In 1798 Malthus’ Essay on the principle of Population sowed a seed that was to germinate in Charles Darwin’s mind as the theory of natural selection, published in the Origin of Species in 1859. Malthus had noted that although a breeding pair generally produces a total of more than two offspring, many populations do not grow as fast as this would imply, if at all, Darwin was impressed by the subtlety of species’ adaptations and saw that individuals differed in the detail of their adaptation and thus their “fitness”, i.e. the perfection of adaptations to prevailing conditions. The variation between individuals arose from the mixing of genetic material involved in sexual reproduction, and from mutation, although the connection between these mechanisms and Darwin’s theory was not realized until 1900 when Mendal’s work of 1865 was rediscovered.
Since populations do not necessarily grow, many of the young born must die, and the variation between individuals facilitates selective death, allowing better adapted individuals to prosper. Traits which confer an adaptive advantage will thus spread, if they are heritable, since those who bear them will become an increasingly large proportion of the breeding population. Natural selection fashions individuals of succeeding generations to be ever better adapted to their circumstances. The characteristics of a species represent the sum of the actions of natural selection on similar individuals.
It is wrong to say that animals behave “for the good of the species”-rather, individuals are adapted to maximize their own fitness, which is equivalent to maximizing the number of their offspring which survive to breed. In fact, selection acts on the genetic material that underlies each individual’s traits, and so individuals actually behave in ways that promote the survival of the genes for which they are temporary vehicles-hence Oxford biologist Richard Dawkins’ now famous term, the “selfish gene.” Sometimes an individual helps its relatives, behaving in a way that appears detrimental to its own interests but is on balance beneficial to its genes, and hence improves its overall (or inclusive) fitness.
Individual mammals behave so as to maximize their reproductive success and since the pattern of reproduction is the core of society, adaptation to this end are reflected in the great variety of mammalian social systems. There is an asymmetry between males and females in this respect: sperm and cheaper to produce than ova, and of course only female mammals bear the costs of pregnancy and lactation. Therefore males may more readily maximize their reproductive success by mating with many females. Females, in contrast, can mother only a comparatively small number of young and so maximize their reproductive success by investing heavily in the quality of each offspring and, in particular, securing the very best (evolutionary fittest) father. Infanticide, as practiced by males of some primates and some carnivores, is a striking example of the lengths to which males will go to spread their genes at the expanse of their rivals’-the death of their rival males’ offspring brings lactating females back into the heat (estrus), in addition to disposing of potential competitors of the infanticidal male’s own progeny.
Females are a resource over which male mammals compete. The stringent natural selection that therefore operates between competing males is called sexual selection. It explains why many mammals are polygynous (one male mates with several females), few are polyandrous (one female mates with several males) and why males are often bigger than females (i.e. sexes are dimorphic). A big male defeats more rivals, secures more females in his harem and thus sires more offspring; if his size and prowess are passed to his sons, they will in turn become successful, dominant males. Therefore, females adapted to behave in a way which enables their sons to prosper will select only the biggest, most successful, males as mates.
The situation is different if the species’ niche is such that a male’s reproductive success is affected by the quality of his parental care rather than simply by the quality of his sperm; for example, amongst members of the dog family the survival of young depends on their father provisioning them with prey, and the male would find it impossible to provide for more than one or perhaps two litters. In that event natural selection favors monogamy and the size and appearance of male and female are less disparate. This explains why greater sexual dimorphism is associated with polygyny, but it is less obvious why sexual dimorphism is disproportionately marked among bigger species. One possible answer is that energy demands are relatively less on larger species and therefore they can afford to invest more heavily in bulging muscles and masculine armaments.