Female frogs prefer multitasking mates, study finds

In looking for a mate, there are so many things to consider. But while humans might struggle to come to a general consensus on the most desirable traits, female tree frogs know exactly what they're looking for. They want a mate that can multitask, according to researchers at the University of Minnesota.

To attract potential mates, male gray tree frogs (Hyla chrysoscelis) let out a series of trills. These vary in length and frequency, ranging from 20 to 40 pulses per call at a rate of 5 to 15 calls per minute.

After recording the calls of 1,000 male frogs in the natural setting, the researchers calculated the average call lengths and rates. They then played an average call back to females, as well as a series of synthesized calls that were longer, shorter, more frequent or less frequent than average. All of the calls were within the normal range for the species -- there were no super calls.

"We showed that females preferred longer calls, and they preferred a faster call rate," lead author Jessica Ward, a post-doctoral researcher in the Department of Ecology, Environment and Behavior, explained to CBSNews.com. The interplay between call rate and call length is call effort.

By simple arithmetic, call effort increases when rate or length increase. The researchers hypothesized that it was actually call effort that the females care about, so they next tested the calls when effort was held constant. The results proved their hypothesis right, showing that effort -- the intersection of two factors -- was more important than any singular factor.

"We didn't find consistent evidence that they prefer longer fewer over shorter more numerous calls or vice versa. When the call effort is constant, they're not showing a preference," Ward said.

The trade off between call rate and duration appears to be based on energy stores. Maximizing call effort requires high energy, leading to the conclusion that, as with so many species, those putting in the most effort were most attractive to the females.

Publishing their research in the August issue of the journal Animal Behavior, they also investigated whether males changed their call behavior when faced with competition. The results were surprising.

"Even though they're performing longer calls, which adds more pulses, they're performing fewer of those calls. So overall, effort stays the same," Ward explained.

"The interesting bit was that they weren't boosting call effort, the major criterion that females were using to discriminate amongst males, so why are they doing this, why are they adjusting behavior? We're saying wait a second, this maybe isn't directed at females at all. Maybe it has another function."

To fully understand why the males would alter their mating calls for any reason other than attracting more ladies, further research is needed.