Apr. 2, 2013 — Male and female blue tits are hard to
distinguish for the human observer. However, in the UV-range, visible to birds,
the male is much more colourful. A closer look at the monogamous mating system
of these birds again reveals that all is not what it seems: in every second
nest there are chicks that are not related to the care-giving father. An
already mated male can increase the number of his offspring by siring
extra-pair offspring in other nests than the one he cares for with his mate.
Emmi Schlicht and Bart Kempenaers of the Max Planck Institute for Ornithology
in Seewiesen investigated whether this could be the driving force behind the
evolution of sexual dimorphism. However, effects of extra-pair paternity are
limited, cuckoldry can even reduce the intensity of sexual selection.
In many species males and females look very different.
Male deer wear impressive antlers and the magnificent plumage of a male peacock
is impressive not only to the hen. In our backyard we can identify the male in
chaffinches or house sparrows easily from the distinctive colouration of breast
and crown. Why do these differences exist? For deer and peacock the answer is
straightforward, at least in principle: a well-endowed male can better defend
the females on his territory, or attract more females in the first place. For
these animals sexual dimorphism has evolved, because such traits help males to
obtain additional offspring. For females sexual selection is weaker as they
cannot increase their number of offspring by outcompeting other females.
However, many bird species pose a challenge for this
explanation by evolutionary biologists. Most bird species are socially
monogamous, in permanent relationships with a partner of the other sex. Both
parents have to work hard to raise their offspring. So why do males in
monogamous species have more colourful plumage than females, if the number of
offspring for both parents is decided by the clutch size of the female?
Paternity analyses have long revealed that not all
offspring are related to the male that feeds them. Therefore, a monogamous male
can have additional offspring if he succeeds in siring additional eggs in the
nest of other females. Is extra-pair mating the key to sexual dimorphism?
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