The study, published Tuesday in the journal Proceedings of the Royal Society B: Biological Sciences, uses a math model similar to one that economists apply to analyse investment strategies to suggest the offspring question depends on the animal's partner.
Author Holly Kindsvater says the research has shown that if males are helping to raise the young, it's better to have many babies of a smaller size. But if the male is a lousy mate, then the female is better off having just a few large babies.
In nature — and as is observed in many school playgrounds — offspring that are born bigger usually have a greater chance of survival because they can compete better against others, says Kindsvater.
So in most cases in nature, a female animal with a desirable, robust mate that helps look after the young would give birth to a few large babies in order to pass on the good genes, the report says.
The assumption is "females should just make large babies when she has a really good mate because those babies are going to be so much more successful," says Kindsvater, a research scientist with Simon Fraser University's biological sciences department.
But she adds that researchers have observed female birds and fish producing large offspring after mating with a "bad" male — that is, one that slacks on parental care.
On the other hand, there are also females that mate with "good" males — those that bring back food and tend the nest — and then give birth to a bunch of smaller babies.
"That was very confusing for people," says Kindsvater. "In a way, it's saying you should invest more in the crappy stock, and it doesn't necessarily make sense."
The confusion cleared up once Kindsvater examined the reproductive patterns using the mathematical model similar to ones used by economists to determine investment strategies.
"We basically recognized that this ultimately is an investment problem and it's going to depend on the male contributions to offspring fitness," she said.
"Once you have insight that females need to invest according to how the males are going to improve her fitness or the offspring's fitness, you can kind of make sense of some of this variation."
Kindsvater's theory is that some fish and birds have evolved in such a way that reproductive efforts have become based on whether a female can maximize her "returns on investment."
Kindsvater says in the animal world, giving birth to many smaller babies takes less effort than giving birth to just a few large babies.
"Our model showed that when males are actually helping offspring grow faster, females can get away with investing less," she said.
"They can make smaller babies with these good males because these good males will help these babies do just as well."
And if a male is not going to stick around to take care of the offspring, it makes more sense for a female to "invest in" only a few big babies, since they would have a greater chance of surviving without the care of a father, she explains.
The study suggests that a female may not always get the mate that she wants — that is a fact of life, says Kindsvater.
But she can at least make the most of the situation by making sure her offspring prosper.
"What we previously might have thought was a mistake could actually still be the best reproductive strategy for that female," Kindsvater said.
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