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TRANSVESTITE snakes may seduce unsuspecting males in order to sap their
competitors’ energy—allowing them to get it on with the real females at
their leisure.

The idea follows the study of male garter snakes, some of which fool other
males into mating with them by secreting female chemicals from their skin. The
reason for this strange behaviour has been a mystery since its discovery 15
years ago. But now Rick Shine, an evolutionary biologist from the University of
Sydney, and his colleagues from Oregon State University, Corvallis, think they
have worked out the evolutionary strategy behind the behaviour.

The team observed red-sided garter snakes (Thamnophis sirtalis
parietalis) in Manitoba, Canada, which had recently emerged from
hibernation. The snakes formed “mating balls”, where several suitors competed
for one female. But over a quarter of the mating balls were actually centred on
a “she-male”. When the researchers studied these impostors, they found that they
were fatter than their male counterparts and crawled more slowly. Their
courtship behaviour was suppressed and they were less likely to couple with
females. “Males that produced female skin lipids became female in behaviour as
well as attractiveness,” says Shine.

To check that the snakes were not getting their hormonal disguise from
contact with females, the researchers rubbed males against females. Males
treated in this way failed to excite other males. However, when they removed the
snakes’ male pheromones by washing them with a solvent and adding female lipids,
the snakes became attractive to males. These “artificial” she-males also lost
interest in sex.

But to the researchers’ surprise, when these she-males were captured and
tested a day later, they had become the most vigorous suitors of all. At the
same time their attractiveness to males had decreased.

The situation seemed inexplicable, until Shine realised that the hormonal
“cross-dressing” was a temporary phase that all males went through upon
awakening from hibernation. Shine and his colleagues suggest that she-maleness
suppresses the snakes’ urge to court females while they are still weak from
hibernation, preventing them from wasting valuable energy on courting when they
are likely to fail. Masquerading as females also confuses other males,
distracting them from true females.

“We call this kind of behaviour `spite’,” says Morris Gosling, an
evolutionary biologist at the University of Newcastle. “The she-males diminish
the costs of courtship at the same time as duping other males.”