Today, like any other, the age-old battle of the sexes continues unabated among the water striders, writes Dick Ahlstrom
It may be St Valentine's Day but you can forget your chocs and flowers if you are a water strider. Males and females of this insect species are locked in an "arms race" when it comes to love, each evolving new weapons to dominate the other in a battle for sexual supremacy.
New young water striders do keep appearing, which means the species isn't under threat, but not before their parents engage in a violent pre- mating wrestling match. The process certainly has nothing to do with affection and goes by the name "sexually antagonistic co-evolution". It is a sexual duel, with one sex gaining the upper hand for a time before evolutionary pressures bring about new adaptations that allow the other sex to take control several generations later.
This remarkable battleground is described by Dr Göran Arnqvist of the University of Uppsala, Sweden and Dr Locke Rowe of the University of Toronto, Canada this morning in the journal, Nature. They relate how the balance of power continually shifts from one insect to the other in a process that one might assume should lead to an evolutionary standstill.
Evolutionary conflict of interest between the sexes is "ubiquitous" and is "a central process of evolution", the researchers write. Such a powerful force should lead to physical changes in a species that give it advantages in the battle for survival.
The researchers looked at 15 species of the water strider, Gerris lacustris, which undertakes a battle royal before mating. The males force themselves on the females, who in turn "try to dislodge harassing males to avoid superfluous and costly mating".
The scientific assumption was that where mating was an antagonistic process, there was potential for an arms race. The result would be evolutionary changes associated with the "weapons" used by one against the other. In this scenario, natural selection should favour any "technological" advantage that would allow one side to win the fight.
Because the battle continues all the time, it is difficult to read the consequences, so the researchers took a different route. They looked at the effects of "small imbalances" that emerged when one side or the other held sway.
Water strider males want to mate all the time - what a surprise - because it improves their chances of passing on their genes. Females that have mated, meanwhile, don't need or want the attentions of rival suitors and so they fight them off for all they are worth. High rates of mating would indicate that "the advantage has shifted towards males", the researchers said. "The converse would be expected for species in which the advantage has shifted toward females."
The physical characteristics that undergo alteration include "clasping and anti-clasping morphologies". Males evolve a flatter abdomen to get closer and hold tighter, while females counter with a longer abdomen to make it more difficult for the male to take hold.
THE researchers looked at four traits: the duration of pre-mating struggles; male struggle success; female mating activity, and female mating rate. They found that success in the arms race indeed had a strong influence on the outcome of sexual interactions.
"As females evolve a relative advantage in arms over males, they are able to dislodge harassing males more rapidly and more successfully in pre-mating struggles. Consequently, females evolve to mate less frequently and spend less time in superfluous matings." When the balance of power shifts towards the males, mating rates increase.
This may carry echoes for human mating practices for much of the year, but the differences between the sexes get somewhat blurred on St Valentine's Day. Maybe it's because of all that chocolate.