And the Krebs and Davies book, Behavioural Ecology, is a classic that helped shaped that field.

Cuttlefish behavior. Over at her website, Karin Bondar highlights new work on cuttlefish, showing that exposing embryos to the odors of different prey can affect the feeding preferences of those animals after they’ve hatched.  There’s also a nice video showing an animal changing color.

Frog teeth violate Dollo’s Law. Dollo’s “Law” (really a generalization) is an evolutionary principle stating that once a trait is lost in evolution, it won’t reappear in exactly the same form.  The idea is that if a feature is lost by inactivating or changing the function of genes that produced it, it’s unlikely that those exact gene sequences will re-appear (or that new genes will mutate) to produce a trait that has precisely the form of the lost one. (Another interpretation is simply that it’s unlikely for a lost trait to reappear in a similar form.)  There have been some exceptions to this “law”, and now we have a new one: a paper in Evolution (yay, us!) by John Wiens of Stony Brook,  showing that a species of frog from the Neotropics (Gastrotheca guentheri) has re-evolved mandibular teeth (teeth in the lower jaw).  These teeth, lost in frogs over 200 million years ago, were regained in the ancestor of this species several million years ago.

G. guentheri (photo from Arkive).

Gastrotheca are called “marsupial” frogs because some females have a brood pouch on their backs where they deposit and carry fertilized eggs. In other species males guard the eggs in the ground, and then take the tadpoles into their pouch, carrying them around until they develop into froglets.

The Wiens paper is here, and there’s a blurb in The New York Times. I’ll try to get a picture of the teeth from Wiens.

h/t: Alexandra Houssaye and Matthew Cobb.

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Feldman, H. 1994.  Domestic cats and passive submission. Anim. Behav. 47: 457-459.

Guibé M, Boal JG, & Dickel L (2010). Early exposure to odors changes later visual prey preferences in cuttlefish. Developmental Psychobiology, 52:833-7.

Houssaye, A., F. Xu, L. Helfen, V. de Buffrénil, T. Baumbach and P. Tafforeau. 2011.  Three-dimensional pelvis and limb anatomy of the Cenomanian hind-limbed snake Eupodophis descouensi (Squamata: Ophidia) revealed by synchrotron-radiation computed laminography.  J. Vert. Paleontology 31:2-7.

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