The enigmatic life & death of toad tadpoles

One of our favorite places to stop while traveling through GA is the Botanical Garden in Athens. Now I hope we may be forgiven for revealing that we do not always focus entirely on the fabulous plants, both exotic and native that are on display there. Indeed we enjoy those but the garden is somewhat unusual in that it includes some very nice deciduous and flood-plain forests. So in the Spring you can expect to find some interesting migrating birds.
While my wife was chasing after a Kentucky warbler calling from the bushes along a power-line cut, I noticed something strange in a drying pond, a mass of wriggling black objects (see photo 1). On closer inspection (see photo 2) it is clear that these were a huge mass of black tadpoles, apparently in their death throes as the pond dried. I immediately concluded they must be American toad tadpoles due to their color, choice of habitat, and time of year. Male American toads (see photo 3) start calling in early Spring when it rains and search out very small isolated ponds that lack fish and most other aquatic critters of any size. Their call is a series of long trills and it attracts females for mating. Long strings of eggs are laid and quickly hatch. Under ideal conditions the larvae grow rapidly and many will metamorphose at a very tiny size and leave the pond before it dries. However if enough rain does not fall to maintain the pond, all or most of the tadpoles may die, as seems to be the likely scenario here.
So why would toads lay their eggs in such a difficult place to survive?
It seems completely counter-intuitive. Yet there is a method in this apparent madness- namely escape from the predation and competition in larger ponds. Only a few types of amphibians (such as bullfrogs and green frogs) are able to survive in ponds with fish. Amphibians such as spotted salamanders and wood frogs breed in fish-less temporary or vernal pools which may contain water for several months. The pools chosen by American toads are generally so small and temporary that few other amphibians and insects will breed there. This seems to be the strategy behind the strange choice of pools they make. Yet the shallow pools used by these toads would seem to make them very susceptible to predation by terrestrial and avian predators; their defense is to be toxic and to associate in large schools of their siblings. Thus when a foolish predator attacks a few tadpoles, it may then avoid eating any more in this group and those that survive are genetically related to the "martyrs."
So consider the remarkable life history of the lowly toad and marvel at the complex interactions involved. Ain't Nature grand?


-Bill Dunson