Mating with stressed males increases the fitness of ant queens

Abstract

Background: According to sexual conflict theory, males can increase their own fitness by transferring substances during copulation that increase the short-term fecundity of their mating partners at the cost of the future life expectancy and re-mating capability of the latter. In contrast, sexual cooperation is expected in social insects. Mating indeed positively affects life span and fecundity of young queens of the male-polymorphic ant Cardiocondyla obscurior, even though males neither provide nuptial gifts nor any other care but leave their mates immediately after copulation and die shortly thereafter.

Principal findings: Here, we show that mating with winged disperser males has a significantly stronger impact on life span and reproductive success of young queens of C. obscurior than mating with wingless fighter males.

Conclusions: Winged males are reared mostly under stressful environmental conditions, which force young queens to disperse and found their own societies independently. In contrast, queens that mate with wingless males under favourable conditions usually start reproducing in the safety of the established maternal nest. Our study suggests that males of C. obscurior have evolved mechanisms to posthumously assist young queens during colony founding under adverse ecological conditions.

Figure 1. Different modes of colony foundation of C. obscurior queens.

Under favourable conditions (left), wingless males (yellow male symbol) develop and mate with virgin queens (black female symbol), which start egg laying within the colony, assisted by workers (grey circles) and other queens (black female symbol). New colonies are founded by several individuals, which leave the mother colony and colonize new nest sites close by. Under stressful conditions (right), winged disperser males develop (brown male symbol) and mate with the virgin queens in and away from the maternal nest. Under such conditions, young queens leave the colony to colonize new nest sites in better environmental conditions and found either alone or together with other queens.

Figure 2. Lifespan of groups of Cardiocondyla queens.

Groups of queens, which all mated with winged males (right, brown symbols), lived on average significantly longer than groups of queens, which all mated with a wingless male (left, yellow symbols), independent of whether queens were reared under stressed (filled dots) or unstressed conditions (open rectangles). Black dots indicate censored data, e.g., groups of queens still alive at the end of the experiment.

Figure 3. Lifespan of Cardiocondyla queens kept with 20 workers each.

Unstressed queens that were assisted by workers lived on average significantly longer when mated with a winged male (brown) in comparison to queens which mated with a wingless male (yellow).

Figure 4. Electrophoresis pattern of the accessory gland extracts after silver staining.

No differences in the protein pattern of pairs of glands of winged males (winged) and ergatoid males (erg.) could be detected.