Thomas langurs picky about female primates?

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.