The evolution of infanticidal mechanisms in male langurs

Langurs are leaf-eating terrestrial monkeys found throughout the Indian subcontinent. Langurs typically live in groups of about 20 individuals: several adult females, their dependent offspring, and one resident adult male. The females are often related to each other, as a set of adult sisters, mothers, aunts, grandmothers, and more distantly related female relatives. When male langurs reach adulthood, they leave their natal troop, a behavior that prevents inbreeding. In consequence, the resident adult male in a troop is unrelated to the adult females. The adult females constitute a breeding "harem" which the resident male reproductively monopolizes. Virtually all offspring in the troop are fathered by the one male. Males fight among themselves for control of the troop and access to the females. Typically the resident male loses, and is replaced by a rival male every 27 months.

Upon gaining control of the troop, a new resident male systematically kills all the infants in the troop. If natural selection favored traits that promote the survival of the species, or the group, this infanticidal behavior would be extremely maladaptive. However, natural selection favors design features which promote their own reproduction, regardless of their impacton the group or species -- a principle that becomes clear when the case of langur infanticide is analyzed.

The psychological design features of a male langur can only be passed onto future generations if the male becomes the resident male of a troop. This is because it is only resident males who have access to reproductive females. Circuits of a male langur's brain which predispose him to aggressively challenging a resident male for control of a harem stand a better chance of being passed on to future generations than circuits which predispose a male to a long and peaceful life on the sidelines. Males, therefore, are aggressive in their quest for the privileged position of resident male, but why should this aggressive behavior generalize so tragically to the infants of the newly dominated troop?

The resident male's tenure is a brief one, 27 months on average. The more offspring a male can sire during his brief reign the more likely it is that the design features of a male's brain will spread in future generations. A major constraint upon the reproductive success of the resident male is the fertility of the troop females. A female that is pregnant by the previous resident male is infertile to the newly resident male for the length of gestation -- a period that lasts approximately 6 1/2 months. Further, langur females nurse their infants for several months and do not begin weaning until about 8 to 10 months, though they may continue to nurse their infants for quite some time thereafter. Lactation, however, has the effect of suppressing ovulation. This guards the female against becoming pregnant and spreading her energy resources too thinly among her offspring jeopardizing their health and hers. Were the nursing infant to die, then ovulation would resume and the female would again become fertile. Gestation and lactation combined can cause a female to be infertile for between 20 and 30 months, potentially the entire duration of resident male's tenure. A male, however, can bring a female back into estrus early by killing her nursing infant.

Given the sexual monopology of resident males, a newly resident male will encounter a population of infants sired by the previous male, not himself. However, mutant circuits of the new resident male's brain are far more likely to be passed on through his offspring, rather than the previous male's offspring. Consider the fate of a design feature of the new resident male's brain which causes him to kill the infants in the troop when he takes over. It promotes its own spread by bringing lactating females back into estrus far earlier than they otherwise would if their infants survived to weaning. Once in estrus, the new male can mate with them. Such a mutation produces more copies of itself than the noninfanticidal design, thereby causing the infanticidal design to spread. Eventually, it spreads to fixation, becoming universal. This happens despite the fact that it lowers the rate of offspring production by the group, injures the females fitness, and injures the preceding male's fitness.

Over a long evolutionary history of male takeovers natural selection would have favored the accumulation and refinement of design features which promote their own spread through infanticide. Marks of such `special design' are seen in langur infanticide. For example, the only males to practice infanticide are newly resident males, for a circuit which predisposed a nonresident male to kill infants would not thereby promote its spread -- after all, only resident males mate and hence reproduce their designs. Likewise, resident males do not kill infants in other troops, but limit their aggression only to the infants of females they may potentially impregnate, females of their own troop. Thirdly, males don't kill adult females, because females are a necessary precondition to reproducing their design. Those mutants who did so were selected out. Fourthly, males don't kill weaned offspring either, since it is only the nursing of unweaned infants that generates the hormones in the mother than inhibits her return to estrus. Weaned and unweaned infants may look identical, except for the fact that they can be observed nursing. Nor do resident males commit infanticide throughout the duration of their tenure. A circuit which predisposed a resident male to kill his own offspring (possessing that same circuit) would defeat its own spread. All infants the male finds in the troop when he first assumes control cannot be his progeny, but so too any infant born much less than 6 1/2 months after the male's takeover cannot be his given that gestation lasts 6 1/2 months. Not surprisingly the male's infanticidal behavior subsides around 6 months after he assumes control: he has a clock which shuts down infanticide as soon as the infants born are likely to be his own.

The program pits males who are non-infanticidal against 1) infanticidal males who cease infanticide immediately after they kill all living infants upon taking over a troop, and 2) infanticidal males who have additional circuitry that kills infants that continue to be born after takeover but were sired by the preceding male when he was still dominant. In nature, parsimony is not the guiding principle of brain design: circuits are added as long as there are mutants for them, and they add sufficient functionality to the system.

In this program, you can change tenure time of the resident male. As tenure time gets shorter, the importance of infanticide increases.

Remember, biological functionality is bizarre: a trait is a gene's way of making more genes. The functionality of a system refers solely to how, on average over evolutionary time, a trait promoted its own reproduction.