Cockroaches may be tiny enough to slip through the smallest of cracks, but just like humans, these eternal pests can get fat on an unhealthy diet.

As part of a decade's worth of research on cockroaches, Patricia Moore of the University of Exeter studied how female cockroaches change their mating behavior in response to their diet, specifically what they eat when they are young.

"We already knew that what they eat as adults influences reproductive decisions," Moore said. But just how the food they consumed early in life shaped these decisions wasn't known.

To find out, Moore and her colleagues picked young female cockroach nymphs and divided them into two dietary groups. Half were fed a good-quality balanced diet of protein-rich fish food and high-carbohydrate oatmeal, while the rest were raised on fish food only.

Both groups were allowed to eat as much as they wanted. The difference in diets "was not quantity but variety," Moore said.

After the last molt, when the nymphs became adults, the team switched the diets of some animals. Half of the cockroaches raised with good quality diet lost their oatmeal, while half of the bugs fed poorly were promoted to a good-quality diet.

Eighteen days after the switch, the diet control ended and some of the surviving cockroaches were dissected. The rest were allowed to live on and reproduce.

The results: While the lifespan of the members of both groups was about the same, the cockroaches on the poor diet were fatter and took longer to mature.

Moore suggests that the poorly fed bugs were storing up excess fat at the expense of their growth in case their dietary options got even worse.

"This was a surprising result," Moore said, "but it shows the importance of a balanced diet for healthy development."

The effects of unbalanced meals continued throughout the cockroaches' lives, even for the few that were switched to good-quality food.

Females that ate a poor-quality diet were less willing to mate and less likely to produce offspring. They were also more picky and spent more time considering possible mates.

The findings, detailed in the June 24 issue of the journal Proceedings of the Royal Society B, found, that "poor diets [during early life] have an effect on the way cockroaches respond to their environment and cannot be reset later on," Moore said.