Female Formosa kukri snakes will defend their food even though doing so shortens their lives, researchers have discovered.
The finding, reported in the journal Science Advances, provides a brutal example of an evolutionary trade-off, in the which short-term food security trumps a long life.
It is also a rare inversion of a pattern common across mammals, birds, insects, amphibians and fish, in which males tend to die earlier than females.
The Formosa kukri snake (Oligodon formosanus) is a specialist egg-eater, with a diet comprising mostly the eggs of small lizards and other snake species.
By far its preferred food source, however, is the eggs of marine turtles, laid in clutches on beaches.
When a store of turtle eggs is located, female snakes will aggressively defend them against all comers – including males of their own species.
To test whether this behaviour impacted on longevity, scientists led by Chi-Ying Lee of the National Changhua University of Education in Taiwan conducted an elegant natural experiment.
Starting in the mid-90s, the scientists tagged hundreds of Formosa kukri snakes resident at two Taiwanese beaches – Little Paiday and Tungching – monitoring their lifespans.
As predicted, the male snakes tended to live longer than the females, but although extreme egg-defending aggression was a clear behavioural difference between the two genders it could not be conclusively identified as the determining factor.
At least, it couldn’t until a storm hit Tungching and obliterated the turtle nesting grounds – a situation that persisted for more than a decade.
With no eggs available, the overall population of the snakes declined – but the females, no longer spurred to aggressive defence, lived just as long as the males.
The researchers conclude that the short, violent lives of the female snakes in egg-rich circumstances confer a species survival that is more important than the individual cost.
“Obviously, reproductive investment on the part of female snakes demands ample supply of energy and nutrients, which could be provided by a high-value food resource such as sea turtle eggs,” they conclude.
“When reproductive output is concerned, the benefits derived from egg consumption could outweigh the costs of territorial aggression … Female snakes might thus be able to maximize their own reproductive output at the expense of their life span.”
