"Infanticide -- A Major Factor in Mammalian Sociology"

I love books on animals. For my birthday last month, I got the beautiful and extraordinarily informative encyclopedia, The Encyclopedia of Mammals, edited by David W. Macdonald. It's an edition with an orangutan on the front -- since a new three volume edition had come out, this older edition was going cheap at Borders. (It's the 2001 edition, ISBN 0-681-45659-0)

One of the hard things to miss, because the book has lots of special sections on it, is the importance of sociobiology as a factor in the evolution of mammals. And a real highlight of contemporary sociolobiological research is the role of what we might call "criminality" in mammal societies. For example, a one page spread on infanticide in mammals in general and a smaller sidebar on infanticide in primates (including us) emphasizes the fascinating role infanticide played in human evolution, possibly being the reason for pair bonding and so on.

Sarah Blaffer Hrdy (rhymes with "birdie") brought about a revolution in the field with the publication of her book, The Langurs of Abu: Female and Male Strategies of Reproduction, in 1977. She showed how infanticide among langurs can be understood as part of a sociobiological strategy of maximizing reproduction and how female promiscuity can be understood as a response to that. (If male langurs don't know who the father is, they are more likely to tolerate or protect an infant on the chance that it might be his.)

The sheer prevalence of "criminal" behavior among the mammals is rather striking. 13% of all gorilla babies are killed by adult male gorillas. About 50% of all male black rhinos die at the horns of another black rhino. In Borneo, about 90% of all copulations among orangutans are non-consensual (what we would call rape among humans).

Now of course I put "criminal" in quotations here, because animals are not moral beings in the same way people are. But the sheer similarity of animal behavior with the criminal behaviors among H. sapiens is striking. Especially if one believes, as any biologist seeking to unify what we know about the behavior of living creatures (and that includes us) must, that this is our past.

Here are some interesting selections from the Encyclopedia of Mammals:

Fatal clashes among rhinos
Males of all species [of rhinos] are given to vicious fights that inflict gaping wounds. Both African species jab at one another with upward thrusts of their front horns. Black rhinos have the highest incidence among mammals of fatal intraspecies fighting: almost 50 percent of males and 33 percent of females die from wounds [inflicted by other black rhinos]. Why they are quite so bellicose is not known . . . Asian rhino species attack by jabbing open-mouthed with their lower incisor tusks, or, in the case of the Sumatran rhino, with the lower canines (p. 480).

Fatal clashes among hippos
About 10 percent of males are territorial; true to their amphibious nature, they defend not a patch of land but a few hundred meters of river or lake shore. They will tolerate other males in their territory provided they behave submissively, but will expect exclusive mating rights over all the females within it. If a bachelor male does not obey the rules and instead challenges the sovereignty of the territory holder, serious fights can break out. These are often bloody affairs that may result in the death of one of the contestants; most of the damage is done by the animals' razor-sharp lower canines, which may reach a length of 50 cm (20 in). (p. 492).

Infanticide among mammals
Among the subtleties of mammalian sociology is the importance of infanticide -- the killing of young by conspecifics [animals of the same species]. Infanticide has been recorded in over 100 species from at least five orders [of the twenty or so in the class Mammalia] and 18 families. Nor is this habit confined to one sex -- infanticide by females is known for 25 species from three orders: lagomorphs, carnivores, and rodents. [And of course from primates as well, as exemplified by Homo sapiens, us.] However, the potential motives may differ between the sexes -- generally males may be seeking to eat the babies or incease their mating opportunities, whereas females may be after foraging or nesting sites.

Possible benefits of infanticide include: a) securing food (male and female chimpanzees eat their victims);

b) eliminating competitors (among common marmosets [a small New World monkey] subordinate non-reproductive female helpers increase the weaning success of infants born to dominants; infanticide by the dominant female provides her with a bereaved helper and removes rivals to her own offspring);

c) stealing resources (infanticidal female rabbits and Belding's ground squirrels seldom eat their victims but do occupy their burrows);

d) sexual selection -- infanticide is adaptive for the infanticidal male that destroy's a rival's offspring and causes the bereaved mother to stop lactating and come into estrus sooner (e.g. chimpanzees, lions, grizzly bears, and lemmings). Perpetrators reduce the risk of killing their own offspring by being able to recognize their odor, or by sparing offspring of females with which they have mated, or females in places where they have mated (in White-footed mice infanticide is generally by newly-arrived males, since resident males are inhibited from killing pups for the 35-40 days after mating, exactly the time needed for their progeny to mature and disperse);

e) parental manipulation, where parents regulate the sex ratio of a litter; and

f) minimizing the risk of accidentally adopting non-kin and so wasting effort (perhaps applicable to pinnipeds [seal lions, walruses, and seals] which commonly kill pups separated from their mothers) (p. xxiv).

