Pregnancy is a tiring affair and as you reach full-term you could be forgiven for avoiding confrontation to conserve energy. However, picking your battles does not seem to be a priority for live-bearing female mosquitofish, Gambusia holbrooki: according to Frank Seebacher from the University of Sydney, Australia, they get more aggressive towards the end of their pregnancy (p. 771).

Female mosquitofish are naturally feisty, vying to become dominant over other fish in order to attract more suitors. However, this behaviour is energetically costly, especially when confrontations can escalate into high-speed chases with the two fish nipping at each other. Seebacher reasoned that given the amount of energy needed to fuel these aggressive encounters, females in the late stages of pregnancy would become more docile as most of their energy would be needed to support the increasing metabolic burden of their growing offspring.

Teaming up with Ashley Ward from the University of Sydney and Robbie Wilson from the University of Queensland, Australia, Seebacher set out to investigate his theory. The trio placed two female fish, one in the early stages of pregnancy and the other in the later stages, in a tank and recorded the outcome of these tense encounters. In contrast to the original theory, the team found that late-stage pregnant females were far more likely to chase their opponents than females that had only recently become pregnant. Consequently, the soon-to-be mums won over 77% of fights. However, this display of superiority did come at a cost, with females in the later stages of pregnancy squandering the remaining resources left over after supporting their unborn young.

Spending all their energy squabbling for dominance could be a risky strategy, as fighting mums-to-be may lack enough energy to defend themselves against predators or even forage for food. However, Seebacher explains that there may also be benefits to fighting. He points out that gaining dominance over other females may increase the victor's access to resources as well as preventing subordinate females from reproducing, thus giving the dominant mum's offspring a better chance of survival. So perhaps soon-to-be mosquitofish mums do know which battles to pick after all!

A. J. W.
R. S.
Increased aggression during pregnancy comes at a higher metabolic cost
J. Exp. Biol.