Men in many cultures sometimes experience "sympathy pains"—signs of pregnancy, such as body aches, nausea, and altered food preferences—when their partner is expecting.
New research demonstrates that marmoset and tamarin monkey dads do something very similar. As their mates' pregnancies progress, they start to gain weight.
Tamarins and marmosets, both small monkeys native to Brazil, are monogamous, according to Toni Ziegler, the study's lead author. Tamarins are particularly devoted, while marmosets are slightly less so.
"Some of [the marmosets] are high responders," Ziegler said, "and others, well, it's, I'll take the baby if you want me to. They're more like humans in that sense."
Ziegler and her colleagues at the Wisconsin National Primate Research Center in Madison found that monkey fathers gain, on average, about 10 percent of their body weight by the time their mates give birth.
Their findings appear in the January 31 issue of Biology Letters.
Adaptive Weight Gain
"The weight gain is an adaptive mechanism," Ziegler explained. "These males that are gaining weight are picking up some type of signaling from the female."
What's more, other physiological changes are occurring at the same time.
"During the last half of pregnancy, [the female] puts off high levels of cortisol"—a stress-fighting hormone—"due to the growth in the fetus's adrenal gland," Ziegler said.
"Right after she shows that increase, the male has a spike in his cortisol level too."
This hormonal change, Ziegler believes, makes the male more responsive to infants.
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