Blog Post 3: Northern Plains Gray Langur

                                                             Blog Post 3: Social Systems

By Nicole Anguiano

Group of Northern Plains Gray Langurs. 

Picture from Robert Harding.

Social Organization: 

Northern plains gray langurs have a very complex and variable social organization, especially when it comes to different habitats. In general however, gray langers can be found in polygynous troops, where one male mates with multiple females, multi-male/multi-female troops, or all-male bands. Polygynous troops and multi-male/multi-female troops have a mean size of 11-64 individuals, while all-male bands have a mean size of 2-17 individuals(Botting, 2019). It should be noted that all-male bands are unusual and are typically a sign of transition; following the takeover of the alpha male in polygynous troops. Furthermore, multi-male/multi-female troops usually turn into polygynous troops during mating season (Gron, 2008). 

Like social organization, group composition within troops and bands are varied. In polygynous troops, you can find one male adult, multiple females, and juveniles. In multi-male/multi-female troops, there are males and females of all ages and sex classes. In all-male bands, you find male adults, male subadults, and male juveniles (Gron, 2008).

Created by me on Google Docs. Horizontal arrows in between group boxes show movement of males into breeding groups. Arrows at the bottom of group boxes describe mating systems. 

Social Structure :

    Hierarchies are found within all 3 types of social organizations. In all-male bands, rank is determined mainly by displacement, when a male joins a multi-male/multi-female or polygynous troop, but can also be determined through fighting, harassment, and copulatory success, or the male's ability to successfully mate with females and produce viable offspring. In troops containing females, the hierarchy within the sex is usually determined by physical condition and sexual maturity (Gron, 2008). The more physically fit a female is, the higher her rank is likely to be, and the younger a female reaches sexual maturity, the higher her rank will tend to be. This is likely because younger females who can reproduce have a longer window of opportunity to mate and reproduce with the ever-changing alpha males.

    Relationships within and between groups change depending on the type of social organization. In all-male bands, males can be peaceful, cooperative, or agonistic. In fact, cooperation between all-male troops has been reported when overtaking alpha males in polygynous troops. Individuals from all-male bands work together to run out the resident male in a polygynous troop, and if successful, will then fight amongst each other to determine who stays in the troop. There are some circumstances where an alpha male in a polygynous troop may allow other males to remain in order to help fight off any outside males that may want to take over the troop in the future (Botting, 2019). 

In multi-male/multi-female troops, males and females tend to be more peaceful and cooperative since there is no real threat of takeover for alpha position. The occasions where you might find more aggressive and submissive interactions are typically between the higher-ranking individuals of these troops. Female-Female relationships are quite positive and they spend quite a bit of time grooming, feeding, and embracing one another (Gron, 2008). When it comes to grooming in  multi-male/multi-female and polygynous troops, females tend to groom males more than males groom females. When females groom other females, typically the lower ranking individuals will groom the higher ranking ones. 

In troops containing females, you will find that they are philopatric and stay in their birth groups for life. Males on the other hand, will leave their birth groups at around the age of 5, when  they reach sexual maturity, and join an either all-male band or multi-male/multi-female troop if given the opportunity (Botting, 2019).

Picture of male gray langurs fighting. Picture from Gron 2008.

Mating System:

Like many other animals, reproduction in gray langurs is linked to food availability. Only after females can access enough food to compensate for the high energy cost of pregnancy, are they able to reproduce. This means that sexual maturity can range from 3 to 8 years, depending on location and resource abundance (Semke, 2011). In some extreme cases, if an area is very low on food availability, females can become seasonal breeders (Botting, 2019). 

    Due to their different types of social organizations and the focus for female monopolization, gray langurs exhibit slight sexual dimorphism. Males, on average, weigh about 13 kg while females weigh about 10 kg. Excluding their tail, males are about 64 cm long, and females are about 58.5 cm long (Semke, 2011). 

During heat, typically during the months of July and October, females gain male attention by head shaking and showing their anogenital region to potential mates (Semke, 2011). Once pregnant, a female is in gestation for about 200 days, resulting in the birth of a single infant, though there have been cases of twins before (Botting, 2019). One very interesting and important female reproductive strategy seen in female langurs is their ability to continue mating during gestation to prevent infanticide by dominant males (Semke, 2011). This strategy occurs to confuse any incoming alpha males, tricking them into thinking the baby belongs to them. 

Infanticide is a common male reproductive strategy when over-taking polygynous troops. This is done because females with infants are not able to reproduce since they have to spend their energy taking care of their young. By killing the female infant, the female is able to return to heat and allow the male to mate with her (Botting, 2019). Infanticide has a profound effect in mating. In troops with high levels of infanticide, mating periods are shorter to ensure that the male who fathers the offspring is more likely to be the one protecting the baby (Borries et al., 2001).

Care System:

    Female gray langurs have a more active role in infant care than males. Mothers take on a very protective role over their young, and are usually the center of attention during their first months for increased protection. In some cases, a week after an infant is born, allomothering can take place and other females will take on the role of mother (Semke, 2011). On average, the mother, or any other individuals taking the role of the mother, will feed their young for about months until they begin eating for themselves. By around 9-12 months, male infants are around their mother about 20% of the time, and before they reach 5 years of age, males are forced to disperse (Semke, 2011). 

A unique parental behavior exhibited by gray langurs is kidnap and sacrifice. Sometimes, females from surrounding troops will kidnap infants, and mothers will often risk their lives in order to retrieve their young (Semke, 2011). While I could not find too much information about this topic (thank you firewalls), this could be linked to infanticide and reproductive strategies.  

Infant gray langur feeding on its mother.

Picture from Gron 2008.

Works Cited: 

Borries, C., Koenig, A., & Winkler, P. (2001). Variation of life history traits and mating patterns in female langur monkeys (Semnopithecus entellus). Behavioral Ecology Sociobiology, 50: 391-402. DOI 10.1007/s002650100391 

Botting, J. (2019). Northern Plains Gray Langur: Semnopithecus entellus. New England Primate Conservancy. https://neprimateconservancy.org/northern-plains-gray-langur/

Gron, K. (2008). Primate Factsheets: Gray langur (Semnopithecus). Wisconsin National Primate Research Center. https://primate.wisc.edu/primate-info-net/pin-factsheets/pin-factsheet-gray-langur/

Semke, R. (2011). Semnopithecus entellus: Hanuman langur. Animal Diversity Web. https://animaldiversity.org/accounts/Semnopithecus_entellus/