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| Meerkats are monogamous |
Any list of the world’s as-yet-unsolved mysteries has to include
these three questions: 1) Why are we here? 2) Why are yogurt containers
sealed to pretty much guarantee spillage upon opening? and 3) How did
monogamy ever develop?
New and divergent replies to the latter conundrum have been offered
in two new studies, which, unbeknownst to each set of authors, came out
in the same week. According to one, primates are monogamous so that
nonrelated males don’t kill their babies. According to the other, that’s
hooey; animals are monogamous because it was the only way they could guard their mates and thus their breeding rights.
Monogamy, as nearly any Ashley Madison subscriber likes to note, is
not “natural.” That is, hardly any species practice it, except for birds
(and, reportedly, cockroaches). Social monogamy, an arrangement in
which two creatures mate and work together to meet their basic needs, is
especially uncommon among the nonavian warm bloods; only about 5% of
the 4,000 or so mammal species on earth hang around with just one mate.
(These include wolves, beavers, naked mole rats and meerkats.) Since
mating with only one female at a time tends to lower a male’s chances of
producing as many offspring as possible, what good, evolutionarily
speaking, can come of being monogamous? Why would mammals end up that
way?
One reason, according to a report in the Proceedings of the National Academy of Sciences (PNAS),
suggests that males stayed with one female to ensure their young were
not killed by another male. It’s one thing to sire a litter of pups and
another to ensure that they actually survive to reproduce to carry on
their genetic lineage. Based on breeding and parenting behaviors
collected from 230 different primate species over several generations,
the researchers determined that males began balancing the need to spread
their gene pool against the need to protect their young from being
killed by other nonrelated males. The attacking males needed to kill the
young so that they could breed with its mother, who would delay
conception of another offspring if they were nursing. So the father hung
around to ensure the safety of his genetic line and to help raise the
young so that the mother could reproduce again sooner.
“This is the first time that the theories for the evolution of
monogamy have been systematically tested, conclusively showing that
infanticide is the driver of monogamy,” trumpeted Christopher Opie, a
research fellow in the Anthropology Department of University College
London, in a statement. “This brings to a close the long running debate
about the origin of monogamy in primates.”
Well, not so fast. Another study, published in the journal Science,
used a similar type of analysis, but across a much wider sample — about
2,500 mammals, or more than half the known species. Those authors,
Dieter Lukas and Tim Clutton-Brock of Cambridge University, found no
correlation at all between infanticide and monogamy. Their results
suggest that monogamy works like real estate: it’s all about location
and supply. “Monogamy develops where females live at low density,” says
Lukas.
Males cannot fend off rival suitors from more than one female at a
time because they’re too spread out. Therefore, they cannot ensure
their young are the ones the female is carrying, so they stick with one
female. “It’s a consequence of resource defense.”
The Science study notes that in mammalian species that are
monogamous, the females tend to be solitary and intolerant of other
females. Unlike ungulates, who are rarely monogamous, these mammals’
nutritional needs are greater, and they therefore shoo off competitors
for the food resources.
Both studies suggest that the third theory often advanced for the
development of monogamy — in which males can assist in raising of the
young — is much less likely. Rather than a cause of monogamy, such
paternal assistance is probably a consequence of the mate-for-life
scenario.
As for human monogamy, the PNAS study is much more
comfortable extrapolating its results. Because humans have such big
brains, their infants take a long time to nurture and are vulnerable for
longer. Therefore human males had a compelling reason to hang around
and protect their child-rearing female until breeding was done.
The Science study is more speculative. “We are cautious on
making any definite statement. Humans are such unusual animals,” says
Lukas. Adds Clutton-Brock: “I’m far from convinced that humans are
indeed monogamous.”
So how important were kids in man’s move toward monogamy? It’s a
fascinating fight, but ultimately whether or not monogamy is natural is
less relevant than whether it’s desirable. When considering behavior,
naturalness is not the most important issue. There’s nothing natural
about reading, using toilet paper or skydiving, but they have their
advantages. Monogamy, in humans, seems to be both an acquired taste and a
learned skill. The question remains whether it’s worth the cost of
learning it.
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