Out there in the big bad natural world, where kids' strength and size change their likelihood of getting eaten, the strength and dedication of dads can affect how many babies a mom will have in her clutch and how big those babies will be.
In nature, there are two broad strategies to reproduction: have one (or, at most, a few) large babies or have a whole lot of little ones. These reproductive strategies are set across species lines. Humans usually have one to a few babies, while many fish will lay hundreds of eggs at a time. But even within a species, there's a bit of variability: broods can be bigger or smaller, the babies stronger or weaker.
According to a new study by Holly Kindsvater and Suzanne Alonzo, who looked at fish, insect, amphibian and bird reproduction, the balance of baby size and baby number can depend on how engaged the dad seems to be.
What the scientists found is that females do a little bit of internal calculus to figure out how helpful their mate seems to be. If the dad is a good dad, bringing food and willing to help with the kids, she'll have more, smaller babies. If dad is a deadbeat, she'll have a few big strong ones. The Canadian Press:
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."
According to the scientists in their study, though, the effects on baby size and number are actually a little more complex than this. They say that the effects depend on how, exactly, the dads can help:
If the male's behavior increases the babies' survival—by defending them, for example—then the female will have fewer, stronger babies. If you have a lot of babies, losing one of them is less devastating. If dad is there to protect them, though, you can put more effort into each and focus on just a few.
If the male increases the babies' growth rate, say, by bringing home food, then the above relationship would hold with the female having more, smaller babies. Smaller animals can't as easily find food for themselves, but if the male is taking care of that, it could be worth it to have more babies.
This finding bucks the assumed trend, that everyone would always want to have the biggest, strongest kids possible. But, when given the opportunity of a supportive partner, the lady fish will relax a little bit, knowing the male will be there to pick up the slack.