A tiny species of Amazonian frog regularly engages in "explosive breeding," when hundreds of frogs congregate in an area, usually small ponds or headwaters, for two to three days and brutally compete to win the attentions of receptive females. Scientists from the Brazilian Universidade Federal do Amazonas observed Rhinella proboscidea over a period of five years and an area of 64 kilometers.
The researchers found that when female frogs are sometimes accidentally trampled to death or drowned under the weight of too many male frogs, the males will extract eggs from the females' bodies by "squeezing the sides of her belly with rhythmic movements of his front and hind limbs" and fertilizing them, Wired reports.
After picking up dead females and dissecting the ones that showed no signs of decomposition, scientists found that none of the dead females still contained eggs, or oocytes. Because the Rhinella frog eggs are deposited in strings, researchers hypothesized that the necrophilic behavior only works for these frogs and not hylids, or tree frogs, because of how the eggs are distributed in the abdominal cavity.
The ratio of male to female is usually about 10 to 1, meaning that if a female dies, the chances of mating drastically lower for the males. Being able to deposit eggs after death was advantageous for the species, in other words.
"In contrast to the conclusions of other studies," the study says, "necrophilia is not a behavioural mistake in R. proboscidea, but rather is a functional behaviour in terms of fitness, with positive effects on the reproductive success of both males and females."
Scientists held onto the eggs deposited after death until after embryos formed to confirm that fertilization happened. While it is beneficial for females to be able to expel eggs post-mortem — the researchers did not know whether the females would survive to the next explosive breeding event, nor if it was still able to reproduce again — they observed female R. proboscidea leaving the area, meaning that many do survive the process.
The study was published in the Journal of Natural History on Taylor Francis Online, where the full text is available.