Social systems

Sexual reproduction in animals generally puts a heavier load on the female side. In mammals, however, this bias in cost of reproduction is far more extensive due to the period of gravidity (pregnancy) and the subsequent lactational period, both of which cannot be taken over by a male. Consider a female mouse suckling six young: shortly before weaning, each young has about half her weight. Thus, she has to nourish and support 400% of her body weight! There is an even higher evolutionary pressure on mammalian females in at least two aspects: females have to forage more intensively, and more effectively, in order to cover their energetic and nutri-

The southern opossum (Didelphis marsupialis) covers itself with a foul saliva that discourages predators before it plays dead. (Photo by Joe McDonald. Bruce Coleman, Inc. Reproduced by permission.)

A polar bear (Ursus maritimus) rolls in the snow in Cape Churchill, Canada. Polar bears keep their coats clean by swimming and rolling in the snow. Clean coats keep the bears warmer. (Photo by Gary Schultz. Bruce Coleman, Inc. Reproduced by permission.)

tional demands of reproduction. Secondly, as each young or litter forms a rather high proportion of her total lifetime reproduction, she is on heavy demand to select her potential mating partner. Male quality is thus very important, and female mate choice can be expected to be even more careful and elaborate than in other vertebrates.

Animal social systems are supposed to evolve in the context of providing each individual with a so-called "optimal compromise" regarding the demands of foraging, predator avoidance, reproduction, and sheltering. We have to accept the fact that a social group (or other social unit above the individual level) is not some sort of super-organism with its own demands and evolutionary history. Instead, each social unit is brought into existence simply and solely if it is catering to the demands of the individual members, and will remain stable only as long as all of its members do not have any option that, regarding this compromise, provides them with better conditions in total. This does not mean that the animals have to be aware of these choices and options. For natural selection to work, it is sufficient if they behave, based on at least some hereditary components of behavior, in the "correct" way, and their reward will likely be to have more, more viable, or otherwise advantageous young. This is the concept of Darwinian fitness—everyone has to put as many bearers of their own genetic heritage into the next generation, and the one with most young reared successfully into the next generation's gene pool is the fittest. What we as humans use to colloquially call fitness (as in going to a fitness studio) is, in the terminology of behavioral ecology, called resource-holding power (RHP), the possibility to defend resources such as a territory, a mate,

Male lions fight for territory and supremacy. (Photo by Karl Ammann. Bruce Coleman, Inc. Reproduced by permission.)
An Australian sea lion (Neophoca cinerea) juvenile acknowledges an adult. (Photo by Hans Reinhard/OKAPIA/Photo Researchers, Inc. Reproduced by permission.)

or food, and provide those resources to one's potential social or mating companions.

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