The guys were all stressed out. There were new infants in the community, and the guys knew from experience that that’s when invaders were likely to come and kill the babies, particularly the male infants.

This annual threat was a defining moment in their lives — it had more impact on everyone than the daily social struggle to be on top, or than any other community crisis, like defending the group against hostile neighbors. Nothing was more stress-inducing than having helpless infants around to protect from marauders.

This drama is wrenching to observe, yet until fairly recently, no one knew it was happening. But then again, it is hard to get inside the head of a male sifaka, a large Madagascan prosimian primate which lives mainly high and unseen in the forest treetops. Sifaka have cryptic faces, devoid of the telling facial expressions of more advanced primates, like baboons, chimps or humans. At first glance, sifaka look thoughtless and simple, but the capacity for complex social drama is there. We can tell from their poop.

A finding published in the current issue of the Proceedings of the Royal Society B details how primatologists have found conclusive evidence of an annual, population-wide increase in anxiety and stress among male sifaka concurrent with birthing. Authors Diane K. Brockman of the department of anthropology at the University of North Carolina at Charlotte and Amy K. Cobden and Patricia L. Whitten of the department of anthropology at Emory University show that a significant rise in stress-related glucocorticoid hormones in male sifaka feces occurs annually and “reflects specific events related to reproduction rather than states or social context during the birth season.”

These results, combined with recent evidence of male infanticide (largely directed towards male infants) suggest a more complicated social dynamic among the prosimians than primatologists traditionally believed to exist.

Verreaux’s sifaka (Propithecus verreauxi), like other lemurs, are primates that are only found on the island of Madagascar and have long been considered to be generally more primitive than monkeys and apes. Sifaka, like many non-primate mammals, have clear annual estrous cycles where the females are only receptive for mating for a brief period once a year, and all the resulting infants are weaned by the following mating season.

However, the primitive appearance of the sifaka may be masking a more sophisticated social animal. Brockman and colleagues have been studying a large population at Madagascar’s Beza Mahfaly Special Reserve for two decades, where they have accumulated a substantial amount of data that points to more advanced social behavior. “Prior to 1994, previous studies of wild populations of sifaka by Alison Richard and colleagues, supported the idea that sifaka males have little or no interest in newborns and they do not commit infanticide,” Brockman notes.

“In 1994, we had our first incidence of a male at our study site invading a neighboring group, expelling the group’s resident males, and mortally wounding an infant and likely killing a second infant,” she said. “This new revelation fundamentally altered our perception of male social complexity, particularly the potential reproductive tactics males might employ during the birth season.”

morevia sciencenews