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THE twins Romulus and Remus should have had it all. Their mother was a
princess and their father, Mars, was god of war. But the brothers were abandoned
at birth. Instead of inheriting wealth and status, they were brought up by a
she-wolf. Later, the twins set about founding a city. But they argued over its
location, and in the ensuing fight Romulus killed his brother, continuing a
family tradition of siblicide.

The story of the founding of Rome may be pure myth, but in the animal world,
examples of brothers and sisters killing one another abound. Siblicide is
practised by a bewildering array of animals, from praying mantises and termites
to wild dogs and pigs. In spotted hyenas, it is so common that, although females
usually produce two cubs, around half of all mothers raise just one.
Prong-horned antelopes don鈥檛 even wait to leave the womb鈥攐ne of the
embryos skewers the other with the tip of its tail.

For evolutionary biologists, siblicide is puzzling. Why should this brutal
behaviour be so common? After all, siblings share half of their genes on
average. Since killing a sibling reduces the chances of these common genes
surviving, natural selection should act to make siblings altruistic towards each
other. Such thinking has led biologists to believe that, where siblicide does
occur, the benefits must outweigh its high cost. In other words, siblicide is
seen as an adaptation, a behaviour moulded by natural selection that helps an
organism to survive and reproduce in certain environments.

However, a closer look at siblicide in the African spotted hyena and the
polar skua in Antarctica has persuaded us to challenge this dogma. We believe
this intriguing behaviour can be explained without invoking cost-benefit
analysis and adaptive arguments, and that there is an alternative explanation
for many examples of siblicide.

The current thinking about why siblicide occurs is set out in the recent book
by Douglas Mock of the University of Oklahoma and Geoffrey Parker of Liverpool
University, The Evolution of Sibling Rivalry (Oxford University Press,
1997). The key idea is that parents 鈥渄eliberately鈥� produce too many offspring.
These siblings then have to compete for limited resources, which in some
environments may lead to siblicide.

Overproducing young could have three possible benefits. First, it might allow
parents to raise the largest number of robust offspring when food supplies are
unpredictable. This idea, known as 鈥渞esource tracking鈥�, was suggested by the
Oxford ornithologist David Lack in the late 1940s. It predicts that sibling
aggression will be greatest when food is scarce, and indeed over the decades
many researchers have found this to be so. The second potential benefit is that,
in hard times, a weaker brother or sister could be a valuable source of food for
its stronger siblings. Finally, extra young may be a kind of insurance policy
against the premature or unexpected loss of an offspring.

Fight to the death

In the classic case of spotted hyenas, siblicide is usually explained as an
example of resource tracking. The killing takes place in the natal den, a burrow
too small even for the mother to enter. The gestation period in spotted hyenas
is longer than that in other hyena species, so young are born with highly
developed canine teeth and motor skills. Intense fighting between cubs from the
same litter starts very soon after birth. The weaker cub is excluded from
nursing, and the result is often starvation, complicated by infections of wounds
that develop after repeated attacks.

Recent research, however, suggests that siblicide in spotted hyenas is not
simply a way of tailoring the number of offspring to the resources available. In
1996, Laurence Frank of the University of California reported that when there
are too many adult males in the hyena clan, mothers intervene between fighting
female cubs. This parental behaviour is likely to reduce female mortality, so
Frank sees siblicide as an adaptation that allows mothers to manipulate the sex
ratio of their offspring.

This idea is plausible, but just because siblicide is now used in this way
doesn鈥檛 mean that the behaviour evolved for this purpose. It might originally
have had a completely different function or arisen as an incidental result of
other biological or evolutionary processes. As Stephen Jay Gould and Richard
Lewontin of Harvard University pointed out two decades ago, not every
characteristic is an adaptation. They urged researchers not to divide organisms
into component parts and construct independent adaptive explanations for each,
but instead to look at the creature as a whole, taking account of the
environment within which it developed.

