"Hanuman Langurs: Evolution and Social Behavior"

How Does Evolution Impact the Social Behavior of Hanuman Langurs?


The Hanuman Langur is an Asian monkey; whose social behavior is uncanny. Their troops consist of both males and females numbering around 125. The head male—of a troop where there is one male—is competing with other males to maintain his position as leader. When a group of males overthrow the leader —so to speak—they kill his offspring. After that “ritual” one male becomes they new leader. That male then mates with the females of the troop and the process is repeated. The males gain a reproductive advantage because the females are at their maximum sexual receptivity when they lose their offspring.

·      This species is vulnerable to infanticide.

·      Males attack the infants only if they were not present at the time of contraception.

·      The males are usually protective of their offspring, however some still kill their own young.

 Ecology, Social Structure & Evolution

·      Ecology pressures that influence sex ratio populations also affect group size and social structure.

·      Increasing the Hanuman Langur troop size might prevent takeover and infanticide.

 One study reports that species such as:  P.ursinus, Propithecus diadema, and Semnopithecus entellus (Hanuman Langur) have over a 10 percent mortality rate of infants. Another species, the Red Howler species, shows that the rate of infanticide increases with group size. However, there was a change in rate when the Red Howler groups became large enough to cause paternal confusion—the males in the species normally do not kill their own young. The confusion causes males refrain from infanticide, which causes an overall decrease in the infanticide rate.

Because the social behavior of the Hanuman Langur depends on ecology...ecological evolution impacts the behavior. The ecology of the Hanuman Langur includes varied troop sizes and varied troop constituents, which evoke different behaviors. As previously stated, troop size impacts the level of infanticide.

Questions for Research

What does the social behavior of hanuman langur imply about evolutionary desires to be a leader or at an advantage by any means?

What does this behavior say about evolution in general?

Hanuman Langur selectivity and evolution…