Most mammal dads wouldn’t exactly win the “Best Father of the Year” award.
Engaged fathers—those who care for their offspring or bring home the bacon so their female mates can focus on childcare—are present in only about 10 percent of mammal species. But for the rare few who do stick around, the rewards can be myriad: new research finds that parenting efforts pay big dividends for offspring and mates alike. Stay-at-home dads appear to boost reproductive success among their mates by enabling them to breed more frequently and produce larger litters, according to a study published this week in the journal Nature Communications.
Previous studies have sought to explain why the males of certain species would choose to buck nature's trend and focus on the family, rather than running off in search of their next fertile mate. After all, caring for young costs these dads time, energy and the opportunity to father offspring with other mates—a drive so strong it leads some male mammals to kill infants of their own species in order to mate with their mothers. So what, exactly, is in it for these family-loving fathers?
Biologist Isabella Capellini at the University of Hull examined the question from a different angle, trying to parse out how the families of males who invested their time and energy were impacted. “We realized there was a gap in the knowledge about what happens when the male remains and helps to raise the offspring. So our question was, 'do the female and the offspring gain any advantages?'” To find out, Capellini and her co-author Hannah West examined a wide-ranging data set of some 529 different mammal species (humans excluded) from rodents to primates, then focused on the 10 percent they found with male care behaviors.
They found that these dads do indeed deliver clear advantages to their families. “When you have male parental care, you have shorter periods of lactation, and you have larger litters if the male provisions the female,” she says. “Both of those things ultimately have consequences for the overall fecundity of the female, and in turn their male partner, because the female can have larger litters or reproduce more often. Therefore the males who are sticking around also have more opportunity to mate with that female, and that compensates for their costs in caring for the young.”
The species with such model fathers aren't necessarily the most cuddly or charismatic of mammals. They include several canid species like wolves, coyotes, and African wild dogs, whose dads invest in feeding and teaching their offspring hunting skills. In several rodent species, males remain with females in the nest until their young have grown, huddling with and grooming their offspring. Among some New World monkeys, like tamarins, fathers often physically carry their offspring around during their first months of life.
The well-cared-for offspring of such dads don't grow bigger, but they do grow faster when compared to species without male help. The study doesn't have enough age-dependent mortality data to say for certain, but it's likely that those faster growth rates increase the odds that young mammals will survive by shortening the window of time when they are most vulnerable to predators or unable to find their own food. “We do know from a lot of other studies, in wolves for example, that when the offspring are a bit bigger prior to winter, they tend to survive the winter better,” Capellini notes.
Hannah West, a PhD candidate with the University of Hull's evolutionary comparative ecology group and a co-author of the study, says that a big key to this relationship is another relative rarity among mammals: monogamy. Monogamous species that exhibit male care “tend to mate with the same female multiple times,” she says, helping the female reproduce more quickly. “Over the course of their lifespan a monogamous pair where the male cares can have more offspring in total.”
Of course, reproductive strategies vary widely across the animal kingdom. One study suggests that among fish, birds, insects and amphibians, deadbeat dads breed bigger, stronger babies in fewer numbers. That’s because moms might have more and smaller babies when they know that the dads will be around to help them survive, the authors suggest.
Dieter Lukas, a behavioral ecologist at the University of Cambridge who wasn't involved in the research, suggests that some paternal behaviors might not boost reproduction, but may simply be more mutually beneficial. “Huddling, defending the group against other individuals or against predators, or hunting together [are cooperative behaviors] that individuals participate in because they provide benefits both to others and to themselves,” he says. “So these behaviors have a mutualistic value. Group huddling makes each participant warmer.”
“It is interesting to see in this study that only the truly costly behaviors [like] giving food away or carrying heavy offspring, led to changes in female reproduction,” Lukas adds.
By these behaviors, attentive dads direct their energies toward allowing moms to build up additional resources, which they channel into faster rates of reproduction. But it appears that moms retain the right to hedge their bets if those fathers somehow turn fickle: “The change that the authors observed in the pattern of female reproduction is mainly in terms of shorter lactation,” Lukas notes. “This is a flexible trait, so that if the male were not to be around, if he deserts her or dies, the mother would still be able to raise the offspring.”