Bigger Is Not Always Better

Advertising

Size does matter but if you are an Anopheles gambiae mosquito bigger is not better. A recent study shows that the average-sized males get the most action.

The males gather in large swarms to attract females, and when one drops by they dart out and grab onto as many as they can. Kija Ng’Habi from the Ifakara Health Research and Development Centre in Tanzania and colleagues duplicated these mating swarms in cages where 30 males of varying sizes competed for the attentions of 10 females.

To the researchers’ surprise the average-sized males had sex a staggering six times as often as the biggest males, despite having lower energy reserves and smaller wingspans, and twice as often as the smaller ones.

The usual finding in animal studies is that being bigger means living longer and having sex more often. However, this study showed that although the largest mosquitoes did outlive their average-sized peers, they are unlikely to have had enough extra mating opportunities to make up for their failure to charm the females.

Lead researcher Heather Ferguson at the University of Glasgow, UK, says the study “addresses one of the central paradoxes in evolutionary biology” - why body size varies so much within a species if big is better. If traits that increase survival differ from those that maximise mating ability, natural selection may be unable to settle on one physical ideal.

Medium-sized male mosquitoes may be better at winning mates because they strike the optimal balance between agility and stamina. They have better technique and can maneuver better than larger individuals and can stay airborne for longer than small ones.

On the other hand, the clincher could be that average males were closer in size to females, a quality favoured by other species of arthropod.

The study has direct implications for malaria control strategies which release mosquitoes that are sterile or genetically modified to resist the malaria parasite. This approach relies on rearing males that can compete successfully for mates in natural populations.

“If you’re going to release GM mosquitoes or inundate them with sterile males, you need to know about their mating behaviour,” says Andrew Read, a malaria researcher from Penn State University.