Rain Stimulates Many Frogs to Breed. But HOW does it work?

Smilisca baudinii from Central America

In my last post here I remarked on the lack of knowledge of how the onset of seasonal rain induces breeding in a number of tropical birds. We know why that happens: the onset of rain presages the explosion of insect life on which young birds are fed but we do not know how that stimulus works.

Amphibians also breed in response to rain and some of them ought to provide an opportunity to discover what it is about rain that signals to the brain and sets in train the neuro-hormonal events that lead to breeding behaviour, egg production and fertilization.

I should make a distinction here between a physiological state of readiness to breed and the actual trigger that sends a signal loud and clear to ‘go’ such that in a matter of hours eggs have been laid and fertilized. It is this latter effect of rainfall I am discussing.

Many tropical frogs can and do breed throughout the year but only do so after rain. Herpetologists make use of this phenomenon to induce breeding in captivity. All sorts of devices have been rigged in the past to mimic the whole manifestation of a tropical downpour: simulated rain; tape recordings of heavy rain and thunder; setting off the simulated shower during a period of decreasing barometric pressure etc. Sometimes such manipulations work; sometimes they do not. When they do work though the results are spectacular; within hours there are masses of frogspawn.

Such set ups in captivity ought to enable the identification of what it is about rain that is important and to determine the senses that are being activated to pass on that signal to the brain. Thirty-five years after I was either breeding or attempting to breed treefrogs from central and northern South America, the use of a ‘rain chamber’ is established practice for some species, like the Red-eyed Tree Frog (Agalychnis callidryas). At this stage I should point out that a rain chamber is a vivarium arranged with a pump to recirculate water through a spray head such that ‘rain’ falls onto the plants and inhabitants below. It should, therefore, be possible to devise experiments to test the factors that might be important. For example, there have been suggestions that a sharp rise in humidity is a sufficient stimulus - easily tested by having a netting divider across the vivarium with frogs but not ‘rain’ on both sides; sealing the vivarium would enable changes in barometric pressure to be tested.

Red-eyed Treefrog from Central America

The outcome of such experiments might have a bearing on the old problem of how frogs and toads find their way to ponds in spring. A famous but now largely forgotten British scientist, Ronald Maxwell Savage (1900-1985) whose ‘professional’ hobby was studying the Common Frog, Rana temporaria, obtained some evidence that frogs can smell the algae growing in fresh water and thus move towards the source of the aroma. Could chemical sensing be involved in tropical frogs? Do they respond more reliably to recirculated aged vivarium water or to fresh rain water at the same temperature? How important is change in temperature? The possibilities seem endless but some pretty simple experiments would I am sure soon uncover which factors could be discounted at least in one species. We do not, of course, know whether the key factor about rain that stimulates breeding is the same in tropical tree frogs as in, say, a desert frog in Africa or Australia.

I was not able to take my breeding of species thought to be susceptible to tropical rain very far for the simple reason that other species I had, particularly dendrobatid or poison-dart frogs, were breeding so well that rearing their young was taking all my spare time. However, one species that sometimes did and sometimes did not respond to an artificial tropical storm was Smilisca baudinii, which has a number of common names including Mexican Treefrog and Masked Treefrog. One spawning by several females goes a long way and I raised a hundred or so past metamorphosis to a size where they could be passed on to others. With those which did not breed after an artificial downpour, an injection of human chorionic gonadotrophin (i.e. as in the human pregnancy test using Xenopus) resulted in fertilized eggs the next day. In other words the frogs had been ready to breed but my ‘rain’ had not been a sufficient or only stimulus.

Given the importance of rain in stimulating breeding in some amphibians as well as in some birds and other organisms, surely it is time we had experimental evidence on what it is doing and how it works.

Smilisca baudinii - captive-bred juvenile