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
Contact: Andrew McLaughlin
University of Bath