Calling the tune

New Scientist, 15 May 1999
ALISON MOTLUK

THE call of a male Majorcan midwife toad keeps females ripening their eggs in anticipation of sex, a new study shows.

Jerry Lea, a postgraduate student at the Open University in Milton Keynes, has studied three groups of female Majorcan midwife toads, Alytes muletensis. All the toads had ripening eggs in their ovaries. To one group, Lea played synthesised versions of the male calls. A second group heard the call of a different species, while the third heard no calls at all.

After a month, females from the first group had many eggs that were ripe and ready for ovulation, while the females in the other two groups had hardly any ripe eggs. Lea speculates that stimulation of the female's auditory nerve fibres causes hormone release in the part of the brain that controls reproductive behaviour.

The finding makes sense given the sex roles of Majorcan midwife toads, says Lea. The males are in short supply because after fertilising the eggs they are celibate for a month while they raise the brood on their own. "The males carry the eggs down the cliff faces to the pools where they develop into tadpoles," says Lea.

Meanwhile, females squabble over remaining males, who advertise their readiness to mate by calling. Some females don't find a partner for the entire breeding season. There's no point in wasting energy ripening eggs that have no hope of being fertilised, Lea says.