overwintering

Written by Dr Andrew Smart, Head of Science and Research at Froglife

People often wonder why they occasionally find tadpoles in their ponds over the winter.  This is an example of delayed metamorphosis, which can be advantageous to some individuals, particularly in northern latitudes. In newts we find overwintering larvae and also examples of neoteny when the larval form is retained and, in some cases, mature and reproduce.

The classic case of neoteny in amphibians is the axolotl, which retain their feathery gills into adulthood, becoming enlarged larvae that develop reproductive organs and can produce large numbers of fertile eggs. Axolotls evolved to live in two lakes in Mexico where the water levels were stable and food levels high, enabling animals, lacking the thyroid hormone that triggers metamorphosis, to survive and evolve a life-strategy that was more effective than having a terrestrial stage.

In frogs and toads, there are a range of reasons why tadpoles might be found in a pond in the winter months. The first to rule out is the risk of the invasive species Lithobates catesbeianus, the American bullfrog, which has a larval stage than can be anything from 1 to 4 years.  Overwintering of this large, predatory species has been found to have a neutral or negative impact on other amphibians in introduced populations in the USA[1]. Thankfully, the bullfrog appears to have been eradicated from the UK since 2015[2].

If you have other ‘normal sized’ tadpoles in your pond, the chances are that a number of things may have happened: in some northern latitudes, frog and toad tadpoles have been shown to adopt different strategies[3], with some animals metamorphosing early while others delay metamorphosis, remaining in the pond to be able to exploit resources early in the spring , outcompeting a new cohort of smaller tadpoles[4].  As well as food availability and temperature, other unknown factors lead to this strategy[5]. This enables them to produce larger metamorphosed froglets or toadlets which have a better chance of survival through the winter. 

So why don’t all anurans follow this strategy?  It can be high risk; extreme temperatures could kill animals during hibernation or ponds could dry out before the end of the late summer period and kill all the ‘delaying’ tadpoles.  Most Anurans have developmental cues that trigger metamorphosis, linked to the proximity of other tadpoles and to reduction in water level; [6] in the case of ‘drying ponds’ reducing water depth[7]^,[8] leads to faster metamorphosis and smaller froglets.

Anurans that develop into ‘large’ tadpoles do have the capacity to develop gonads but in the case of Xenopus (the African clawed toad) ‘giant tadpoles’ that occur develop gonads but struggle to release eggs and sperm[9] and in any case, a ‘giant tadpole’ would be an unlikely animal to participate in amplexus. A similar situation occurs in Pseudis paradoxa, the paradoxical frog, which has evolved a ‘giant tadpole’ that metamorphoses into an adult rather than a froglet.[10] The tadpoles have developed gonads that are close to maturity but the authors point out that life stages in Anurans are very different and that the opportunity for full neoteny is unlikely because of the need for behavioural changes to enable successful reproduction.

What about Caudata? There is evidence that some individuals, usually young adults, may move into ponds in the autumn and overwinter[11].  Examples of palmate newts overwintering in ponds in Scotland are well recorded[12]^,[13], again a strategy to enable a more robust metamorph emerging in the summer to face colder winters. 

There are now several well recorded cases of neoteny in smooth newts – in a Norfolk swimming pool, numbers of neotenous newts were found, resembling female smooth newts but with feathery gills[14].  Two of these animals were  removed for examination and very quickly metamorphosed to adult newts.  The authors of the study suggested a number of possible outcomes suggesting that a change in water quality (increase in iodine) or change in water depth could have triggered the change.

Examples of delayed metamorphosis and also total neoteny have also been recorded at a site in Cambridgeshire, where newt larvae have been seen in large numbers in autumn and early spring, after presumably hibernating in the pond. One of the newts was taken to captivity and laid fertile eggs[15]. This animal did rise to the surface for air,  a behaviour not normally linked with neotenous newts, which have fully functioning gills. As in many cases of neoteny, the records were incidental and unable to be investigated in detail.

So overwintering tadpoles and newt larvae may be a way of amphibians having two different life strategies within a population, some animals taking the risk in the pond, some taking the risk overwintering as a smaller sized individual. Neotenous individuals may be taking advantage of a stable environment with significant food sources and low levels of competition as an alternative strategy, but to really investigate this further we need more information on the habitats where these animals are found.  If you know of a site with neotenous animals, please let us know by emailing: info@froglife.org

References