Some trees, including those with seeds favored by red squirrels, have years of very high seed yields, followed by years of lower seed production, creating what Stan Boutin, a biologist at the University of Alberta, calls a "swamp and starve" strategy. Usually, the size of a predator population depends on how much food is available. When resources are plentiful, they reproduce more. The problem is that the food supply may dwindle by the time their young are born.
To determine if increased food triggers the second litter, Boutin and his colleagues offered part of the North American red squirrel study population extra food, but there was no change in the number or size of litters compared with the population that did not receive more food. "It wasn't just resource tracking," Boutin notes. "Something else was going on."
By monitoring red squirrels and seeds in Canada's Yukon Territory, Belgium and Italy, the researchers report in this week's Science that they determined the animals can somehow predict which years will be boom years for seeds. They are not sure what cues the squirrels to have more young but they speculate that the squirrels may be eating buds off the trees the preceding fall and that the extra plant hormones from them may trigger the extra litter born the next summer.
"It's a fascinating twist on the life histories of mammals," says Tim Karels, a terrestrial biologist at California State University, Northridge, who was not involved in the study.
The researchers also noted that North American squirrels did something their European cousins did not. The New World females became pregnant before they were done nursing the first litter. In most mammals, ovulation is suppressed during lactation. Boutin suspects this block is circumvented by a break in lactation, but acknowledges that eggs fertilized during the late winter could be stored within the females' bodies and to be implanted later in their uteruses to develop.
If the cue that initiates increased reproduction is isolated, Karels says, then it could eventually be used in agriculture to prod other mammals into having more offspring.