The difference in size between males and females of the same species is all down to the battle for a mate, according to a study of shorebirds published by British scientists.
The findings, published in the Proceedings of the National Academy of Sciences, are the first explanation for a rule identified over forty years ago by German scientist Bernhard Rensch.
Rensch’s rule, as it has become known, says that the ratio between the sizes of the sexes is related to body size with very few exceptions throughout the animal kingdom - for example, male gorillas are much bigger than female gorillas, whereas male rats are only slightly larger than female rats.
In this new research, scientists from the Universities of Bath, Oxford and East Anglia, carried out complex statistical analyses of the mating behaviour, body size and ecology of more than 100 different shorebird species from around the world.
They found that in larger shorebirds the battle between males for a mate is highly competitive and larger size offers an advantage over other potential suitors as they battle aggressively on the ground. The evolutionary result is that male Ruffs, a large shorebird, are about twice the size of females.
For smaller species, such as Dunlins, battles take place in the air and agility and smallness become more important factors. The result is that Dunlin males are smaller than the females.
Although biologists thought that sexual selection must be playing a role in driving Rensch’s rule, until now nobody had been able to prove exactly how it worked. Although the research was conducted on shorebirds, the researchers are confident that that the same driver is in operation in everything from mites to primates throughout the animal kingdom.
Dr Tamas Szekely from the University of Bath who led the project, said: “We have known about Rensch’s rule for many years but didn’t have a clue what drives it. Many biologists had considered sexual selection to be the most likely answer, but nobody really knew how.”
“Once we started analysing the data the story shaped up perfectly. I believe that sexual selection is the driver behind Rensch’s rule throughout the animal kingdom and we are already finding this is the case in other bird species we are studying such as Bustards,” said Szekely. “There’s no reason why it would not apply in many other animals from gorillas living on the rainforest floor to monkeys living high up in the canopy too.”
Shorebirds, gulls and auks are the ideal group to test theories explaining Rensch’s rule as the body mass of male shorebirds ranges from 59 per cent to 169 per cent of female body mass, encompassing nearly the entire range exhibited by the world’s 9,700 species of birds. Shorebirds also encompass the full range of mating systems including social polygyny (males have more than one female at a time), social monogamy (one partner at a time), and polyandry (females have more than one male at a time). The researchers collected data on body size, mating behaviour, ecology and life histories from existing literature and analysed them with a suite of statistical tools.
The researchers also found that where there is high competition between males, the difference in the ratio between the sizes of the sexes increases, explaining the changing ratios identified by Rensch. In contrast, in species with low intensity of male-male competition, females tend to be the larger of the sexes.