Published by Pelagic Publishing
www.pelagicpublishing.com
PO Box 725, Exeter, EX1 9QU
Amphibians and reptiles
Naturalists’ Handbooks 31
Series editors
S. A. Corbet and R. H. L. Disney
ISBN 978-1-907807-45-9 (paperback)
ISBN 978-1-907807-46-6 (ePub)
ISBN 978-1-907807-47-3 (mobi)
ISBN 978-1-907807-48-0 (PDF)
Text © Pelagic Publishing 2013
All rights reserved. Apart from short excerpts for use in research or for reviews, no part of this document may be printed or reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, now known or hereafter invented or otherwise without prior permission from the publisher.
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library.
Cover photographs: great crested newt (Photoshot Holdings/Alamy), grass snake (Fred Holmes); common frog (Erik Paterson, www.erikpaterson.co.uk)
Foreword
Acknowledgements
1 Introduction
2 Basic biology
3 Ecology and conservation
4 Surveying and monitoring
5 Studying amphibians
6 Studying reptiles
7 How schools can help
8 Identifying species found in Britain
Key I Adult and immature newts
Key II Newt eggs, larvae and metamorphs
Key III Frogs and toads (adults and metamorphs)
Key IV Frog and toad spawn
Key V Frog and toad larvae
Key VI Adult and hatchling limbed lizards
Key VII Adult snakes
Distribution of amphibians and reptiles
9 Working with amphibians and reptiles
10 Useful addresses and links
References
Index
The uncoiling rush and sliding olive of a grass snake, the still, unblinking cinnamon eye of a silver grey adder and the emerald flash of a darting sand lizard through a cushion of heather are amongst the most tantalising and thrilling glimpses of our native wildlife you can find. The slow, deliberate posturing of a dragonised male smooth newt fanning his female, the perverse fervour of spawning common toads as they roll in cold ecstasy on a spring night or that first encounter on a wet sandy slack with the call of the natterjack are all encounters to be treasured. And yet our reptile and amphibian fauna are sadly too often neglected by naturalists. They are happened across rather than sought and sidelined behind birds, butterflies and the likes of orchids. This is such a shame, as they are both beautiful and fascinating animals.
I entertained a huge crush on these groups as a child, indeed I still count grass snakes as one of my favourite animals. Peering into murky or dark ponds, sneaking through heather and gorse looking for their resting spots, or twisting quickly to hear the characteristic rustle of a fleeing snake or lizard was all of my life between about eight and ten. I still have the first sloughed adders skin that I delicately teased out of some brambles all those years ago.
This book, by one of the UK’s foremost authorities, seeks to make this small guild of underestimated creatures more accessible through simply highlighting the ease of identifying our reptiles and amphibians and fuelling a greater interest in their behaviour and ecology. They are not enigmas, they are actually quite easy to find and with a modicum of practice equally easy to watch and enjoy. I seriously hope you take up the challenge because, unfortunately, a number of these species are either rare or in decline and it is only through engagement, actually meeting an animal, that any lasting affinity for it can be forged. This passion is essential to motivate and maintain our conservation efforts. So, charge your torches, get your wellies out of the cupboard, don’t forget your binoculars and camera, and most important of all, pack your children into the car. Then get out, and get to grips with our fabulous snakes, lizards, frogs, toads and newts.
Chris Packham
Thanks to Tony Gent of Amphibian & Reptile Conservation for suggesting this book, to Nigel Massen and Sally Corbet of Pelagic Publishing for advice and support, and to Ronn Altig, Brian Banks, Lee Brady, Tim Harry (Chaffinworks), Richard Newton, Charles Snell, Paul Wells, Julian Whitehurst, John Wilkinson, Andrew Williams (Critterzone) and Wolfgang Wuster as well as contributors to ARC’s photo archive (Neal Armour-Chelu, Chris Dresh, Tony Gent, Chris Gleed-Owen, Fred Holmes, Howard Inns, Rodger McPhail, Nick Moulton and Angela Reynolds) for supplying photographs. Additional images were supplied by: Erik Paterson (www.erikpaterson.co.uk/), Kelly Minars (Flickr/greaterumbrage), Keri Leaman (Flickr/birderkeri), Quentin Scouflaire (Flickr/ sqfp.info), Duncan Hull (Flickr/dullhunk), Alan Martin and Steve Ogden (www.wildlifeinsight.com).
Author royalties from this book have been donated to Amphibian and Reptile Conservation.
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?
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.
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.
Features of the natural world that contribute to human wellbeing, such as pollination, or the uptake of nutrients by reedbeds
Bearing live young (so not laying eggs)
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)
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.
