Picky eaters

Natural History, April, 2005 by Stephan Reebs

Vegetarians must balance their diet, because few plants can supply all the essential nutrients. The herbivores of the animal world do a balancing act as well, seeming to know instinctively what to eat, and in what proportions. Carnivores, however, should be unconcerned about balancing their diet, because most parts of an animal's body provide a fairly complete set of nutrients. Yet a new study shows that some invertebrate carnivores choose their prey carefully, day by day.

David Mayntz, a zoologist at the University of Oxford, and several colleagues studied three species of carnivore--the mobile ground beetle (which can select what's worth chasing), the "sit and wait" wolf spider (which can choose where to wait in ambush, but must content itself with the traffic of the hour), and the web-building desert spider (which has no control over what arrives at its web of the week). All the animals received nutritionally unbalanced meals for one or two days. Some beetles got a powder rich in lipids; the rest got a powder rich in proteins. Some spiders got fat (lipid-rich) live fruit flies; the rest got lean (protein-rich) ones. Then the beetles were given a choice of both types of prey, the wolf spiders were given only one prey type but could choose how much to eat, and the desert spiders were given half an hour to deal with one prey type as they saw fit.

The beetles and the wolf spiders ate more of the type they'd been deprived of. The desert spiders did something even more interesting: they compensated for their last meal's imbalance by adjusting the amount of nitrogen (protein) and carbon (lipids) they extracted from the flies they ate for lunch. (Science 307:111-13, 2005)