Gardens shelter snakes and lizards

I know that my garden supports a small number of frogs and toads. While the frogs were quick to lay claim to my new wildlife pond, spawning in the first spring after the pond had been dug, the toads show a more terrestrial existence, most often appearing on the patio on damp nights to frighten the dogs. The fact that our garden is walled on three sides and located in the middle of town, is probably sufficient reason for the lack of any other amphibians or reptiles. It is known that only the common frog readily penetrates the urban landscape, possibly because of its greater dispersive powers or, more likely, because it is assisted by householders moving spawn from one pond to another.

It seems that the range of reptiles and amphibians you can attract into your garden is, to a greater extent, determined by where your garden is located and the nature of the habitats that surround it. There are some gardens, recently built on former heathland sites in Dorset, that have visiting Adders and Smooth Snakes, the latter a rare species these days. Other rural gardens might support a thriving population of Great Crested Newts or Grass Snakes, and I know of gardens in Norfolk that have both. Less common in most Norfolk gardens is the Slow-worm, not a worm but a legless lizard, with a beautifully patterned body (in the case of the adult males).

Recent research has underlined that the features you have within your garden can also have an influence on which amphibian and reptile species you can support. Open compost heaps are used by Grass Snakes, the elevated temperatures within the mound of decomposing vegetable material help the Grass Snake’s eggs to develop, while stacks of logs and piles of rubble provide shelter for this and other species. There is a garden just down the road from here where the owner has placed sheets of corrugated tin at various points around his large, wooded garden, He now has an impressively large population of Grass Snakes and Slow-worms.

If you are an active gardener like me, then you are more likely to stumble across one of these garden-dwelling reptiles or amphibians but in many gardens they will go unnoticed. Having said this, cat owners sometimes find that their pet has delivered some unfortunate frog, Slow-worm or Grass Snake, which would otherwise have gone unnoticed. The impact of cat predation on some of our reptile and amphibian species might be quite significant, at least in terms of the numbers killed each year. There are likely to be other risks (lawn mowers and garden chemicals to name but two) so the garden can be a difficult place for some species.

In awe of snakes

The other week I visited the garden of Dave Shearing to take part in some filming we were doing to promote a new survey of garden reptiles and amphibians. The garden was a wildlife haven, a formal area stocked with bird feeders and then a gate through into open woodland complete with nestboxes, wildlife pond and corrugated tin plates. The tin provided shelter for Grass Snakes and Slow-worms, both of which proved to be present in the garden in good numbers.

With a careful approach and lifting of the tin it was possible to take a close look at these two less commonly encountered creatures. While some of the snakes made a swift retreat into one of the many woodpiles that dotted the garden, others remained and seemed just as inquisitive about us as we were of them. By being able to watch the snakes in this way I was really impressed by the range of adaptations they displayed.

Snakes are not unique amongst the reptiles in being limbless but this feature does form the cornerstone of their anatomy. The long thin body places constraints on its internal structure and many of the internal organs are highly modified (or absent - most snakes have just a single functional lung). Many organs are elongated and paired organs are often arranged one behind the other, rather than side-by-side; the left kidney, for example, sits behind the right. Each of the vertebrae (and there are usually between 160 and 400 of these) has a pair of ribs attached to it. The ribs themselves articulate and can be swung forward, allowing the snake to flatten out its body to soak up more of the sun’s warmth (I often see Adders doing this) or to allow the passage of a large prey item down the digestive tract. The skull is also adapted to aid the passage of large prey, the two lower jaws not joined at the chin and allowing them to move independently of one another as well as pull apart. This is why even some of the smaller individuals we watched could have tackled an adult frog.

The survey of garden reptiles and amphibians is being organised by the British Trust for Ornithology, Froglife and the Herpetological Conservation Trust. The researchers want to find out which factors influence whether or not particular species of reptile and amphibian will use a garden. As such, they want to hear from anybody who has a garden, regardless of whether or not it contains any reptiles or amphibians. A free survey pack and identification chart can be requested by calling the BTO, by emailing gbw@bto.org or by writing to Reptile Survey, BTO, The Nunnery, Thetford, Norfolk, IP24 2PU.

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.