With this in mind, we have reassessed the role of siblicide in spotted
hyenas. This extraordinary animal has attracted scientific interest from the
time of Aristotle. Stranger even than its high rate of siblicide is the bizarre
sexual morphology of spotted hyena females. Females have a pseudopenis and
pseudoscrotum that are superficially indistinguishable from the genitals of
males. There is strong evidence that sexual morphology, brain organisation and
behaviour have a common developmental origin, which is why we believe that
siblicide cannot be understood in isolation.

In spotted hyenas, the same hormones that have a central role in the
differentiation of internal and external sexual structures and brain
organisation, also play a key part in the processes leading to siblicide.
Androgens, through their conversion to oestrogen and testosterone, are key
precursor molecules in pathways that lead to the differentiation of testes and
ovaries. The same androgens also influence the aggressive behaviour of female
hyena cubs. It is no accident that young female spotted hyena, with their high
levels of these hormones, have been implicated as the prime initiators of
sibling conflicts. Viewed from this perspective, siblicide itself need not
necessarily be adaptive in the spotted hyena but can be explained simply as a
byproduct of the hormonal state of the developing cubs.

This led us to question the origins of siblicide in birds, which have
dominated investigations into the behaviour鈥檚 evolutionary significance.
Siblicide occurs in a wide range of species, from pelicans, raptors and boobies
to skuas鈥攐ur particular speciality. The established view is that siblicide
in birds serves a variety of purposes, but all ultimately ensure that the
highest-quality young survive. The evidence does not always support this.

Take polar skuas. They live and breed in and around the Antarctic, one of the
most extreme environments on Earth. These predatory seabirds are monogamous, and
each pair typically produces two eggs. However, those living farthest south
rarely manage to raise more than a single chick. Shortly after hatching, the
siblings fight during feeding time. The smaller chick starves to death or is
killed by its nestmate, though in some cases a neighbouring adult will finish it
off.

This might seem like 鈥渄eliberate鈥� overproduction of young by parents, as you
would expect if this siblicide were an adaptation to allow the fittest offspring
to survive. But even in areas where pairs usually manage to raise two chicks,
they still produce only two eggs. In fact, all types of skua produce two eggs.
This is simply a characteristic of a large group of species, which probably
originates from a common ancestor.

What鈥檚 more, if the purpose of siblicide is brood reduction in extreme
environments, then sibling aggression should decrease as resources increase. It
does not. Working with Euan Young at the University of Auckland, we tried
supplementing the diet of polar skua chicks by providing adults with more food.
We found there was no reduction in siblicide. So for the polar skua, siblicide
cannot be an adaptation triggered by food availability.

Why, then, do polar skuas kill their own siblings? A close look at hormones,
social context, the relative ages of offspring and their sex reveals parallels
with hyenas. Most unusually for birds, the female polar skua is larger and more
aggressive than the male. What鈥檚 more, aggression is arguably the single most
important factor influencing chick mortality. So the question is whether this,
rather than adaptation, explains siblicide in these birds.

If females really do have the edge in size and aggression, then siblicide
should skew the sex ratio in their favour. Testing these predictions is
difficult, though, because birds, particularly young birds, are notoriously
difficult to sex.

To overcome this hurdle we have been working with colleagues including Andrew
Sinclair of the University of Melbourne and Joy Halverson of Celera AgGen, a
company that specialises in DNA testing, to develop a technique that will allow
us to determine the sex of a chick from a single feather. The next step will be
to use this to discover the sex ratios of skua chicks and the sex of chicks that
kill鈥攖he crucial information we need to show whether siblicide in polar
skuas is adaptive or not.

Whatever the outcome of this research, our work will be a success if it
persuades biologists to question the assumption that every facet of life has
evolved to enhance survival. As the renowned evolutionary biologist George
Williams warned almost 35 years ago: 鈥淸Adaptation] should be used as a last
resort. It should not be invoked when less onerous principles, such as those of
physics and chemistry, or that of unspecific cause and effect, are sufficient
for a complete explanation.鈥