Confirming what many have long suspected, scientists have found that male monkeys of two different species get heavier when their mates are pregnant.
The roughly 10 percent gain in male girth occurs in common marmosets and cotton-top tamarins, both squirrel-sized primates known for their monogamous lifestyles and devotion to good parenting.
Since marmoset and tamarin dads are heavily involved in infant care, they may be stocking up on pounds during pregnancy in preparation for the rigors of fatherhood, says Toni Ziegler, an endocrinologist at the University of Wisconsin-Madison's National Primate Research Center. Ziegler and her colleagues reported their findings today in the journal Biology Letters.
The knowledge that expecting primate fathers also experience biological changes can help scientists better understand what governs human fathering behavior, Ziegler adds. "We're interested in what motivates dads to be good parents because there are so many men who just aren't good fathers. This work could help to tease apart what makes a good dad."
In the last few decades, scientists have noted weight gain and other symptoms of pregnancy in human men too, but the phenomenon has never been systematically studied. Known as the "couvades" effect-from the French word meaning "to incubate or hatch" - researchers have generally explained sympathetic pregnancy symptoms in men as entirely psychosomatic events.
But the UW-Madison work helps "to realize that this phenomena that so many people know about, is actually real with a possible evolutionary purpose behind it," says co-author Shelley Prudom, a research specialist at the UW-Madison Primate Center. The scientists took monthly weight measurements for 29 common marmosets and 29 cotton top tamarins, of which 14 marmoset males and 11 tamarin males were expecting new offspring. Marmosets gestate their young for five months while tamarins normally gestate for six.
"The males somehow cue in to the cascade of hormonal changes going on in their pregnant mates," says Ziegler. That cue triggers changes in their own reproductive hormones. Rising levels of the lactation-inducing hormone prolactin, for instance, most likely cause the weight gain in expecting male primates. Levels of estrogen and testosterone also rise higher.