While reading Why Zebras Don’t Get Ulcers by Robert M.
Sapolsky, I came across an interesting footnote regarding monogamous mammals.
Sapolsky writes that humans probably shouldn’t be included among the ranks of monogamous
species (33). This assertion surprised me at first. To say that humans are not
meant to associate primarily with one partner goes against what I, and many
living in Western societies, would define as the normal way to reproduce.
Sapolsky does not elaborate on this issue, as it is clearly a complex one;
however, there has been significant research done on the subject.
Homo sapiens are not the only mammals that
display monogamous behavior. One recent study states that 9 percent of all
mammalian species are socially monogamous—defined as having one mate at one
time or for life (Lukas & Clutton-Brock 2013). In terms of reproductive
success, it may initially seem disadvantageous for organisms to limit the
amount of individuals they mate with. After all, isn’t the goal to have as many
offspring as possible? Emlen and Oring speculate monogamy might have arisen in
environments where it was difficult, or impossible, for an individual to
reproduce successfully with multiple mates (1977). In such instances, organisms
are better off staying with their initial mate throughout a mating season or
much longer (Emlen & Oring 1977).
More recently,
this hypothesis has been revised to argue that monogamy observed among humans
derived from common ancestors that lived in environments with solitary females
coupled with territorial males whose ranges overlapped (Lukas &
Clutton-Brock 2013). In these cases, it would behoove males to remain with a
single mate and invest time and energy into one relationship to ensure the
successful development of their offspring. Competition among males for multiple
mates in an environment such as this would induce a lot of stress on
individuals. Ultimately, monogamy in humans is thought to have arisen from the
necessity to guard mates and/or the advantages of having two parents to provide
for and protect offspring (Chapais 2013).
From my
research, it appears that human beings are justified in living monogamous
lifestyles. It is entirely possible that hominids reproduced more successfully
when mating with a single partner and, thus, adopted this method of
partnership.
References:
Chapais, B. 2013. Monogamy, Strongly Bonded Groups, and the
Evolution of Human
Social
Structure. Evolutionary Anthropology.
22: 52-65.
Emlen, S., Oring, L. Ecology, Sexual Selection, and the
Evolution of Mating Systems.
Science. 197(4300): 215-223.
Lukas, D., Clutton-Brock, T. H. 2013. The Evolution of
Social Monogamy. Science.
341(6145):
526-530.
Sapolsky, Robert M. Why Zebras Don’t Get Ulcers. 3rd
ed. New York: St. Martin’s
Press; 2004.
Matt,
ReplyDeleteVery interesting post! I definitely agree that humans, along with the other animals that practice monogamy, are justified in living monogamous lifestyles. Your post led me to do a little bit more research into human monogamy, and I found a great article talking about how monogamy conferred not just individual benefits in reproduction, but societal benefits as well. suggesting that monogamy, as you mentioned, limited within-group competition. This decreased competition then theoretically allowed people to live together in increasingly larger societies, eventually leading to the formation of large nations. This would then give the said society an advantage in competing with other groups. The same authors talk about how monogamy was also advantageous for males and females in a society if this monogamy allowed resources to be passed to offspring (which would be advantageous when resources are limited) and when the females grant higher fidelity in exchange for investment in their children. I really liked your post, and I like how some of the things you mentioned--such as decreased competition because of monogamy--apply to the evolution of species as well as of societies.
Reference: Fortunato, L., & Archetti, M. (2009). Evolution of monogamous marriage by maximization of inclusive fitness. Journal of Evolutionary Biology, 23(1): 149-156. doi: 10.1111/j.1420-9101.2009.01884.x
Very interesting post on a very interesting topic! One of the other theories behind social monogamy in humans is the prevalence of male infanticide in that particular species (Opie et al. 2013). In most species with dependent young, females are the drivers of evolution because of the incredible amount of time and care that infants need (Hrdy 1979). With all of that effort put into caring for young by the females comes that much less effort that can be devoted to reproducing with other males, leading to a dearth of males that get to have their genes passed on. Thus, from this perspective, male infanticide is considered a beneficial evolutionary move. After all, if a male starts killing infants that he thinks might be not be his in order to stop a female from pouring resources into passing on what might be another male's genes, then females are more likely to stay monogamous with the males that they mate with in order to ensure the survival of their infant (Opie et al. 2013). The males are not going to kill offspring that they are sure that theirs, but the females are not willing to chance the survival of their offspring. Thus, primates are socially monogamous. It's one of the many theories that are at play currently on this topic.
ReplyDeleteReferences:
Hrdy SB. 1979 Oct. Infanticide among animals: a review, classification, and examination of the implications for the reproductive strategies of females. Ethology and sociobiology. 1(1): 13-40.
Opie C, Atkinson QD, Dunbar RIM, Shultz S. 2013 Jun. Male infanticide leads to social monogamy among primates. PNAS Anthropology. Early Edition: 1-5.
It is very interesting that male infanticide is a factor that influenced species to adopt monogamy, but I can definitely see how that makes sense. At the end of your post, though, you imply that all primates are socially monogamous. The study I cited by Lukas and Clutton-Brock states that only 29% of primates are socially monogamous. This is partly why I find this issue so interesting. It seems plausible that homo sapiens could've just as easily evolved to not be socially monogamous. It makes one wonder if the way many humans mate is natural or just a derivation of our societal structures.
DeleteOh no, I did not mean to imply that all primates are socially monogamous! It's just interesting that a history of violence towards other male's offspring induces females to be monogamous with their mates. Chimps, for instance, are incredibly violent, and thus females tend to be more monogamous. In bonobos, however, sex is used for both pleasure and reproduction, and males do not get jealous, so therefore there is more of a polyamorous feel to their communities.
ReplyDeleteI actually read up on potential causes for this, and one interesting study was published recently that links testosterone production to mating competition between rival males. Bonobos show a constant level of testosterone throughout infancy through adulthood, while chimps have a decrease after infancy and relatively low production until a massive spike in adolescence (Wobber et al. 2013). This often has a lot to do with mating competition (Wobber et al. 2013). In addition, the low testosterone found in male bonobos can correlate with a higher rank in their society. Males that have lower testosterone than their more aggressive counterparts were more likely to form friendly relationships with unrelated females, which served to increase their rank in the hierarchy (Surbeck et al. 2012). So some primates have found ways around it, and they are usually societies in which females and males occupy equal status.
References:
Surbeck M, Deschner T, Schubert G, Weltring A, Hohmann G. 2012 Mar. Mate competition, testosterone, and intersexual relationships in bonobos, Pan paniscus. Animal Behaviour. 83(3): 659-669.
Wobber V, Hare B, Lipson S, Wrangham R, Ellison P. 2013 May. Different ontogenic patterns of testosterone production reflect divergent male reproductive strategies in chimpanzees and bonobos. Physiology & Behavior. 116-117: 44-53.