Amphibians and reptiles

1 Introduction

1.1 Interest in amphibians and reptiles

Amphibians and reptiles have a long history of both fascination and, sadly, repulsion in the common imagination. Although often considered together by naturalists and the public alike, in fact these two groups of animals have little in common apart from being vertebrates. Perhaps the simple fact that none of them are warm and furry (mammals), feathery fliers (birds) or streamlined swimmers (fishes) is sufficient to explain why most people think of them collectively. In the not too distant past even professional zoologists made the same mistake. Until the early nineteenth century, for example, lizards and newts were not properly distinguished. We still have the scientific discipline of herpetology which continues to pool amphibians and reptiles together, and herpetological societies around the world that do the same thing. The number of enthusiasts for these animals has remained relatively small compared with the other vertebrate groups and so far there simply aren’t enough to warrant the separation that taxonomic distinction justifies. This book continues the tradition of treating amphibians and reptiles together, partly for an additional reason. In Britain there are, even combined, far fewer species than in any of the other vertebrate groups. Fortunately what they lack in species diversity the British amphibians and reptiles more than make up for in intrinsic interest and accessibility for study.

In general amphibians and reptiles were largely ignored by early naturalists, including the pioneering taxonomist Carl Linnaeus who declared a profound dislike of them. Even Gilbert White gave scant attention to frogs, newts, lizards or snakes in letters describing wildlife around his Selborne parish in the eighteenth century, an area that was and still remains home to all the British species. These people didn’t know what they were missing. Happily things have changed dramatically over the past fifty years or so and there is now a much greater interest in these fascinating vertebrates, to the extent that since 1989 there have been regular world congresses of herpetology attended by hundreds of scientists from every corner of the planet. What has triggered this change of fortune?

Sexual selection

Sometimes regarded as a form of natural selection, sexual selection is an evolutionary process that depends on competition among (usually) males for access to females, or on preferences among (usually) females for particular male traits

The answer to that is a mix of intriguing recent revelations. Every schoolchild learns about frogspawn, tadpoles and the dramatic events of metamorphosis resulting in tiny frogs and toads around the pond edge. But it has become clear that this only represents one method of reproduction, albeit the commonest type in temperate countries. In the tropics things are very different. Some frogs lay eggs singly in the small volumes of water held among the leaves of bromeliads, plants that grow on tree branches high in the forest canopy. These frogs return later to feed their tadpoles with unfertilised eggs. Others rear tadpoles in their mouths, on their backs or even embedded within their skin. Females of one Australian frog, now extinct, swallowed their eggs and allowed the tadpoles to grow in their stomachs before regurgitating fully-formed froglets. It turns out that frogs have a greater variety of reproductive methods than any other animal group. And it’s not just the basic biology that has attracted attention. Darwin recognised the importance of sexual selection in evolution and frogs demonstrate this in quite extraordinary ways. In many species the strength or frequency of male croaks makes all the difference to an individual’s chances of attracting a female and therefore of successful reproduction. In newts there is a comparable situation: the large and striking crests of males in the breeding season are used to impress potential mates in elaborate underwater displays (Duellman and Trueb, 1994*).

Reptiles are equally remarkable in their breeding behaviour. Male sand lizards turn a beautiful iridescent green in spring and compete for females by a combination of aggressive displays and physical fights. Amazingly, the intensity of the green colour can be temporarily enhanced during these encounters to increase intimidation of a rival, or maybe to impress nearby females. These lizards have provided some of the best examples of the benefits of multiple paternity in nature. Females usually mate with several males and those that do this tend to have the fittest offspring. For all British species of amphibians and reptiles, spring is the season when they are most visible because this is when breeding occurs with its associated extrovert behaviours. Each species has its own special effects. Male adders ‘dance’ together in power struggles, preoccupied with winning the contest and utterly indifferent to anybody watching them. ‘Balls’ of grass snakes thrashing around in the herbage are made up of several males wrestling for access to a (usually) much larger female somewhere in the middle. And so on. Vitt and Caldwell (2009) summarise much of the knowledge and excitement concerning both amphibian and reptile biology.

*References cited in the text appear in full on page 165.