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 the animal. Although it is a substantial advance on earlier techniques, PIT tagging also has disadvantages. Usually the amphibian or reptile has to be caught again because the recorder only functions close up; PIT tags are expensive and too big to inject into young amphibians and reptiles; and there is still some risk of infection, although they come in sterile packages.
Digital photography has revolutionised the tracking of individuals because many of our amphibians and reptiles have unique markings of some kind that can be caught on camera. The spot patterns on the backs of lizards, scale arrangement and colouration around the heads of snakes, belly spots of crested newts and wart or back stripe patterns of natterjack toads have all provided ‘personal’ photographic fingerprints. This can even work for very young animals such as newly-metamorphosed natterjack toads, although patterns do sometimes change a bit as animals grow, requiring careful checks if identification is to be reliable over long periods. Another advantage is that photography is minimally invasive and sometimes individuals can be recognised by taking pictures in the wild (for example of basking lizards) without having to catch or handle the animal at all. It’s not perfect – some species such as smooth and palmate newts have very few features that might be used for individual identification. But there’s room for much more study along these lines, as will be discussed in later chapters. Software for automated screening of photographs is also developing rapidly, taking away the tedium of having to search large picture libraries by eye to identify recaptured individuals.
An animal that eats both plant and animal material
The second recent development, of special relevance to amphibians, is the vogue for installing garden ponds, which have become increasingly popular since the 1960s. Mostly these ponds are made for ornamental reasons and usually they are stocked with fish, but this has not prevented them from becoming important breeding sites for some of the commoner amphibian species. This may be because most garden pond fish are goldfish or other varieties of carp, and these are omnivores rather than dedicated predators, thus allowing some amphibian larvae to survive. Common frogs have benefited the most from garden wetlands, followed by smooth (and possibly palmate) newts. Common toads crop up less often and usually only in the largest ponds. Great crested newts are rare; even carp may be too much for them, and in any case they also prefer quite big ponds. Gardens are also key sites for at least two of the introduced species. Alpine newts occur almost exclusively in garden ponds, and so do the much rarer midwife toads, which survive in just a few towns in England as far as we know. By the late 1970s about one garden in seven in the Brighton area had a pond and about half of these were used by at least one species of amphibian. This amounted to several thousand breeding sites for common frogs, smooth newts and common toads (Beebee, 1979). A similar picture has emerged in other towns and cities in Britain wherever anyone has bothered to look.
Fig. 1.3 A garden pond used for breeding by several species of amphibians
What all this means is that equipped only with a garden pond (Fig. 1.3) it is possible to study the breeding behaviour and larval development of amphibians within a few metres of your back door. Such convenience makes intensive study remarkably easy and there is scope for much more work using this resource. If there are local populations a new garden pond will probably be colonised within a couple of years, but there’s no need to wait that long if other ponds with frogs or newts are available. Transferring small amounts of spawn (Fig. 1.4) or tadpoles (frogs) or a few adults (newts) can initiate a new population within weeks of finishing the pond construction.
Unfortunately gardens, even those managed with wildlife in mind, are generally less useful for studying reptiles. This is probably because there is usually too much disturbance for species that need to bask in daytime (that is, for most of them) and there are high concentrations of effective predators. Cats and blackbirds are remarkably adept at catching lizards. Grass snakes sometimes visit garden ponds but by far the most successful garden reptile is the slow-worm. These lizards sometimes occur in large numbers and survive because they are so secretive (though cats still catch a few), but this also makes them hard to study. Nevertheless there is the prospect of carrying out interesting work on this species in gardens, perhaps more effectively than in the wider countryside, simply because it’s possible to work so intensively close to home.
The primary focus of this book is to demonstrate how easy it is to carry out original work on amphibians and reptiles even without professional training and equipment. Despite the increased interest in both groups there is still a lot left to discover and much can be done with minimal apparatus. After some background material on the biology, ecology and conservation of amphibians and reptiles (chapters 2 and 3), the book introduces some topics on which further research is needed, and describes the hows, whys and wherefores of carrying out new studies on these intriguing and very amenable subjects. Of particular importance is the contribution that amateurs can make to the survey and monitoring of amphibian and reptile populations across Britain, a national exercise devoted to finding out how the status of each species is bearing up under modern pressures on the countryside (chapter 4). Chapters 5 (amphibians) and 6 (reptiles) outline various types of study that could make substantial contributions to knowledge about all our species, and chapter 7 proposes collaborative studies, in which schools could make a particularly valuable contribution. Chapter 8 consists of keys for identification, and is followed by some essential information including the requirement for licences for handling the animals, data analysis and publishing results in scientific literature (chapter 9), and useful addresses and links (chapter 10). Finally there is a reference list of relevant work cited in this volume. Other books that cover the biology and identification of British amphibians and reptiles in detail include a New Naturalist volume (Beebee and Griffiths, 2000), a well illustrated recent guide (Inns, 2009) and a publication dedicated to crested newts (Jehle, Thiesmeier and Foster, 2011).
