The American toad eggs that were laid about one-two weeks ago in our farm pond have now hatched and the young tadpoles are clustered in a tight group along the edge of the pond in shallow water (see photo).
This raises several questions- why do they do this, and how do they avoid predation, by birds in particular?
Given that it was a relatively warm day for the VA mountains (at 3 PM it was about 69F in the shade and in the mid 70’s in the sun), I suspected that this behavior of seeking out the shallows might be related to thermoregulation, or raising their body temperature to grow faster. I cannot of course answer this definitively but the area where there were gathered was at 76-77 F, about 10 degrees warmer than the nearby deeper water. This is a huge difference and it seems obvious that this could provide a significant advantage in speeding up development in a species that breeds in ponds that can dry up quickly. It might also yield larger tadpoles and thus larger toadlets at metamorphosis, a likely advantage for subsequent survival on land. The black coloration of these tadpoles could be an adaptation for warming up more quickly.
But how can these tadpoles get away with clustering together in shallow water where they would be susceptible to predation, especially by birds? I recall that toad tadpoles are thought to be toxic, and thus at least partially protected from predation. In addition they group together in sibling clusters so that if a naive predator decides to eat some tadpoles, the sacrifice of some will benefit their close relatives if, by causing sickness, it teaches the predator not to eat more. The grouping or schooling behavior could also function to confuse predators and make it harder for them to select a specific prey item.
So here again we observe that within such a simple thing as a group of tadpoles, there is amazing complexity and natural design.
Bill Dunson, Englewood, FL & Galax, VA