Why females care more, theoretically speaking

Using mathematical models, the researchers found that if the only initial difference between the sexes is the size of the sex cells they make (sperm by males and eggs by females), evolution does not favor females becoming more attentive parents.

"Although an egg is a much larger parental investment than a tiny sperm, there is no propensity for females to care more as a result," said Academy Research Fellow, Dr Lutz Fromhage, from the University of Jyväskylä. But he added, "There is, however, also no evolutionary force favouring equal care by both sexes." This new finding refutes earlier theories that concluded that equal care by both parents will evolve.

Although females tend to care more than males, there is much variation among species. In many fish, for example, only males guard eggs and defend babies, but in mammals females usually care alone. Dr Fromhage said the study, published in Nature Communications, would lead to a more solid theoretical foundation to understand how male and female parental care evolves.

So why do females provide more care? The researchers propose that another process is important: investment in being sexy, hence mating sooner, might trade-off with the ability to provide care efficiently. Taking this balancing act into account, evolution favors ever more care by the initially more-caring sex. Eventually this sex might end up caring alone. "One factor that could set the ball rolling is an inevitable difference in the certainty of parentage of males and females," said Prof Michael Jennions from the Australian National University, "with many more sperm than eggs, it is often hard for a male to be sure that he is the father. So males might initially care a little less."

Many researchers have put forward arguments to explain why females care more than males, but this new study provides formal confirmation based on solid maths.