A female strawberry poison frog faces an abundance of choice when it comes time to breed. The forest floor is full of bright red males trying to attract her with their songs, and wrestling with other males to defend their territories. She could pick a suitor based on his size or health. She could weigh up the quality of his territory. She could judge him on the depth, volume or length of his croaking, any of which could indicate how strong he is.
Or she could just mate with the first male she finds.
That, rather anticlimactically, is exactly what happens. For all the effort that males put into attracting a partner, the only factor that seems to matter to the females is who’s nearest. And according to Ivonne Meuche from the University of Veterinary Medicine in Hanover, this strategy makes perfect sense for these frogs.
The strawberry poison frog (Oophaga pumilio) has become something of a celebrity among scientists studying frog behaviour. It’s easy to find because of its bright colours and tendency to hop about in the day. And it has lots of sex. On average, a female will only go for 4 to 5 days between partners.
The frogs practice ‘lekking’—a style of mating where many males call at the same time, allowing females to choose between them. Each male defend a small territory, and each female wanders across many of these. When she chooses a mate, the two partners face in opposite directions while she lays eggs and he fertilises them.
Meuche’s team followed 20 female frogs in the rainforests of Costa Rica to see which males they mated with. They compared the qualities of these victors to those of every other male within the females’ home ranges. They also compared the two males that were closest to the females on the morning of their egg-laying days.
In an earlier study, one of the team, Hieke Prohl, suggested that males mated with more females if they called more often and at a lower pitch. But this time, they found that females were completely oblivious to the males’ territory size, weight, length, health or calls. Instead, they just went straight for the nearest one who was calling.
You could argue that females are making pickier choices the night before, so that they’re waking up near their favoured partners on egg-laying days. But in other studies, the team showed that the females’ whereabouts don’t depend on the males but on the availability of food.
They also checked their results by using two speakers to play recordings of males with different call rates and pitches. Forty-five females heard these calls and none of them seem to care about the calls themselves. They just went for the closest speaker.
This is unusual. Lekking is almost synonymous with female choosiness, although some other frogs also use a “closest-male-wins” strategy. It presumably means that they sometimes mate with dud partners while there are prime specimens calling a bit further away.
But Meuche thinks that the females aren’t fussy because there are big costs to shopping around. In this corner of Costa Rica, female strawberry poison frogs outnumber the males. Males are in short supply, and if they’re with another female, they stay silent and cannot be found. If a female rejects a male, she might not be able to find another partner, much less a better one. If this happens, she’ll lose an entire clutch. On her egg-laying days, she has to find a mate within a certain time or she’ll just lay unfertilised eggs that never develop.
The team also found that the males are all much of a muchness. They compete so intensely for territories that those with good ones, which put them closest to as many females as possible, will probably also have good genes. Females have a good chance of getting a high-quality mate even if they grab the closest one.
Reference: Meuche, Brusa, Linesenmair, Keller & Prohl. 2013. Only distance matters – non-choosy females in a poison frog population. Frontiers in Zoology http://dx.doi.org/10.1186/1742-9994-10-29
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