Sex is a risky business for many animals. Those who take part in 'explosive breeding' -- where many males gather and compete for a small number of females over a few days -- have it particularly tough. Males can become exhausted from the competition and search for a scarce mate, or from trying to dislodge other males from receptive females. The females themselves can be unintentionally crushed, drowned or simply exhausted under the weight of their many suitors.
But now scientists have discovered that the tiny Central Amazonian frog Rhinella proboscidea has developed an effective, if unsavoury, way of making the most out explosive breeding despite its inherent dangers. Writing in a recent issue of the Journal of Natural History, Brazilian researchers have observed the frog indulging in what they call a 'functional necrophilia strategy'. In other words, males of the species have been observed extracting oocytes from the abdomens of unfortunate females killed in mating struggles, and then fertilizing them.
While unpleasant from a human viewpoint, from an evolutionary perspective taking advantage of oocytes from dead females minimises the losses both partners can experience as a result of explosive breeding. The male is able to breed successfully despite not having had access to a live female or expending too much energy in the battle to secure one; the female's eggs are fertilised even though she herself has expired. The existence of this strategy also suggests that there may be possible selection in favour of stronger and more persistent males in explosive breeders; these sorts of males may be not be advantageous to the females themselves -- and indeed their determined behaviour can often kill them -- but the 'functional necrophilia' strategy ensures that the species continues despite a lack of live females.
The authors write that 'although necrophilia has been reported in other species of anurans, this may be the first case where the necrophilia brings a direct fitness gain, generating descendants. In contrast to the conclusions of other studies, necrophilia is not a behavioural mistake in R. proboscidea, but is rather a functional behaviour in terms of fitness, with positive effects on the reproductive success of both males and females.' This study is a fascinating glimpse into a species' struggle to survive under the rain-forest canopy -- however strange it might seem to us.
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