Fig. 1.4 Frog spawn (Chris Dresh)
It’s in their life histories that amphibians and reptiles differ most strikingly. All amphibians need water for reproduction. In the tropics this requirement can sometimes be accommodated without resort to ponds or streams (usually by adults carrying tadpoles on or in their bodies), but in Britain access to a wetland is essential for all our species. Unshelled eggs (spawn) are deposited in water, without which they desiccate and die very quickly. The eggs may be laid singly and wrapped in the leaves of pondweeds, as in the newts, or laid in clumps or strings as in frogs and toads. Depending on temperature, development of embryos is complete within one to two weeks and the larvae (tadpoles) become free-swimming soon after hatch. An astonishing feature of great crested newt eggs is that 50% inevitably die before hatch, the result of a genetic anomaly not seen in any other British species. Larval development continues in the pond for as little as four weeks for natterjack toads in warm springs but occasionally for up to a year in the case of newts. This aquatic phase of amphibian life history is too well known to merit detailed description here but remains one of their most fascinating features, highly accessible to study and still open to new discoveries, as indicated in later chapters. Sooner or later, usually around midsummer for frogs and toads but in August or September for newts, the larvae metamorphose into miniature versions of the adults, and these juveniles immediately leave the water. The final hormone-triggered transformation is rapid and normally accomplished within a few days.
Anuran (frog and toad) tadpoles are omnivorous, consuming microorganisms in the water column but also grazing sediments on the pond bottom or even from beneath the surface film. Urodele (newt) larvae are entirely carnivorous and feed on whatever they can catch that is small enough to eat, especially crustaceans such as Daphnia. After leaving their natal ponds young newts carry on feeding on small invertebrates, whereas frogs and toads make a dramatic transition from omnivore to carnivore, now also pursuing invertebrate prey. For around two years, sometimes longer, juvenile amphibians of all species remain almost entirely terrestrial while they grow to adult size. Great crested newts are an exception because young animals occasionally return to ponds for a while in spring, presumably because these are a good source of food. In summer and autumn juveniles and adults alike venture out of their hiding places on warm, damp nights to hunt the small animals that provide all their food. In winter they seek shelter from frosts, usually underground but occasionally (especially in the case of frogs) at the bottom of ponds. Some time during the spring adults of all species migrate to their breeding ponds for spawning while juveniles remain on land.
Adult frogs, toads and newts can survive in captivity for many years, much longer than mammals or most songbirds of comparable size. Ages of over twenty have been recorded for some species, including common toads and great crested newts. In the wild, lifespans are typically much shorter. Although there is considerable variation among species and locations, a seven or eight year old amphibian is at the top end of normal life expectancy and the probability of surviving from one year to the next is commonly around 50%. Individuals in high altitude populations tend to live longer than those in the lowlands, possibly because they are active for fewer months each year and the hibernation period (when they are not exposed to predators) is relatively long. All this means that many individuals have more than one chance of reproduction and amphibian populations normally consist of overlapping generations.
Hardened by deposition of calcium salts, as in bones
Laying eggs
The life history of a reptile is very different. Reproduction does not involve water because eggs, where present, are shelled and partly resistant to desiccation. They are not however as impermeable as bird eggs, being leathery rather than calcified on the outside. This means that egg-laying requires conditions with at least some moisture if the embryos are to survive. But even within the small British list there are distinct differences in reproduction mode. We have only two native species that are oviparous, notably the sand lizard and the grass snake. The lizard uses holes dug in sand whereas the snake selects warm spots, ideally with heat supplemented by fermentation of decaying vegetable matter, such as piles of dead leaves or compost heaps. Eggs develop through the summer and hatch in August or September. Our other species are viviparous, giving birth to fully formed live young after the eggs have completed development within the mother. This strategy is adopted by viviparous (common) lizards, slow-worms, adders and smooth snakes. Often the juveniles emerge still encased in a membrane but wriggle free from it almost immediately. Viviparity in reptiles means that the mother can, by basking, maintain much better control over development than is possible where eggs are left to take their chances. On the other hand, the extra basking needed in summer may expose females to elevated risk of predation. Viviparity becomes increasingly frequent among reptile species at high latitudes where temperature may be most critical to reproductive success. It was surprising to discover that in some reptiles, especially turtles, incubation temperature influences sex determination of the offspring, but this has not been demonstrated in any British species.