Little engines that could

Natural History, July-August, 2003 by Stephan Reebs

It's hardly news that for many species, raising offspring takes a lot of energy. Add to that the wide variations among individuals, and you might well ask, What accounts for differences in energy, and do they affect reproductive success?

Consider an extreme case: the European beewolf, a species of wasp. To provision her young, the female stings and paralyzes honeybees that each weigh more than she does. Then she lofts the bees one at a time back to her nest-in-progress. During her lifetime a laid-back mama beewolf might hunt down five bees and lay five eggs, giving each of her offspring just one bee to see it through the larval stage. But a supermom might hunt four fat bees for each of as many as thirty-four eggs, more than twenty times the workload of her laid-back counterpart.

The biologists Erhard Strohm of the University of Wurzburg and Wiltrud Daniels of the University of Bayreuth, both in Germany, decided that the beewolf was just the critter they needed to prove a direct connection between reproductive success and the ultimate source of animal energy: the mitochondria within every working cell. Specifically, they looked at the folded membranes on the insides of the mitochondria in the beewolves' flight muscles. Why? The denser the mitochondria's inner membranes, the faster the production of energy.

Strohm and Daniels orchestrated mating, breeding, and honeybee-hunting opportunities for a group of female beewolves. Each wasp was then killed, its weight and fat reserves (good determinants of reproductive success in other species) were measured, its age recorded, and the mitochondrial density and mitochondrial-membrane density of its flight muscles examined at high magnification.

The only factor that correlated with the beewolves' rate of bee killing was the membrane density. And the more bees a developing larva had to munch on, the more likely it was to survive. In beewolves, at least, being a supermom pays off. ("Ultrastructure meets reproductive success: Performance of a sphecid wasp is correlated with the fine structure of the flight-muscle mitochondria," Proceedings of the Royal Society of London B 270:749-54, April 7, 2003)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).