Ectotherm

An animal dependent on environmental temperature and unable to generate its own body heat

It’s not just their sex lives that have increased interest in amphibians and reptiles. These animals play pivotal roles in food chains, both as predators and as prey. All are ectotherms with no need to use energy for maintaining body temperatures, so they can get by with very little food. One consequence of this is that many species exist at much higher densities than comparably small mammals like mice, voles and shrews. The sheer numbers of toads emerging after rain on a warm summer evening give an idea of how important they must be both to their main prey (various invertebrates) and to animals that prey on them, of which there are a lot. Abundant and diverse amphibian and reptile faunas are good indicators of habitat quality and biodiversity in general and have attracted the attention of ecologists for that very reason.

All of which leads on to a related point, the realisation that amphibians and reptiles have declined around the world over recent decades more dramatically than any other vertebrates (Stuart and others, 2004; Beebee, Wilkinson and Buckley, 2009). We obviously need to know why this is happening and try to minimise the damage. Whatever the causes are, there is no reason to suppose that other groups, including mammals like ourselves, will be perpetually immune to them. Arguably this is rather a gloomy reason to be interested in these animals but the importance of the subject is beyond dispute, and study of declines can provide surprising insights in unexpected ways, such as the impact of new ‘emerging’ diseases on wildlife populations. These studies can also have early practical consequences, for example by showing conservation organisations how best to improve a habitat. Sometimes such management, designed for a particular species, has unexpected extra benefits for others. Pond restoration for natterjack toads on a Hampshire heath, for example, revived populations of shoreweed (Littorella uniflora, a rare plant) and the spangled water beetle (Graphoderus zonatus) at its only British locality.

Finally it’s worth mentioning that amphibians and reptiles can sometimes provide direct benefits to humans, another source of interest in these days of ‘ecosystem services’. The toxins in toad skins have been investigated for their antibiotic properties (so far without much success) and crocodiles are farmed for their leathery skin and its contribution to the handbag trade. None of which, of course, is of much concern to most naturalists.

Ecosystem services

Features of the natural world that contribute to human wellbeing, such as pollination, or the uptake of nutrients by reedbeds

Viviparous

Bearing live young (so not laying eggs)

1.2 Engaging with amphibians and reptiles

The first questions you are bound to ask if you are thinking of studying these creatures include: where do they live and how can I find them? The good news is that few places in the British Isles are distant from at least one or two species of amphibians or reptiles. It will not be necessary to travel very far to locate some of the commoner species, notably smooth, palmate and great crested newts, common frogs and toads (Fig. 1.1), viviparous lizards, slow-worms, adders and grass snakes. All of these animals are habitat generalists and can be found, with luck, in a broad range of landscapes including farmland (though not so often where it is under intensive management), open woodland, heaths, commons, moors, sand dunes, parks and gardens. Of course there’s more to it than just turning up somewhere and hoping for the best, and not all the common species occur in all these habitats. The rare native species (natterjack toad, pool frog, sand lizard and smooth snake) are by definition not so easy to find. Native pool frogs have a very restricted distribution, currently at just one site in Norfolk. The other three rarities are heathland and sand dune specialists with very limited ranges, mostly in Surrey, Hampshire and Dorset for the reptiles but additionally at sites in East Anglia and around the Irish Sea coast in the case of natterjacks. Several additional, non-native species of amphibians and reptiles have been introduced into Britain over the past two hundred years but only three have become at all widespread. You might find noisy marsh frogs in low-lying ditch systems in various parts of southern and eastern England, especially in Kent and Sussex. Brilliantly coloured alpine newts now exist as many small colonies, mainly in parks and gardens, over much of Britain. And wall lizards thrive in an increasing number of favoured sites, such as south-facing cliffs and walls, mostly in southern England.

Fig. 1.1 Adult female common toad Bufo bufo (Erik Paterson)

Looking for amphibians is very different from searching out reptiles, with one exception (described below). Common frogs, toads and newts spend most of their lives hiding in vegetation, in burrows or under stones and logs well away from water. This is where they hunt their prey, invariably at night, throughout the summer and autumn months. In winter they become even more secluded, usually in frost-free refuges well below ground. To find amphibians in these phases of their life cycle, the only options are to turn over likely hiding places (any sort of debris, natural or man-made, preferably on at least slightly damp soil); or to search at night with a powerful torch. This only works well if the habitat is fairly open, such as grazed pasture, heath or mobile dunes.

