WHY do the females of some species mate with as many males as possible? The strategy seems to reduce the chance of choosing a male that鈥檚 infertile, closely related or genetically inferior, but that may not be the whole story. Research on the most promiscuous vertebrates ever studied 鈥� Bibron鈥檚 toadlets 鈥� suggests that promiscuity can also lead to a safer home to bring up the kids.
Phil Byrne at Monash University in Clayton, Victoria, Australia, and Scott Keogh at the Australian National University in Canberra studied toadlets (Pseudophryne bibronii) living in a coastal region of New South Wales. As with many birds, fish and other amphibians, the males are responsible for building the nest and tending to the brood.
In this species, the male creates a shallow depression in the soil, into which the female lays her eggs. The embryos develop into tadpoles inside thick jelly capsules, then enter a state of suspended development until the nest floods, which might not be for weeks or even months.
Advertisement
The biggest threat to the tadpoles is that they鈥檒l dry out and die before the flood arrives, so a female must choose males with nests that will retain a high level of soil moisture. What鈥檚 more, the nest needs to be in an area that floods sufficiently and at just the right time in the season.
Predicting exactly which nests will be best for the job is tricky. Byrne and Keogh discovered that the most successful females hedge their bets by laying eggs in as many nests as possible. All the females they studied mated with at least two males, and some with up to eight. The females that mated with the greatest number of males had significantly more tadpoles hatch than those that mated with just a few (Proceedings of the Royal Society B, ).
鈥淭he female Bibron鈥檚 toadlets that mated with the most males had significantly more tadpoles hatch鈥�
鈥淭his is a pretty novel explanation for female promiscuity,鈥� says Byrne. It might also hold for other egg-laying species in which the males build nests in unpredictable environments, such as certain fish, he says.
The work does suggest that, in this species at least, paternal nest quality could be an important driver of the evolution of female promiscuity, says Leigh Simmons, an evolutionary biologist at the University of Western Australia in Perth. 鈥淭he importance of bet-hedging as a contributor to female fitness has been largely neglected since early theoretical models suggested it could not work,鈥� he adds, so Byrne and Keogh鈥檚 results demand a rethink of bet-hedging theory.