Infanticide among primates
Infanticide has since been document in many primates, among them several lemur species, howler monkeys, leaf monkeys, the guenon monkey group, and savanna baboons, as well as among mountain gorillas and common chimpanzees. Infanticide is usually, although not always, carried out by males, and happens most often when new males move into a group. On a day-to-day basis, infanticide is rare and not easily witnessed -- only about 60 episodes have been well described to date -- but its apparent rarity should not obscure its significance. In primates such as red howler monkeys, mountain gorillas, and chacma baboons, infanticide is a major source of infant mortality, accounting for 25-38 percent of infant deaths. In other words, about 13 percent of the infants born in mountain gorilla and red howler monkey groups are killed by males. (p. 392).

Infanticide not only has fascinating origins; its repercussions are also far-reaching. One compelling possibility is that the cohesive social bonds that exist between males and females evolved as a deterrent to infanticide. Around the time a chacma baboon gives birth, she typically seeks out a particular adult male within the group and establishes a "friendship" with him. She sticks close by the chosen male, following him around incessantly, grooming him much more than he grooms her, and allowing him alone to touch and handle her baby. Why do nursing mothers associate with a male in this way? The answer, in chacma baboons at least, may be to gain protection from infanticide, for a female's male friend is more likely to actively defend her offspring than other group males. (p. 393).

Fratricide/sororicide among spotted hyenas
Spotted hyenas are unusual amongst carnivores in that cubs are born with their eyes open and their teeth erupted. Litter mates, which are normally twins, engage in high levels of aggression within minutes of birth, which quickly leads to the establishment of a dominance hierarchy between siblings, allowing the dominant cub to control access to maternal milk. Sometimes this aggression will lead to the death of the smaller cub. This appears most likely to happen when resources are in short supply and probably insufficient to sustain two cubs (p. 144).

Aggressive war and raiding among chimpanzees
Male chimpanzees also cooperate with each other in aggression between communities, which takes two forms. When parties from neighboring communities meet during the course of normal activities, they usually display excitedly and may charge at and chase each other . . . Sometimes males patrol the boundaries of their ranges and even make incursions into those of neighbors. Patrollers are conspicuously silent and wary and are looking for neighbors. If they hear or meet some, their response depends largely on relative numbers: they quietly leave, or even flee, if outnumbered . . . but they attack when they greatly outnumber their opponents. Attacks are severe and can be fatal; males are known to have killed adult and adolescent males, infants, and even nonfertile adult females.

Lethal aggression by male coalitions is unusual in mammals; among primates it occurs only in chimpanzees and humans. (p. 411)

Hunting [of other species for meat], male bonding, cooperative intergroup raiding, and the making and using of tools are all traits that chimpanzees share with humans and that might also have characterized our last common ancestor. If so, we need to explain why male bonding and intergroup raiding are absent in bonobos [the so-called lesser chimpanzee of the Congo]. (p. 412).

Rape among orangutans
Subadult males try to associate with potentially receptive females whenever the opportunity arises, but a female that is ready to conceive seeks out the local dominant adult male, which generally succeeds in preventing most subadult males from mating with her. The nonpreferred males, adult or subadult, therefore tend to force matings when they encounter a lone female, often viciously biting the fiercely resisting female to restrain her.

Female-initiated courtships with the dominant adult male last a few days in Borneo, but may last weeks in Borneo. Probably related to this is the fact that over half of Sumatran matings are cooperative, whereas some 90 percent of those in Borneo are forced. (p. 423).

For the benefit of any non-human mammals who might read this, now and in the future, I of course concede that this is of course only part of the story. Cooperation, devotion, and care are common in social mammals.

In the sphere of mating, for example, some mammals show quite different behaviors, forming monogamous pairs, such as the South American titi monkey (xxii), or the mara, a big rodent of South America that lives in strict monogamous pairs that come together to raise their young in communal creches (pp. 676-677). Most wolf and wild dog packs are centered on a alpha male and female pair that alone mate. Even stranger are the colonies of naked mole rats where only one "queen" breeds, and in which the "workers" care for her young (p. 693).

But note that there is no tendency toward "nicer" social behavior as intelligence increases, in fact, to the extent that social behavior involves group activity the interweaving of "naughty" and "nice" seems to grow increasingly tight and extreme.