Richard Wells wrote:
The notion that snakes are capable of expanding their ranges over large bodies of water - particularly salt water - is taken for granted, but there is a paucity of direct observational data to support the idea.

Me:
I think you have neglected the sea snakes. Harry Greene wrote in his book Snakes: "...the Yellow-bellied Seasnake (Pelamis platurus) survives waif dispersal with sufficient frequency to colonize the eastern Pacific. The latter species occasionally washes up on the coast of Siberia and as far south as New Zealand and Cape Horn but can neither survive long nor reproduce in such cold areas.

I think you were probably thinking about only terrestrial species when you made that statement. You are probably correct that most snakes cannot cross long stretches of ocean (unlike the Yellow bellied sea snake). Most island snakes have close relatives on a nearby continent, as Wulf pointed out by citing such examples as Lampropeltis zonata. Other species of snakes, including other species of Lampropeltis (e.g. L. getulus) have also colonized islands, probably when these islands were connected to the mainland. The colonization of North America by colubrid snakes was probably made via land bridges between Europe and North America and between Asia and North America. South American species of colubrid snakes may have entered this land mass via a southern land bridge, perhaps across Anarctica.

The Madagascan boas, however, probably could not have gotten there without swimming long distances. Ditto the colubrid snakes found on that continent.

Snakes are good swimmers. The two oldest known fossil snakes both have hind limbs, and are found in shallow marine deposits. A marine, mossasauroid origin can probably explain the nearly world wide distribution of snakes. Once evolved, snakes could have swam or be carried by ocean currents to far corners of the world. There are two recently published papers which support a marine, mossauroid origin for snakes. The first one is

Rest, J.S., J.C. Ast, C.C. Austin, P.J. Waddell, E.A. Tibbets, J.M. Hay, D.P. Mindell. (2003). Molecular systematics of Reptilia and the tuatara mitochondrial genome. Molecular Phylogenetics and Evolution, 29:289-297

Fig. 4 of this paper shows that snakes are nested within the lizards; snakes are not the sister group of lizards. Monitor lizards, in fact, are more closely related to snakes than they are to the green iguana or skinks. Thus the mossasauroid origin of snakes have support from molecular systematics.

A second paper which also supports a marine mossasauroid origin for snakes is:


NATHAN J. KLEY 2001. Prey Transport Mechanisms in Blindsnakes and the Evolution of Unilateral Feeding Systems in Snakes. AMER. ZOOL. 41:1321–1337

In this paper, it is shown that the blindsnakes have evolved a proprietary feeding mechanism that is almost certainly derived, i.e. not the primitive condition found in the ancestor of the snakes. Thus the macrostomatan condition is probably the primitive condition in snakes, as the 95 million year old marine fossil snakes with legs suggest.

These recently published papers strongly support a marine, mossauroid origin for snakes. As one can see, the yellow bellied sea snake ended up as far north as Siberia and as far south as New Zealand. Even though the oldest known fossil snakes are found in the vicinity of Israel, there is no reason why they or their descendants could not have dispersed widely, even globally, rather quickly early in the evolutionary history of snakes the same way that the Yellow bellied sea snake has dispersed.

I think many snakes are fully capable of taking a swim and ending up alive on islands, just as most birds are capable of flying across open water and ending up on an island. The reason why birds do not make those flights is probably similar to the reason why snakes do not regularly take a swim across the sea. It is not the fear of flying or swimming. It is probably because animals have instincts that prevent them from migrating aimlessly across long stretches of unknown terrain, because such migrations would most likely be unsuccessful, since unsuitable habitat and/or fierce competition would likely await the migrant at the other end of the expedition. Homing instincts have evolved in many animals to ensure that they do not stray far from optimal habitat. It is perhaps homing instinct that has prevented the colonization of islands by many snakes, not the fear of swimming. For those who have taken a swim, perhaps few have been successful because of competition from natives. The exceptions of course are the Madagascan boas and colubrids.

Richard Owen, a famous nineteenthcentury British anatomist, claimed snakes can "outswim the fish and outclimb the monkey." --Harry Greene

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