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Slide show: Learn about the many forms of monogamy in the animal kingdom.
Credit: National Science Foundation
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Does
true monogamy exist in the animal kingdom? Why do the males and females
of some species look alike, but others don't? What is research on
voles, of all species, revealing about human monogamy? These and other
curious questions about monogamy are answered by Bruce Cushing, a
behavioral neuroscientist at the University of Akron and an NSF program
director.
Credit: National Science Foundation
DiscoveryAnimal Attraction: The Many Forms of Monogamy in the Animal Kingdom
A Valentine's Day special on the science of monogamy
February 13, 2013
Ever have a relationship that qualified as
"faithless love"? If so, you're in good company: Almost all adults in
the animal kingdom have also experienced, if not a faithless love, then
at least a faithless pairing.
Faithless pairings are so common in
the animal kingdom because only a handful of animal species practice
true monogamy--defined as pair bonding between a male and female, which
exclusively mate with one another, raise offspring together and spend
time together.
The pair bonds of some monogamous species may
last for the long-term, even perhaps for a lifetime. Those of other
species may last for only the short term, perhaps for only a single
mating season.
Who's your daddy?
All
expressions of true monogamy--whether characterized by short-term or
long-term pairings--have long been considered to be a rarity in the
animal kingdom. Nevertheless, since the advent, in the 1990s, of DNA
fingerprinting--which is similar to paternity tests used in the
courts--scientists have discovered that true monogamy is even rarer than
previously believed.
As it turns out, many species that were once
considered to be truly monogamous really practice what is known as
social monogamy. This form of monogamy is defined as pair bonding
between a male and female, which mate with one another, raise offspring
together and spend time together, but may nevertheless occasionally mate
outside of their pair bond.
Scientists call such outside matings "extra pair copulations."
DNA
fingerprinting has revealed that even swans--those icons of love and
fidelity--may participate in extra pair copulations, probably during
quick, furtive trysts. What's more, about five to six percent of pair
bonded swans ultimately "divorce" for unknown reasons.
Looking the other way
The
frequency of extra pair copulations among socially monogamous species
begs the question: Why would any socially monogamous species tolerate
promiscuity?
No one knows for sure. But one theory is that females
may tend to pair bond with males that are particularly good providers
and offer potential stability, but are lured into extra-pair copulations
by males that offer "something else" not provided by their pair bonded
partner.
That "something else" may be superior genes, as
reflected in the male's physical features, such as his weight or
resistance to disease, or his control of particular resources, such as a
large territory.
On the other side of the pair bond, males may
seek extra-pair copulations in order to increase their chances for
reproductive success--even if it turns out that their pair bonded
partner is sterile or genetically unfit in some way; through
promiscuity, a male may fertilize multiple females, and thereby avoid
putting all of his genes in one basket.
How rare is rare?
Some statistics on the frequency of monogamy in the animal kingdom:
- Not
a single mammal species has, thus far, been definitively shown to be
truly monogamous. (Nevertheless, individual pairs of mammals may be
truly monogamous.) Scientists now estimate that only about three to
five percent of the approximately 4,000+ mammal species on Earth practice any form of monogamy.
-
Before the advent of DNA fingerprinting, scientists believed
that about 90 percent of bird species were truly monogamous. But
paternity testing suggests that the reverse is true: Scientists now
believe that about 90 percent of bird species are socially monogamous,
and that true monogamy among birds is the exception rather than the
rule.
- Some insects, including cockroaches, are monogamous.
- Any form of monogamy among fish and amphibians is exceedingly rare.
Because
of the paradigm-shifting revelations produced by DNA fingerprinting,
many scientists are now reluctant to classify any species as truly
monogamous until it has undergone rigorous DNA fingerprinting.
Possible reasons for monogamy
The
ultimate purpose of life for each individual animal on Earth is to
reproduce, and each individual that reproduces successfully helps
perpetuate its species. Building on these facts, some scientists
believe that monogamy evolved in species whose members are more likely
to achieve reproductive success through pair bonding than through
promiscuity.
Such species may include those whose populations are
relatively small and dispersed: in such cases, the male's investments in
monogamous pair bonding may yield more offspring than would his
investments in repeatedly searching for hard-to-find females.
Another
theory: monogamy may have evolved in some species in order to support
their special care-taking needs. Consider, for example, Emperor
Penguins.
Until an Emperor chick becomes independent of its
parents, it must be protected in its colony from the harsh Antarctic
elements and from predators by one parent, while the other parent
travels back and forth to distant seas to feed itself and gather food
for the chick--dual responsibilities that a single mother could not
possibly fulfill on her own.
Therefore, monogamy may have
evolved in Emperors in order to support the intense parental cooperation
needed by Emperor chicks. This theory is supported by the fact that
once Emperor chicks become independent of their parents and thereby
outgrow their need for cooperative parental care-giving, the
overwhelming majority of Emperor parents (about 85 percent) permanently
part ways. (Adult Emperors practice serial monogamy, and usually form a
new pair bond every breeding season.)
Also, some scientists believe that monogamy may have evolved in some species because their young can
be cared for by both of their parents. Such species include bird
species whose young survive on food brought to them by both of their
parents, which are equally equipped for the task. Because the monogamy
of such species supports fatherly care-giving, and thereby promotes
reproductive success, the evolution of such species apparently favored
some form of monogamy, as the theory goes.
By contrast, baby
mammals must be fed via breast-feeding--a need that obviously can only
be fulfilled by females. So, almost by definition, the males of most
mammal species are generally unequipped to help feed their young.
Therefore, such species would not necessarily benefit from a social
structure that supports fatherly care-giving, and so their evolution
would not necessarily have favored monogamy, as the theory goes.
However,
theories about the evolution of monogamy that are based on its support
for fatherly care-giving are countered by the fact that the males of
some monogamous species do not typically help care for their young-even
though the reverse is apparently true: All species in which males
typically help care for their young are monogamous, as far as we know.
The joy of monogamy
While
environmental factors may influence the evolution of monogamy, so too
may genetic factors. Some possible genetic influences on monogamy have
been discovered through
recent research
on prairie voles, which form lifelong social attachments.
Specifically, this research identified special hormone receptors located
in the reward centers of the brains of male prairie voles. Such
special receptors may give the voles a sense of pleasure from monogamy
and taking care of young, and thereby help promote these behaviors.
This
research also involved transferring the special hormone receptors of
prairie voles to other vole species that are promiscuous and do not form
social attachments. The result: the promiscuous voles became
monogamous, like prairie voles.
What's more, the prairie voles'
special receptors are very similar to receptors found in the brains of
humans and bonobos. Bonobos, or pygmy chimpanzees, display empathy and
maintain strong social bonds. By contrast, these receptors are not
present in the brains of common chimpanzees, which are less empathetic
and more-aggressive.
These results suggest that the special
hormone receptors may influence species-to-species differences in social
structure. In addition, individual variation in these special
receptors among human males may help explain some of the individual
variation among men in their attitudes towards commitment, monogamy and
marriage.
Probably because varied and complex combinations of
genetic and environmental factors influence the reproductive behavior of
each species, virtually every species that practices true monogamy or
social monogamy expresses their monogamy in a unique way. (
See slide show.)
Learn more about the biology of love and other animal emotions in an
online chat featuring NSF program director Diane Witt.
Related Websites
NSF press release on research on monogamy in voles:
The National Science Foundation (NSF)
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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