To a female frog, the mating season must sound like a cacophony of rock, rap, and country-and-western tunes as males of all species croak and bellow for attention. So how does she find Mr. Right--and most importantly, Mr. Right Species? It turns out that she doesn't evaluate each call but rather blocks those that are biologically meaningless to her.
Researchers already knew that female túngara frogs (Physalaemus pustulosus) that are ready to lay eggs and thus searching for mates pay more attention to the calls of their own species. Placed in a confined space and given a choice between a speaker emitting the "whine-chuck" of a túngara male or the call of another species, the females invariably hopped toward the túngara speaker. But male túngara frogs showed no such discrimination; they amped up their own calls whether they heard another túngara male or a male from another species.
Kim Hoke, a neuroscientist at the University of Texas, Austin, suspected that males and females process the calls differently. To find out what part of the brain is involved, Hoke and colleagues tested 60 wild-caught túngara frogs (30 males and 30 females) that were ready for mating. Each frog listened for 30 minutes to a recording of a male túngara calling, to a different species, or to silence. They were immediately euthanized and their brains frozen and treated with a radioactive marker to reveal neural activity.
The simple auditory regions of the lower brainstem looked the same in the males and females, says Hoke. Thus, both sexes heard the calls of all species. But deeper in the frogs' brains--in an area called the laminar nucleus, which is important for making behavioral decisions--there was a marked difference. Here, the females' brain tissue showed activity only after listening to the calls of a túngara male. The males' neurons, meanwhile, were activated regardless of the species that made the calls.
The females are essentially blocking out the calls of the nontúngara males, Hoke says. "It's as if they don't hear them at all." That makes sense, she adds; females need to choose a mate carefully, because they produce fewer eggs than males do sperm. The scientists report their findings today in Biology Letters.
"It's an exciting study," says Jason Miranda, a neuroscientist at Emory University in Atlanta, Georgia. Miranda says the findings will give researchers a new area of the brain to target when investigating questions about the development and evolution of sex differences in social behavior. "It's a fantastic steppingstone," adds Catherine Dulac, a molecular neuroscientist at Harvard University who studies the neurogenetic controls of sexual behavior of mice.