Thomas langurs picky about female primates?

By Serge A Wich

MEDAN, North Sumatra (JP): Tucked away in a far away corner of lndonesia's chain of islands, lives a medium-sized primate -- the Thomas langur (Presbytis thomasi) -- that has fascinated biologists for more than 10 years and are only found in the magnificent rain forests of Aceh and North Sumatra.

These monkeys, and many other species of animals and plants, are officially protected within the boundaries of the Leuser Ecosystem managed by Unit Management Leuser (UML) and their future can only be ensured by protection of this area.

Primatologists have, in recent decades, attempted to make sense of the variety in primate social organization. Primates are found in monogamous pairs as well as large groups with many males and females. This variety is thought to be determined by predation pressure and food distribution. Recently, male aggressive behavior against females and infants has been added to this list and is currently the focus of much research.

In a large number of primate species, including the Thomas langurs, males from outside established groups enter into these groups to attempt to, and often with success, kill young infants. This behavior is known as "infanticide". After committing infanticide these males often take over the group by ousting the resident male and luring females away in order to form a new group.

At first it was thought that this was merely a side-effect of aggressive male behavior without any specific purpose. But slowly it has became apparent that by killing the infant of a female, the female becomes receptive again sooner than she would have if her infant would have remained alive until the age of weaning. By taking over a group by ousting the resident male and killing the infants fathered by this male, the new male is assured that the females will become receptive sooner and will only mother his infants, which carry his genes and not that of the previous male.

One of the most important questions to be answered, in order to understand primate social organization, is why females do not move to a better protector male before they lose their infant to an infanticidal male from outside of the group. Losing an infant, in which already so much time and energy has been invested, is of course a disaster for a female. Can a female not simply assess when their male is getting weak and choose a stronger one that can protect her infants properly?

Unfortunately females face a dilemma. At the time of conception, her male might still be a good protector but maybe somewhere during pregnancy or the first year of life for the infant the male becomes weak due to old age which allows a greater possibility that an infanticidal male can enter the group and kill the infant. Therefore, a female would have been better off not having the infant of this male and should have moved toward another male before conception. Even when a female would have made the difficult decision of moving away to another male there is still another problem to tackle. Which male should she choose?

These are exactly the questions that researchers from Utrecht University, in the Netherlands, and the National University, in Jakarta, are trying to tackle through research on the Thomas langurs.

Thomas langurs live in fairly small groups consisting of one male with up to seven females and their offspring. Groups have fairly stable living areas and meet other groups on a regular basis. These between-group encounters can be with neighboring groups also consisting of one male and several females, but can also be with groups consisting only of males.

These "all-male-bands" consist of mostly young males that are looking for opportunities to start their own group by either taking over other groups or by attracting females from other groups. Both groups where there are neighboring males who already have some females but are interested in even more, and males in all-male-bands somehow have to show females that they are a better protector for future offspring than their current male if they want to attract females.

How then can a female assess which is a good protector male? We are trying to answer these questions by measuring factors that are known to be indicators of "male quality" in other animal species.

These factors include characteristics of the loud call that Thomas langur males make and that can be heard so frequently over distances of up to one kilometer in the forest. In other animal species it has, for example, been demonstrated that females choose the male with the lowest call. Also, hormone and parasitical infection levels are being assessed from male faecal samples. Hormonal stimuli, such as testosterone, are related to male development, and possibly, their fighting capabilities as well as immune system. Since a strong male is probably also a healthy male, a male with many deleterious parasites is also likely to be a less healthy male.

Male behavior during between-group encounters has been recorded in detail. The aggression a male displays during these encounters, in combination with its attempts to commit infanticide, are probably also a good indicator for females of both the strength of the neighboring male and their own male.

The curiosity that researchers and laymen alike have always had for primates is obviously linked to the similarities humans have with primates. Although female choice in humans should, in an ideal world, be based on true love this is not always the case. Money and social status are sometimes also factors that women find important when looking for a partner and it happens to be that these factors are obviously important for the chances of rearing children successfully and partly determine the "social protection" a male can offer to the children. This somehow seems to resemble the choice that female monkeys are likely to make for a better protector male.

Although it is debatable whether infanticide is found in humans there are suggestions that indicate that similar processes as those found in monkeys are still with us.

For instance, it has been found that when a woman divorces and brings a child from her former marriage into a new marriage, this child has less chance of doing well than under normal circumstances i.e. with the biological father. This is thought to be related to the disinterest a stepfather may have in a child that is not his and therefore is not going to propagate his genes into future generations. Interestingly, it has been also found that women that have children with a new husband also tend to neglect them in terms of costs spend on food and education. These findings have also been interpreted as being evidence for our "selfish genes" that rather invest in a child from the new husband with greater potential for succeeding than on a child that has less potential. A lot more work remains to be conducted before we can conclude that the same basic drive which underlies infanticide in primates also operates among humans.

In any case, the interplay between research on monkeys and apes on the one hand, and humans on the other, is likely to help us understand our own behavior. It is therefore very unfortunate that humans themselves are rapidly destroying the forests in which our closest relatives live and thereby further eliminating the possibilities to study behavior that can allow us to learn more about ourselves.

The writer is a researcher from the University of Utrecht, in the Netherlands. At present he is doing research in the Leuser Ecosystem that lies in North Sumatra and Aceh. The Leuser Management Unit (a conservation project funded by the Indonesian government) and the European Union has helped him facilitate these research activities.