Smaller Testicles Linked with Caring Fathers
Source: Smithsonian National Zoo, Jochen Lü
Tamarins and Marmosets
June 14, 2011 --
No matter the species, human or otherwise, it's tough being a parent. In honor of Fathers' Day, take a moment to get acquainted with some of the animal world's best dads. With a baby face like that, it's probably not hard for a father tamarin to love his offspring. Marmosets and tamarins, small monkeys in the subfamily Callitrichinae are native to the Amazon rainforests and are known for being good helpers for mothers. Females generally give birth twice a year to twins and it's the dad's job to care for the young while the mother recuperates. Tamarin fathers, such as the one pictured above, are known to carry their young around on their back and bring them to the mother for feeding.
Thomas Kokta/Getty Images
It’s a cold, cold world out there -- literally -- if you’re an emperor penguin chick hatching in the sub-zero temperatures in Antarctica. But these chicks have quite the devoted dad to keep them warm. Emperor penguins mate in the winter. The female lays an egg, rolls it onto dad’s feet, and then promptly takes off- leaving dad to care for the egg. Emperor penguin dads must fast for more than 100 days as they wait for their chicks to hatch. They endure extreme cold and wind, and have even been seen huddling in large groups doing "the wave" to keep warm. If mom doesn’t return in time for the hatch, the dad will produce a curd-like substance in his esophagus to feed the chick. Chicks are carried around on the parents' feet, as they depend on warmth from their parents to survive while they grow from their thin baby down to full feathers. Childcare is generally split between the two parents as they take turns heading out to sea to feed. Talk about a warm and loving dad.
Source: Smithsonian Zoo, Kevin Schafer/CORBI
Male rheas may not be loyal or devoted to one female, but they’re definitely loyal and devoted dads. Male rheas build a nest and then mate with multiple females that then lay all of their eggs together in the same nest -- sometimes amounting to more than 50 eggs. Dad is tasked with incubating and raising the young. The massive, flightless rhea is famously aggressive in guarding its brood and has even been known to attack female rheas that get too close to its young. Male rheas will keep their chicks close for about six months. It’s also common to see the chicks nesting and nuzzling into their dad’s feathers for warmth and protection.
Source: Project Seahorse, Lawson Wood/CORBIS
Seahorse dads might have it easier than most because of their unique appearance. They can camouflage with their surroundings; they have built-in pouches to carry their babies; and they even have eyes that can move independently of each other. (Remember all the times it seemed like your parents had eyes in the back of their heads?) But what makes seahorse dads some of the best in the ocean is the unique birthing process of the species. The dad is the one that takes on the burden of pregnancy. The female seahorse lays her eggs in the male’s pouch. He fertilizes them and then provides them with the nutrients they need to grow, in a womb-like fashion. Pregnancy lasts anywhere from two weeks to a month, depending on the water temperature: the hotter the water, the shorter the pregnancy. The birthing process generally takes place at night and lasts for several hours as dear old dad pushes out 100-200 babies, on average. While the young are on their own after that, we still think seahorse dads deserve to make this list for taking the wonders of childbirth off the hands -- er, tails -- of mom.
Source: San Diego Zoo, Barbra Leigh/CORBIS
While being grounded is every human kid’s nightmare, literally caging its young in a nest is part of what makes the greater hornbill such a great dad. Greater hornbills are native to Asia and prominently identifiable by their huge golden horn, called a casque. After mating with a female, the male hornbill will help the female enclose herself and their eggs in the hollow of a tree for safekeeping from predators. The male feeds the female through a small opening while she incubates the eggs. Generally, only one out of two eggs will hatch. Shortly after the chick hatches, the mother breaks her way out of the nest. But these parents aren’t done yet. With the help of the chick, they rebuild the “wall,” shutting him or her in. Mom and dad bring food by regularly, feeding the chick until its old enough to break out of the nest and live on its own.
Men with larger testicles tend to be less involved fathers than those with smaller testes, a new study suggests.
The findings, detailed today (Sept. 9) in the journal Proceedings of the National Academy of Sciences, are correlational, so they can't say exactly why the trend exists but only that there is a link.
But men who produce more sperm have bigger testes, and sperm production is extremely energy intensive for the body, so it may be that fathers "face a trade-off between investing energy in parenting and investing energy in mating effort," said study co-author James Rilling, an anthropologist at Emory University in Atlanta. [Sexy Swimmers: 7 Surprising Facts About Sperm]
Scores of studies have shown that children with involved and caring fathers tend to do better emotionally, socially and educationally.
So Rilling and his colleagues were interested in understanding what makes some men stellar dads and others AWOL.
A 2011 study in the Philippines suggested that men who have high testosterone levels are more likely to marry. Even so, those men who are eventually more involved in day-to-day child care duties — such as changing diapers, running the bath or kissing scraped boo-boos — see their testosterone levels drop more than men who remain aloof after having children.
But testosterone has many roles in the male body, so it wasn't clear whether the drop in the male hormone occurred because men were investing more in parenting than in mating.
Rilling and his colleagues surveyed 70 married men ages 21 to 55 who had between one and four children about their involvement in caregiving. Only four of the men routinely did more caregiving than the mothers.
The researchers then used magnetic resonance imaging to scan the brains of the fathers while they looked at images of their children. The team also scanned the participants' testes to assess volume and measured testosterone levels from blood samples.
Men's testes' volume varied considerably — from a little more than a tablespoon in volume to about a quarter cup.
Men with bigger testes had a more hands-off parenting style, and the reward centers of their brains activated less when the men viewed their children's pictures. These fathers also tended to have higher testosterone levels.
The findings are fascinating and provocative, said Sarah Hrdy, an anthropologist at Citrona Farms who was not involved in the study.
Steven Puetzer/Photographer's Choice/Getty Images
"For me, the take-home message is, there is this tremendous potential for nurture in human males, albeit a potential that in different cultural and social situations is not always being tapped," Hrdy told LiveScience.
But the study also raises a number of questions, added Hrdy, who is professor emerita at the University of California, Davis; associate in the Peabody Museum of Archaeology and Ethnology at Harvard; and A.D. White professor-at-large at Cornell University.
For instance, no one knows how testicular volume changes over time (although testes do tend to shrink with age).
In addition, scientists aren't sure whether men who make more sperm are genetically wired to be detached dads, or whether early life experience or the act of caring for children leads men's bodies to invest less in sperm-making, thereby causing their testicles to shrink, she said.
Another possibility is that the trend reveals a trade-off that varies with men's mating strategy, said Robert Martin, a biological anthropologist and author of "How We Do It: The Evolution and Future of Human Reproduction," (Basic Books, 2013).
"A man can either invest in looking after the child of one wife, or he can invest more in sperm production if he has several wives," Martin, who was not involved in the study, told LiveScience.
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