U P S T R E A M • S P R I N G 2 0 0 0
Female flies boost abdomens for males
The current issue of prestigious British natural science journal Nature published a review of a paper by Stroud Center entomologist David Funk and a colleague from the University of Delaware, Douglas Tallamy. Their paper appeared in February’s Animal Behavior journal. This is what Nature had to say about the paper:
When it comes to picking a mating partner, the females of most species usually get to do the choosing. Sometimes, as in the case of the long-tailed dance fly Rhamphonyia longicauda (pictured right), the roles are reversed. David Funk and Douglas Tallamy show in Animal Behaviour (February 2000), that even here the females seem to be holding all the cards.
Female dance flies cannot hunt for prey for themselves, and instead rely on nuptial gifts presented to them by the males in exchange for mating. This trade-off takes place in female leks, where females gather together and wait for the males to bring them food. In contrast to many species, this means that it is the males that get to do the picking. But Funk and Tallamy reveal that the females have evolved a clever way to increase their odds of being chosen.
The males prefer to mate with females with swollen abdomens. The likely reason is that abdomen size is perceived to be a good indicator of egg maturity - for related species, R. sociabilis, multiple regression analysis revealed a significant relationship between these two variables. Funk and Tallamy propose that, given that males want their sperm to be the ones that fertilize the egg, waiting until the eggs are mature is a good way to ensure that this happens.
But female R. longicauda cheat. The females puff up their abdomens - probably by ‘swallowing’ air - so creating the impression that their eggs are more mature than they really are. (Multiple regression analysis showed no significant relationship between egg maturity, as judged by egg size, and the size of the inflated abdomen.) It seems that the males of this species are fooled.
The leks take place for a short time between dusk and dark. Together with the fact that the leks occur under a clear patch of sky in an otherwise dense canopy of trees, this allows the females to maximize their strategy. The males hover beneath the females, which appear as dark silhouettes against a bright background of sky. All the males see is the expanded shaped of the abdomen, and they have no further clues to the actual state of the females’ eggs.
All of this means that females whose eggs are not yet mature can gain the sustenance needed to complete egg development at least once. It seems that, contrary to appearances, it is the females who still retain the upper hand in this mating game.
