Steve's Herpetological Blog

An insight into the life of Steve, his research and the many books he reads


#SciFri: What haven’t I seen as many frogs this year?

A number of people this year have asked me why they haven’t seen as many amphibians as they usually would. I’m sure many of you are used to seeing your pond overcrowded with frogs, jostling for a chance to mate. Or maybe you know of a large toad population nearby and you see them on their annual migration. If you usually see a number of amphibians then I suspect that like my friends and acquaintances that contacted me, you must have been left scratching your heads. In case you’re wondering why too, I thought I’d go into some detail about why there seems to be so few amphibians about. I’m sure that many of you won’t be surprised that the cold start to the year has something to do with it!

Amphibians use a number of environmental cues in order to time their emergence from their winter slumber, such as the day length, air temperature and lunar cycle. Whilst they haven’t got a watch or a calendar, they do a bloody good job of keeping track of time and usually know when to emerge. Once of the issues that I personally think has lead to a reduced number of amphibians moving this year is down to temperature. Typically speaking, amphibians will make their annual migrations back to ponds when the air temperature is above 5°C. From mid-February onwards, you can expect to see toads moving back to ponds, followed by frogs and newts. However, this year, there were very few evenings in which the air temperature was sufficient to allow this. Instead, it’s likely that most individuals (probably the males who haven’t invested a heavy metabolic cost in producing a large clutch of eggs), have instead gone to ground and decided that breeding this year wasn’t for them. Now this isn’t a conscious decision but instead one where the environmental cues don’t meet the criteria their biology needs).

Common frogs (Rana temporaria) breeding in a pond a few summers ago

On top of this, amphibians don’t breed every year. I’ve already mentioned the huge metabolic investment that comes with breeding and as an ectotherm, this is quite the price to pay. Therefore, the population of amphibians at breeding time tends to follow a four-year cycle (in my experience of common frogs and common toads), which may also explain why so few amphibians were seen this year. It may be, that it is the year when the numbers of amphibians heading to ponds to breed reaches it’s trough before picking back up again. This combined with the poor weather, artificially deflated the numbers reaching ponds, making it seem like a number didn’t make it for potentially sinister reasons. The mild and wet winters we’re having, will have an effect on populations. Animals won’t hibernate properly and they’ll use up their fat reserves faster, which may have an impact on survival if they’re not able to feed sufficiently enough.

I’m sure there are many other reasons why some amphibians seem to have been absent this year, but these are the two reasons that make most sense to me. Hopefully they do to you too. If you’ve got any questions about either, please leave a comment below and I’ll do my best to answer them.

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