Frogs: Singin' in the Rain

Over a number of articles I have looked at whether or not—and if so how—rain triggers breeding in birds and amphibians.

If rain does trigger breeding how does it work? Is it indirect, as Maxwell Savage proposed, by stimulating the growth of algae, the smell of which attracts frogs to breeding ponds as well as acting as the trigger for breeding when they do arrive? Similarly, Ron Murton and Nigel Westwood wrote in 1977: ‘…what still needs to be determined is whether the appearance of rain or clouds causes any of the Australian desert birds to indulge in display that in turn stimulates the hypophysial-gonadal axis into activity. At present, the indications are that those species which breed with the onset of rain respond to food supplies that are made available within a day or two of rain falling’. Or is it direct, rainfall itself or an associated change in pressure, humidity or temperature, directly perceived? And how does simulated rain trigger breeding so quickly in tropical frogs kept in captivity?

The croaking of frogs is, of course, associated with breeding, as males arrive at their breeding ponds, for example. However, I was reminded by a recent article by Gordon L. Miller in Archives of Natural History that in frogs and toads there is a phenomenon, recorded since the time of the Ancient Greeks, for which there is still no satisfactory explanation: ‘Rain Calls’. Rain calls are those emitted outside the breeding season but in response to impending rain. There are two questions that spring to mind immediately. What environmental change or changes is responsible? Since it can occur before the rain actually falls as well as during and shortly afterwards such factors as a rise in humidity or a decrease in atmospheric pressure have been suggested. The second question, the ‘why’ one, remains unanswered. What was in it for the frog I saw in the garden croaking loudly a few minutes after a shower of rain in late summer? Is it, has also suggested many times, a sign that the frogs are returning to breeding condition but that further development is arrested by the low temperatures of late autumn and winter. But croaking outside the breeding season would seem to bring only disadvantage—why croak when doing so could draw the attention of predators? I will not continue because I did note that no satisfactory explanation exists.

Cicero, as noted by Miller, drew attention to the phenomenon over 2,000 years ago:

There is within frogs a kind of natural force for giving signs, sufficiently clear in itself but too dark for human comprehension.

What the phenomenon of rain calls does show is that some environmental factor acts as a short-term and powerful trigger for frogs to start croaking. Surely it is not beyond the wit of science to devise simple experiments to find what that trigger(s) is(are). Tropical frogs in artificial climate- and pressure-controlled ‘rain chambers’ would seem a good way to start (see here).

The frogs (Rana temporaria) that hang around in our garden after breeding have been known to
croak after a shower of rain. This one, disturbed by gardening, took to the bird bath.

Miller GL. 2021. The rain calls of frogs and the reigning paradigm of American herpetology. Archives of Natural History 48, 42-61.

Murton RK, Westwood NJ. 1977. Avian Breeding Cycles. Oxford: Clarendon Press.