Monkey Dads Gain Weight With Their Mates, Study Says

Nicholas Bakalar
for National Geographic News
February 6, 2006
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.

Maria B. C. Sousa is a professor of psychobiology at the Federal University of Rio Grande do Norte in Brazil.

Sousa, who was not involved in the study, suggests that the hormonal changes in male monkeys are a response to the behavior of females during gestation.

"Our studies using captive marmosets have shown that females have preferential access to food when they are pregnant, because of their aggressive behavior toward males," she said.

"The hormonal changes in males could be a consequence of the behavior of the females."

The latest findings, according to Sousa, "are very interesting when you put the data in the perspective of evolutionary biology. They probably indicate that these animals are adapted to be good fathers."

Packing on the Ounces

The researchers worked with a group of 29 tamarins and 29 marmosets. Eleven of the tamarins and 14 marmosets had pregnant mates.

The group also included most of the animals' mates: 11 pregnant tamarins and 9 pregnant marmosets.

As controls, the study used 6 male marmosets and 7 male tamarins without a pregnant mate or any offspring. The animals were weighed monthly.

Adult marmosets weigh on average a little under a pound (410 grams). The marmoset dads in the study gained between 2.4 and 20.4 percent of their regular weight over the five months of their mates' pregnancies.

The tamarins, starting at an average weight of 1.2 pounds (556 grams) added between 1 and 8 percent of their weight by the end of the study.

The control males gained no weight at all.

Heavy Babies

Male tamarins and marmosets need the extra weight, the researchers say. They participate fully in childrearing, carrying the baby around even more than the mother does.

For up to two months after birth, by which time the babies are about one-fifth the weight of their parents, the dads carry them practically everywhere they go.

When weaning begins, fathers share food with the infants, teaching them which foods are preferred. The cost in energy is high.

Although the researchers were unable to measure weight loss because of certain limitations of the experimental setup, Ziegler thinks the extra fat may start to disappear during this period.

"In tamarins, it has been reported that males lose weight while they're carrying infants," she said. "So they may be fattening up before the birth for what's going to happen after."

The researchers worked with captive monkeys, but Ziegler believes the effect may be even more marked in wild populations.

"The weight gain might be more in the wild," she said. "And the weight loss would probably be more, since they have to move around so much more in search of food as well as carrying these infants."

Ziegler sees the research as having significance beyond the behavior of monkeys.

"I'm interested in what makes a good dad," she said. "What is it that causes some males to have greater responses than others? What is it that motivates some fathers to be good parents and others to be not that involved?"

Important questions—for marmosets, tamarins, and humans.

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