More than Kin and Less than Kind: the Evolution of Family Conflict

  • Douglas W. Mock
Harvard University Press: 2004. 352 pp. $27.95 0674012852 | ISBN: 0-674-01285-2
Credit: CHRISTIAN DARKIN

The private life of the Verreaux's (or black) eagle, a spectacular raptor of the drier parts of Africa, does not exactly embody Victorian family values. The female nearly always lays two eggs, a few days apart, which hatch into two chicks, one a little bigger than the other. As soon as the smaller hatches it is turned upon by its sibling, which attacks it viciously, the assault almost invariably leading to death. In one nest that was continuously watched by the ornithologist Valerie Gargett, the bigger chick pecked its nest-mate 1,569 times in its brief 72-hour existence. Throughout this period, the parents did nothing to intervene. Ornithologists used to call this phenomenon ‘cainism’ but today use the less poetic, but less sexist, term ‘siblicide’. Obligate siblicide occurs in some other raptors as well, and in species of boobies, penguins, egrets and pelicans. Less extreme forms of siblicide are much more widespread in birds: usually the parent produces a hierarchy of chicks and, when conditions are less than perfect, the runt dies, either because its siblings passively deprive of it food, or through more direct attacks.

In More than Kin and Less than Kind, Douglas Mock surveys sibling conflict in birds, and gives a more limited number of examples from other groups of animals and plants. He charts how biologists' views of siblicide have evolved over the years: it has been considered simply an aberration, and also a means of population control for the benefit of the species. The latter explanation became untenable when evolutionists in the 1960s realized the importance of thinking about how selection operates at the individual level. David Lack's view prevailed: parents overproduce young so that they can capitalize on bumper food years when everyone survives, with siblicide being an efficient mechanism to adjust litter size in poor years.

This still does not explain obligate siblicide, which for a long time was controversial. But now there is a general consensus that obligate siblicide is an insurance mechanism to guard against egg infertility or the very early death of a chick: in the species in which it occurs, an infertile single egg may mean a completely wasted breeding season.

Although extreme forms of sibling conflict receive most attention, Mock also discusses more moderate forms of squabbling (informed, perhaps, by his own experience as the youngest of four brothers), as well as conflicts between parents and offspring, and between mothers and fathers.

Mock has a lively and engaging style, and the skill to explain complex ideas from kin selection and related fields intelligibly without being patronizing. He is best known for his work on siblicide in egrets and herons, and his accounts on the one hand of long hours of fieldwork with the birds steadfastly refusing to behave as his hypotheses predict, and on the other of sudden insights from watching family interactions in the nest, are both inspiring and ring true.

The one area where I think he goes astray is parent–offspring conflict, which he views as an interesting theory that, with a few notable exceptions, has failed to produce much insight. The theory is based on Robert Trivers' argument that natural selection operates differently on genes expressed in offspring and parents. Mock argues that in a “really compelling case” of parent–offspring conflict, selection would produce the optimum outcome for offspring with a corresponding reduction in parental fitness. I think this is far too narrow an interpretation, and results from a failure to distinguish between the genetic battleground identified by Trivers, and the resolution of the conflict that we see in nature.

For example, it has been argued that the presence of parent–offspring conflict means that the only evolutionarily stable way in which young can communicate their requirements for food to their parents is by using costly signals. This would explain why the begging calls of baby birds may be energetically expensive to produce, or risk attracting predators. If true, the theory has told us something new about a situation where the optimum outcome for parents is achieved, though at a cost. Mock says little about this type of honest signalling by offspring, and in places is, I think, unfairly dismissive. This is a shame, as these ideas can explain many of the same phenomena as sibling conflict, and the two processes almost certainly operate together. Behavioural ecologists such as Mock have pointed to the difficulty of testing honest-signalling theory, but I think this should be viewed as a challenge rather than a reason to dismiss it.

With this one caveat, Mock has done a superb job in bringing a large area of contemporary behavioural ecology to both a biological and a general audience. It will stimulate a new generation of fieldworkers and inspire theoreticians to try to understand the complex interplay of natural selection acting simultaneously on different members of the family. For a general reader, it will explain how modern evolutionary theory is tested in the field and give a real flavour of the life a field biologist. It deserves to be read by everyone interested in the evolution of family life.