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An occasional series of notes based on field observations throughout the year

March 2010 After the coldest winter for over thirty years, the question is whether our reptiles and amphibians have managed to survive the prolonged snow and frost, buried deep underground or beneath the icy surface of ponds, or whether they might have suffered the sort of population crash which is likely to have happened to the Dartford warbler, a bird which shares the heathland with our rarest reptiles, but which is forced to remain active through the winter, foraging for any insects which it can find. This is where reptiles and amphibians can sometimes be at an advantage, since provided they can see out the cold in a frost-free environment, they can subsist on their fat reserves til spring. Finally, the cold weather relents, and I begin to revisit each of London's four adder sites, to see which animals have made it through the winter, and more specifically, to make 'emergence counts' of the animals as they bask at their hibernacula, soaking up the welcome sunshine, which makes them more obvious than later in the season. In an early spring, adders can be seen in numbers before the end of February in London, but this year it's the middle of March before they first appear; as usual, it's the males which are first to be seen, and one visit at the end of March produces 40 animals, of which no fewer than 34 are males!

Three male adders, having survived the cold winter, conserve warmth by huddling together, while a fourth joins them (photo copyright Will Atkins)

March 2010 - Amorous amphibians...

So, if reptiles seemed to have survived the cold winter well, what about amphibians? After the thaw many reports came in which described dead frogs floating in garden ponds. Most of these will be male frogs, which often hibernate underwater in the silt at the bottom of ponds. This allows them to intercept females (which hibernate on land) as soon as they arrive at their breeding ponds at the very start of the spring. Unfortunately, this strategy can come at a cost - should ice form for long periods on the pond, the depletion of oxygen and the build up of gases such as carbon dioxide can kill the males, hence the bloated bodies which make for a distressing sight when the ice disappears. The good news is that some males always chose to hibernate on land, and, like the females, they seem to survive cold winters well. So the late-comers are favoured in colder winters, with those males hibernating underwater being lucky in mild winters. A balance is therefore maintained between the two behaviours. A week or so after the dead frogs are revealed, mating begins - this time in late March, a good two or three weeks later than in recent years, again indicating the prolonged nature of the winter. However the frogs soon make up for lost time, with several hundred clumps of spawn soon being produced in suitable ponds - even quite small garden ponds. The photo below was taken at a village pond in Hertfordshire which has probably been a traditional spawning site for frogs and toads for centuries.

Look out - he's behind you! A pair of common frogs is inspected by a lone male (photo copyright Will Atkins)

Unlike male frogs, the male toads hibernate on land, along with the females. A couple of weeks after the frogs have spawned, toads make their migrations to traditional lakes and larger ponds to breed. At this time, thousands are run over by traffic as they attempt to cross roads, but in Epping Forest in Essex, for example, toads can be in the fortunate position of being able to travel from wintering sites to breeding ponds without having to cross a road. A visit to the Forest in late March showed that their numbers remain high at Fairmead pond - a typical breeding site for toads, in that it's a big waterbody with fish present. Toad tadpoles are distasteful to fish, and hence toads can spawn in larger waterbodies which might be avoided by frogs. In this way toads can avoid competing with frogs, whose tadpoles (having hatched earlier in the spring) can outcompete toadpoles. Since adult toads also possess distasteful chemicals in their skin (bufotoxins), they can be active in broad daylight at the pond, as this photo below illustrates:

Dozens of toads spawning at Fairmead pond, Epping Forest, March 26th 2010 (photo copyright Will Atkins)

Amongst the large breeding aggregations of toads, it's not uncommon to find balls of mating toads, consisting of a single female which has been grabbed by many males. The unfortunate female can often drown as a result of the unwelcome attentions of the males. At the centre of this toad ball there was a single very large female - perhaps her large size (and hence large number of eggs) made her a hyper-attractive target for the males - at any rate, I removed a dozen males from her, before putting her in a quieter corner of the pond...

The large female toad which was the (literal) centre of attention of the males: (photos copyright Will Atkins)


 

 

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Mid April 2010 - Adders Dancing

 

By the middle of April, most of the work counting adders at the four London sites has been done, and the male snakes have been joined by the females, which emerge later. After a couple of weeks under the clear blue skies, males are in breeding condition and shed their skins, transforming them from an earthy brown ground colour to the silver-grey against which their trademark black zig-zag is particularly evident. Their behaviour also changes - driven to find, mate with and defend a female, they become less wary than usual, to the extent of slithering over shoes, and coming towards a camera lens, in their quest to find a female. As for the females themselves, they sit quietly under cover, releasing a pheromone which draws in suitors from some distance. If two males find a female, then one of the top UK wildlife spectacles, the dance of the adders, can result. At one London site I was lucky enough to see two well-matched males fighting over a female, a combat which took them twisting and turning around a small patch of heather for around ten minutes. Each male tries to push the other down, in a trial of strength like an arm wrestle (except without the arms...) The winning male will probably be the bigger, or, if evenly matched, the male which was already guarding the female. Having driven off the rival, the process of courtship begins in earnest, with the male's body twitching and jerking rapidly, and his tongue flickering all over the female's body. The top two photographs show males in combat, and the lowest photo shows a mating pair (all photos copyright Will Atkins)

 

 

 

 

Mid April 2010 - Grass snake Goings-On

 

Harder to spot than emerging adders and generally harder to study, it's only in recent years that some of the interesting behaviour of grass snakes has become known. For example, that matings in late autumn seem to be a fairly frequent occurrence, although whether this leads to eggs being fertilized and then laid at the start of the following spring is debatable. It has also become clear that grass snakes, like their US cousins the garter snakes, often form 'mating balls' consisting of several male snakes all struggling to be the lucky one which hooks up with the female and gets to pass on his genes. There's none of the ritualized combat of the adder shown here. Another interesting fact is that grass snakes appear to be able to breed more or less immediately after hibernation, without requiring the lying out period of adders, (in which males and females come into breeding condition). This means that grass snakes can be breeding at any time from late March onwards. Finally, again unlike adders, they will happily grab any passing prey item - frogs and toads, during the breeding season, whereas adders wait til the reproductive part of their life cycle is over before considering feeding. All in all, grass snakes seem more flexible than adders in their reproductive behaviour. At one of London's best grass snake sites, fortunately protected as a National Nature Reserve, over twenty animals can be seen in April in a single visit; below are photos of 3 tails entwined (the best shot I could get of a mating ball deep in hawthorn and bramble scrub!), a grass snake and slowworm basking together (the species are often found together and represent no threat to each other) and finally a slate grey animal in contrast with the more usual olive green or brownish animals (all photos copyright Will Atkins)

 

above - count the tails! there are two male grass snakes trying to mate with the larger female

 

a large male slowworm uses a grass snake as a convenient basking platform

 

an unusually coloured slate-grey grass snake, complete with a wolf spider using it as a basking platform