‘Bigger not always better’: Sino-US study explains mating success of ‘Mr Average’, at least among tree frogs

Females of certain animal species prefer small- or medium -sized mates to more physically intimidating or “alpha” males, according to a joint study by scientists in China and the United States.

The study was conducted on frogs but the findings could apply to other animals and even humans, experts said.

It was led by Professor Cui Jianguo at the Chengdu Institute of Biology in southwestern Sichuan province and undertaken in conjunction with the University of Maryland in the United States. The institute falls under the auspices of the Chinese Academy of Sciences.

“Bigger is not always better … females have apparently evolved to prefer mates of an average body size,” Cui and colleagues reported in their paper, published recently in the peer-reviewed journal Plos One.

They based their findings on a series of experiments on small serrate-legged tree frogs that are commonly found in the moist shrubland and freshwater marshes of China’s Hainan, the largest tropical island in the South China Sea. The frog’s scientific name is Philautus odontotarsus.

The female of the species grows to a length of 43mm but males only reach 33mm.

According to natural selection, only the fittest - and often strongest - survive. As such, scientists were curious why the frogs never outgrew their female counterparts.

Moreover, the laws of natural explanation fail to explain why males of many species are not stronger or bigger than they currently are.

If females are biologically programmed to choose the strongest mate, then males carrying these genes should theoretically keep on replicating, causing the males to continue growing over successive generations without limit.

Biologists have come up with various theories to explain this in recent decades - one theory posits that the extra weight would make them sluggish and more vulnerable to predators, another contests that giant males produce lower-quality sperm - but a definitive answer remains lacking.

The Sino-US team discovered that the male tree frog attracted the attention of females by croaking at various frequencies. The larger the frog, the lower the dominant frequency of their calls.

The researchers decided to study the females’ bias for mates of a certain size by conducting repeat experiments at Mountain Diaoluo National Nature Reserve in Hainan.

They entered the reserve with a sound-attenuating chamber and a pair of portable field speakers. Over 100 female frogs were caught and placed in the chamber so their responses to the male’s croaks could be observed and measured.

The team found that their subjects were more likely to approach the field speakers when they heard sounds at an average frequency, which would in real life have been produced by average-sized males.

There was significantly less interest in the higher and lower frequencies, produced by callers big and small, respectively, they said.

It was assumed that the frogs - either consciously or unconsciously - were familiar enough with the males of their species to be able to identify the mating calls with the size of the caller.

The researchers also found that larger females preferred smaller males on an inversely proportionate scale: the bigger the females got, the smaller they liked the bodies of their mates to be.

Curiously, the reverse also proved true - small females opted for large males - but to a lesser degree.

When it came to mating season, the barrel-chested males proved the losers.

“One possible explanation for these preferences is that such choices tend to increase … the likelihood that male offspring will be of intermediate body size since body mass is hereditary,” the paper reported.

To achieve this goal, “large females would be expected to mate with small rather than large males, while the small females should avoid mating with small males”.

Zhou Hongzhang, an evolutionary biologist with the Institute of Zoology in Beijing, said the same rules may also apply to humans.

“Big men are rarely regarded as sexy, but shorter men are,” said Zhou.

“Handsome male celebrities of a medium size also tend to have more female fans,” he added.

However, these rules clearly don’t apply across the board. Big cats like lions that need to defend their territory physically are unlikely to see the merits of mating with the runt of the litter.

Cui’s team speculated that the female tree frog’s preference for “Mr Average” was partly due to a unique sexual behaviour among certain amphibians and crabs known as amplexus (the Latin word for “embrace”).

This sees the male use his front legs to grasp a female, who carries him on her back during the mating process. She must then take him to a suitable breeding ground where she can lay her eggs, which can be up to 30 metres away.