Neither of these approaches guarantees success, but all species are relatively easy to find when they are mating and spawning. For this reason most people engage with amphibians during the spring months when they resort to water for breeding. However, choosing the best ponds to search requires some knowledge of what amphibians are looking for. Large ponds with fish are often selected by toads but are unlikely to be good for any of the other species. Great crested newts in particular usually avoid fish ponds because their larvae are very vulnerable to fish predation. Smallish, fish-free ponds close to good quality terrestrial habitat (so not set in the middle of intensive arable farmland) are the best bet for all the other common species. For natterjacks, very shallow, temporary ponds on heaths and dunes are the favoured spawning sites.

Timing is also critical. Common frogs and toads are ‘explosive’ breeders and only visit the pond for a week or two in early spring, with toads usually arriving a little later than frogs. The precise timing varies a lot across Britain. In parts of Cornwall frogs regularly spawn before the end of December (hardly spring!) while in the highest Scottish mountains they may wait until early April. Newts are less problematic because they stay in the ponds much longer, for at least several weeks, often starting in February or March and remaining until May or early June. Natterjacks behave like newts in this respect. They have a protracted spawning season, usually concentrated in April and May, so much later than common frogs and toads.

The above-mentioned exceptions to the generally elusive nature of amphibians are the introduced marsh frog and its close (but rarer in the UK) relatives, pool and edible frogs, collectively referred to as ‘water frogs’. These amphibians got their general name because they stay in or close to water all year round and can be seen sunning themselves, just like reptiles, on the banks of ponds or ditches from spring through to autumn.

Tadpoles offer further opportunities for engaging with amphibians. Those of frogs and toads are often easy to see in ponds and are present from spring to early summer, or late summer in the case of water frogs. Newt tadpoles are less obvious because they hide in pondweed but are also present for a long time, typically from late May until August or September. Sometimes they even overwinter in the breeding ponds.

Fig. 1.2 Typical reptile habitat (Tony Gent)

Refugia

Hiding places

All British reptiles are active in daytime throughout most of the year, apart from the winter months when they hibernate underground. However, unlike amphibians they do not congregate in particular places (like ponds) for reproduction. This makes them generally harder to find. Spring is again the best period because this is when all our reptile species emerge from hibernation. They mate soon afterwards, and spend a lot of time basking in the early spring sunshine, and then engage in elaborate courtship rituals in which males in particular are bolder and less watchful for predators than they become later in the year. With experience it is possible to predict likely places for spotting reptiles, mostly when they are basking to warm up. Weather conditions are critical. In spring almost any time of day is suitable, provided there is some sunshine and preferably little wind. As the weather warms up, though, basking only occurs very early or late in the day. As with amphibians, refugia may also be useful, especially large flat items such as pieces of corrugated iron or roofing felt. Snakes and slow-worms often hide under such refugia, but viviparous and sand lizards rarely do so, and those species are best found when they are warming up in the sun.

The best places to look for most reptiles are south-facing banks with low-growing vegetation such as heathers or short grass, together with some scrub (Fig. 1.2). Basking animals seek out small patches of open ground surrounded by vegetation into which they can escape quickly when disturbed or threatened. However, grass snakes may also profitably be sought in and around ponds and ditches where they hunt their amphibian prey. In areas where grass snakes are known to live, compost heaps can be good places to look if the timing is right. Females congregate to lay eggs there in early summer, and later on the newly-hatched snakes emerge, sometimes in large numbers.

1.3 Special aids

A couple of relatively recent developments have proved especially useful in the study of amphibians and reptiles for both amateur and professional investigators. The first of these has been the advent of high quality digital photography. With reasonably cheap cameras that come with inbuilt zoom and macro lenses it is now possible to take superb, detailed photographs of many types of wildlife. Amphibians and reptiles lend themselves very well to such photography because most species can be approached easily in the wild or caught and handled for close-ups. Just making a picture collection of the various species, including different behaviours and life stages, is rewarding in itself.

This is not, however, the main reason why digital photography has become so valuable in amphibian and reptile research. For many purposes it is useful to identify individual animals if they are seen or captured more than once. This kind of information allows estimation of home ranges and survival over months or even years. In the past this could only be achieved by marking the amphibian or reptile in some way, often with tags wrapped round part of the body or even by clipping off the ends of toes in an individual-specific pattern. This level of interference is undesirable for many reasons, not least of which is the possible risk of increasing mortality (for example from infection where toes are clipped). Passive integrated transponders, PIT tags, have also been employed with amphibians and reptiles. These are injected under the skin and each has a specific code that can be read by holding a recording device next to