Thursday, 3 May 2007

A leisurely lunch

There is one particular stretch of the local river that is often frequented by grass snakes. No doubt, this reflects the availability of suitable prey, plentiful cover and the nesting opportunities provided by decaying waterside vegetation. On occasion these snakes may be seen, or more likely heard, as they slip away into the water following the approaching footsteps of a passer-by. However, thanks to the eagle-eyed attentions of one of my work colleagues, one particular snake was spotted the other day, unable to slip away because it was in the process of swallowing a sizeable toad.

Grass snakes are not sit and wait predators but are active hunters of toads, frogs and newts. As such, they associate with ponds and rivers and will readily take to the water, swimming with lateral undulations of their body and sometimes diving underwater (where they can remain for up to 30 minutes). This particular individual was a large snake and, since it is known that large grass snakes prefer to take larger prey, it was not really a surprise to see it tackle such a large toad. The toad in question was very much alive and had inflated its body by gulping in air. This would make it more difficult to swallow but was very unlikely to alter the outcome of the contest. Grass snakes have sharp recurved teeth and this would make it impossible for the toad to extricate itself from the deadly grip.

Grass snakes do not really have any special adaptations useful for overcoming their prey, so the victim is typically swallowed alive and head first. With larger items the snake opens its jaws as wide as it can and then works them sideways along the prey. Very slowly, the victim is consumed. Unfortunately for the toad, death does not come quickly, and it may survive in the gut of the snake for some time, only slowly succumbing to suffocation or the action of the digestive juices. There are records of amphibians, swallowed in this way, surviving; grass snakes will often regurgitate their most recent meal if captured or threatened and there are cases of regurgitated frogs hopping away, seemingly unharmed.

The size of this snake suggested that it was a female (females are larger than the males) and, this being the case, she is likely to consume another four toads over the course of the summer. Despite being smaller, male grass snakes will average eight toads a year. This is because the female grass snake does not feed for some six weeks during pregnancy and the egg-laying period. In this particular case, the female may have been struggling with her leisurely lunch but it would keep her going for some time.

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