Primate Friendships, human mating systems and Bipedalism

There are “pleasure” chemicals in the brains of old human couples (e.g., oxytocin) that are released when they are interacting and feeling happy together that are not the chemicals associated with sex. Our endocrine system wasn’t built overnight.

Literature on primate behaviour in general and human evolution in particular has got me thinking about this male infanticide issue had how it has distorted our view of human evolution. It seems that virtually all the papers for the last 30 years on how and why humans became bipedal and upright assume that males are brutish thugs who only want sex from the females and whose closed relationship is to exchange food for it, and to combat other males over it.

When I began researching a leaf monkey, Trachypithecus cristatus, I read in a secondary reference book that, when a male from outside the troop deposes the troop's alpha male, "He immediately kills all the infants" to make sure that the progeny of the troop are all his, and to bring the lactating females into estrus so he can mate with them. Yet when I dug deeper into the literature, I found that male infanticide has never been observed in this species. It was based on a single 1979 study in which male infanticide had been inferred because all of the infants had disappeared when checked three months later. It was inferred because of a theory, then sweeping the wildlife behaviour literature, that males kill infants not their own to increase the likelihood of their own progeny being born and surviving: the "male infanticide" hypothesis.

I've just now got a stack of 20- and 30-year old primate books from the library, and in one of the chapters about a Trachypithecus species, the two males in a troop (as well as the females) run to grab up the infants for safety when danger threatens. In my own species (the subject of three papers, one published, one in press and one in prep.) male infanticide has almost never been observed and where it occurs is infrequent--perhaps once every several years, in a well-studied group of Trachypithecus leucocephalus, as reported to me by the author of a detailed study. This is contrary to the male infanticide paradigm.

On the contrary, in my own studies on T. cristatus and other studies on at least two other species of this genus, males care for infants and the most aggressive they get is to bark or swat at a juvenile who is pestering them.

In fact, recent reviews of the issue have cast doubt on the extent (number of species) to which male infanticide applies, and the evolutionary significance when it does.

The over-emphasis of male infanticide hypothesis, and the flip side, the females' "infanticide avoidance hypothesis", has worked its way into ideas about homonid evolution. I just can't reconcile seeing Homo sapiens males love and care for their own and other juveniles (how many dads coach their kids' hockey or basketball teams, or take their daughters to ballet? Almost every one I know), and have deep relationships with their wives that go far beyond sex, with this brutish Home erectus / early Homo sapiens paradigm of "food for sex" and "kill all the infants who are not mine" paradigm. I think we’ve been too influenced by baboons, and not enough by gibbons. Somehow we got to be upright, monogamous in complex social systems, and large-brained. I think the archaeologists agree now that upright/bipedal came first and the large-brain/complex social systems came later. But my feeling –nascent though it is—is that we really have to overthrow this male infanticide/food for sex paradigm before we can begin to explore how human relationships developed, and even to understand what they are.

While searching literature on this yesterday, I came across a paper (Niemitz, C. 2010. The evolution of the upright posture and gait—a review and a new synthesis. Naturwissenschaften 97(3): 241-263) that synthesized all the theories about how hominids began walking upright. The author had discounted the hypothesis that the need to carry infants was involved in hominids first walking upright.

I think it is wrong to reject infant carrying as a force in bipedalism, which always assumes that women were doing the carrying and they did it with the infant on their hips or in their arms. But consider:

1. Humans are the only primates who carry infants on our shoulders. The infant grabs the head. Human neonates’ hands are far too weak to securely grasp hair or any other part of the body, but they automatically and effectively grasp the head of the carrying adult.

2. Go to any park and you’ll see families with the dad carrying a toddler on his shoulders, perhaps leading a second by the hand, while the mom cradles an infant on her hip or in a snuggly, perhaps leading a second.

3. Humans vertebral columns can carry a heavy load for an extraordinary distance (many km) and we like doing it so much that, even when not forced to, we do it for fun-go to any national park with wilderness and you’ll see backpackers hiking long distances. Why did we need such strong backs if we carry infants, weapons, food etc. in our hands and arms?

4. ...but we can’t carry even small loads with a slight stoop. Lumbar pain would have been a powerful motivation to stand straighter when carrying.

5. Home erectus could not have walked all the way to China without a way to transport juveniles too large (or too many) for the mom to carry and/or lead, but too small to keep up. Don’t forget that neoteny was progressing: the kids were getting heavier just when our predecessors were roaming farther.

The above has obvious implications for grouping behaviour and mating systems, which in turn are integral to brain size. it’s a package. It is time to reconsider the evolution of bipedalism in the light of research that shows primate males to be more caring and less homicidal of infants. Maybe they got more from their mates than sex; maybe they gave more than food. It is time to conder this along with development of monogamy, societal grouping and dispersal.

Key papers consulted in this essay:

Hrdy, S.B. 1974. Male-male competition and infanticide among the langurs (Presbytis entellus) of Abu, Rajasthan. Folia Primatologica 22: 19-58.

Brotoisworo, E. 1979. The Lutung (Presbytis [Trachypithecus] cristata) in Pangandaran Nature Reserve: social. adaptation to space. Kyoto University, Kyoto.

Van Schaik, C.P., and Kappeler, P. 1997. Infanticide risk and the evolution of male-female association in primates. Proceedings of the Royal Society B: Biological Sciences 264(1388): 1687.

Borries, C., Launhard, K., Epplen, C., Epplen, J.T., and Winkler, a.P. 1999. DNA analyses support the hypothesis that infanticide is adaptive in langur monkeys. Proc. R. Soc. Lond. B 266: 901-904.

Palombit, R.A., Cheney, D.L., Fischer, J., Johnson, S., Rendall, D., Seyfarth, R.M., and Silk, J.B. 2000. Chapter 6 Male infanticide and defense of infants in chacma baboons. In Infanticide by males and its implications. Edited by Carel van Schaik, and Charles Helmar Janson. Cambridge University Press, Cambridge. pp. 123–152.

Zhao, Q., and Pan, W. 2006. Male-immature interactions seem to depend on group composition in white-headed langur (Trachypithecus leucocephalus). Acta Ethol 9: 91-94

Zhao, Q., Tan, C.L., and Pan, W. 2008. Weaning age, infant care, and behavioral development in Trachypithecus leucocephalus International Journal of Primatology 29(3): 583-591.