All the better to hear you with: populations diverging based on female ear characteristics

Robin M. Hare, W. Jason Kennington, & Leigh W. Simmons

This is a plain language summary of a Functional Ecology research article which can be found here.

In most animal species, males fight or intimidate each other for access to females and their energetically costly eggs. Sometimes, as in some seahorses, females fight or intimidate each other for access to males and their specialised brooding pouches. But in southwest Western Australia, a species of bushcricket does something different again. Sometimes males compete for access to females, and sometimes females compete for access to males. The whole system is regulated by food: these bushcrickets feed on spring flowers and the males provide nutritive food gifts when they mate, weighing some 20% of the donor’s body mass. When flowers are scarce, females compete for access to males and their nutritive gifts, which also contain sperm that fertilise the female’s eggs.

A female bushcricket (Kawanaphila nartee) perches on a kangaroo paw (Anigozanthos manglesii). Kangaroo paws are a preferred source of pollen for females early in the breeding season, when pollen is scarce and females compete for access to calling males. This female’s ear is just visible as a speck behind the front leg (credit: Robin Hare)

Males sing to females to advertise when they are ready to mate. We previously found that female ear size, a proxy for hearing sensitivity, is advantageous in helping females reach calling males ahead of their competitive rivals. But this finding was restricted to a single population of the bushcrickets in Kings Park, a botanical garden in Perth. In this study, we collected male and female bushcrickets from throughout their range, some six degrees of latitude or over 400 km, and measured how much ear size differed between populations. We found that females collected from another Perth park called Koondoola had significantly larger ears than females from several other populations.

We suggest bigger ears are helpful for females at Koondoola because the flowering vegetation at that park is diverse but not very abundant, meaning that females are probably more competitive for a longer period there than at other parks. Moreover, different ear sizes in different populations speak to different strengths of selection, which could potentially lead to the formation of new species. What is remarkable about this finding is that the differences in ear size are driven by sexual selection on females, which are rarely considered to be under this kind of selection. We encourage further research on female animals, which have historically been overlooked.

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