faithful to just one mate for life, but many
-- at least theoretically -- subscribe to the
idea of monogamy.
Scientists are still asking: Where did this
tendency to form lasting relationships
with just one other person come from?
Two studies published this week address
the question of why social monogamy
evolved in some mammals. But they
don't agree on the answer: One says
dads stick around to protect their kids
from being killed; the other says it's
because of resource distribution and the
female diet.
Why, or why not, monogamy
About 90% of bird species demonstrate
social monogamy; this is true for less than
3% of mammals.
Living in pairs makes sense for birds
because successful child rearing requires
both parents to incubate and provide food
for the babies, which hatch from eggs. On
the other hand, in mammals, the fetus
grows inside the mother, and she lactates
to feed the baby -- activities in which
males don't play a role.
"We would predict that males should have
additional time and additional energy,
which they might use to increase their
fitness by mating with additional
females," said Dieter Lukas, a
postdoctoral researcher in the
department of zoology at the University
of Cambridge. "Why then do males in
some species stick around rather than try
to find additional females?"
Typically, male mammals mate with
multiple females in a single breeding
season, a system called polygyny. If a
male animal's evolutionary goals are to
survive and reproduce, monogamy
represents a problem; staying with one
partner limits his seed-spreading
potential.
In primates in particular, however, about
a quarter of species display social
monogamy. The phenomenon appears to
have developed about 16 million years
ago, which is relatively late in the history
of primates, according to a new study led
by Christopher Opie at University College
London.
Ancient primate could be a missing link
This breeding structure is also seen in
wolves, jackals, beavers, meerkats,
spiders, shrimp and many other animals.
So there must be an important
evolutionary reason (or reasons) that
males of some species stay with just one
female partner until one of their deaths.
But what?
Because of male infanticide?
Opie's study, published in Proceedings of
the National Academy of Sciences,
focused on primates, a subgroup in which
humans fit, along with relatives such as
chimpanzees and gorillas. It's important
to note that the researchers used "social
monogamy" in this context to mean
"living in pairs."
Researchers analyzed the family tree of
primates and used a statistical technique
to gain insights from what we know about
the genetics and behaviors of 230
primate species. They posited that there
are three possible explanations for social
monogamy in this group.
One is parental care -- that the father
sticks around to help carry the children;
for instance, some monogamous New
World primates often give birth to twins.
Another idea is that females are spread
out in the territories that they occupy,
meaning males have a hard time claiming
more than one female.
The explanation that these researchers
favor, however, is that a male that lives
with a female mate can protect their
offspring from other males, that might
want to kill these children.
Why? Mammals usually don't get
pregnant while they are lactating, so a
male that is not the father might increase
his chances of mating with the mother if
he kills the offspring while she is still
weaning these children. Infanticide is not
advantageous in seasonal breeders,
however, since everyone will have to wait
until the next breeding season for
copulation.
Although parental care and dispersed
female ranges are also traits of social
monogamy, male infanticide is the one
that appears to have preceded,
historically, the shift to social monogamy,
the researchers say.
"Our analyses suggest that socially
monogamous species are much more
likely to have low male infanticide rates,"
study authors wrote.
The risk of infanticide goes down because
one or both parents can defend a child
against another invading male, they
argue. It also appears that species
practicing social monogamy have a
relatively shorter lactation period, which
allows fertility to resume more quickly
and reduces the incentive for other males
to swoop in and kill offspring.
This may be an uncomfortable thought,
but researchers suggest that male
infanticide may have put pressure on our
ancestors to stay in long-term couples.
There is evidence that Australopithecines
and early modern humans displayed
monogamous behavior. In humans in
particular, researchers have proposed that
the transition to social monogamy
depended on females choosing to stay
faithful to males -- and that may be
because they wanted to protect their
children.
But Maren Huck, primate researcher at
the University of Derby in the United
Kingdom, doesn't entirely buy this story.
She finds the classifications of key
characteristics of various species in this
study "questionable." Given that, she said
in an e-mail, "It would be premature to
confidently claim that infanticide was the
key factor in the evolution of social
monogamy in primates."
Because of territory and food
resources?
The conclusion of Opie's group's study
also diverges from what a different group
of scientists concluded in a separate study
in Science this week. The second study
looked at the family trees relating to
more than 2,500 mammals. This
represents a much larger cross-section of
animals than Opie's team's analysis, but
this time, humans were not included.
"I'm far from convinced that humans are
really monogamous," said Tim Clutton-
Brock, professor of ecology and
evolutionary biology at the University of
Cambridge.
Lukas and Clutton-Brock view "social
monogamy" somewhat differently: as a
breeding male-female pair ranging
together, with or without their offspring,
associating with each other for at least
one breeding season.
They suggest that in some mammalian
species, females started finding higher-
quality foods in areas that were farther
apart from each other. As a result, the
females would aggressively defend
territories that contained this food,
keeping other females out.
"As they then started to spread out
further, that's when males seem to have
changed their mating strategies," said
Lukas, lead author of the study.
Other species that came to specialize in
more abundant food, particularly in the
grasslands, evolved to be polygynous,
Clutton-Brock said.
The Cambridge team did a separate
analysis of primates -- using more species
than Opie's group did -- and did not find
the same association between social
monogamy and male infanticide as the
other study. They haven't pinpointed the
discrepancy, but the independent
research groups may have classified
species differently, or it may have to do
with different sample sizes, researchers
said.
Huck also had issues with some of the
assumptions regarding various species in
this study. It also appears, she said, that
these results focus on mating
monopolization -- breeding with only one
mate -- rather than how some animals
evolved to live in pairs.
"Unless we are clear about the
classifications and what exactly we are
talking about, we cannot be clear in what
the results we get out of a complicated
evolutionary model actually signify," she
said.
What about humans?
Clutton-Brock cautioned against drawing
any definite conclusions about humans
from the study but said it is possible that
the dietary and resource patterns his
paper described could have something to
do with the evolution of human breeding
strategies, as well as a need for extended
paternal care in our species.
Given that other great apes are
polygynous and that human males and
females differ so markedly in their
average body size and longevity, Clutton-
Brock says "the ancestral condition for
humans is probably polygyny."
Clutton-Brock and Lukas said they were
unaware of Opie's team's study until just
a few days ago; otherwise they would
have hooked up beforehand to compare
notes.
It's not too late. The Cambridge scientists
say they intend to connect with Opie's
group to trace the origins of their
differences, which may in turn put them
closer to the roots of human
togetherness.
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