Female toads can discourage unwanted suitors by inflating their bodies, thereby making it difficult for males to cling on during copulation and allowing rivals to knock them off, researchers have discovered.
Male frogs and toads frequently advertise for women with loud calls but females approaching vocal males can be intercepted by nefarious hijackers who won’t take no for an answer. Frog fights may ensue when multiple individuals attempt to mate with a female.
Now Benjamin Phillips, of Vrije University in Amsterdam, and his colleagues show that the female cane toad (Bufo marinus) has a previously unnoticed method of picking her man: blowing up.
“Why has it remained undetected?” they ask in Biology Letters. “We suspect that inflation by the female during male–male rivalry has been interpreted as a simple response to being pushed, kicked and occasionally flipped over by amorous suitors. Here we show that such a defensive response can be co-opted by selection for mate choice.”
They surgically prevented nine females from inflating and placed them in an ‘arena’ with three males. Initially, they allowed the smallest male to mount the female and then the competition was on. Smaller males were never successfully displaced from un-inflatable females by their larger rivals, but females that could inflate often shrugged off their weedy first dates for a buffer option.
Phillips and colleagues also measured the force needed to remove a male from inflated and un-inflated females with a spot of induced necrophilia. First they killed four females frogs and replaced their insides with balloons. Then they allowed males to mount these models and tested how much force was required to pull them off when the balloons were inflated and un-inflated. Males were much more easily detached from inflated females.
This demonstrates not only a new method of toad mate choice but also suggests there may be a wider trend of females co-opting defensive manoeuvres into sexual selection.
Image: inflated female with (presumably unwanted) male mate / Royal Society