4.2 Altruism -'1 I. 44 SECTION2 Evolutionand BehavioralEcology

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3 44 SECTION2 Evolutionand BehavioralEcology Figure 4.2 Male Hanuman langur monkeys in India, can act agressively especially toward young which they may killwhen taking over groups of females from other males. the male guarding the eggs will sire a new clutch with the female egg stabber and guard the new clutch (Ichikawa, 1995). In human society, individuals also rarely act for the good of the group and instead tend to act selfishly. This causes manyenvironmental problems, such as overgrazing and overfishing, a phenomenon known as the tragedy of the commons (See" Applied Ecology: The Tragedy of the Commons.") If selfishness is more common than group selection, then we must look for explanations of phenomena like altruism and the existence of nonbreeding castes. I II Altrui6tic behavior doe6 occur in nature and i6 often a660ciated with kin 6election. estrus much faster-in 9 months as opposed to 25-than they would if the cubs were spared-hastening the day when males can father their own offspring. A male's reproductive life in the pride is only two to three years before he in turn is supplanted by a younger, stronger male. Infanticide ensures that the male will father more offspring and that the genes governing this tendency will spread by natural selection (Bertram, 1975). The mothers of the deceased cubs derive no advantage from the infanticide, but being smaller than the males, they are powerless to stop them. This episode of infanticide is not an isolated example: Male langur monkeys in India (Fig. 4.2) also kill infants when they take over groups of females from other males. Sarah Hrdy (1977) showed how the elimination of nursing offspring allowed the mothers to mate with the new male and have his babies. In those rare instances in nature when males of a species take care of the offspring, females are known to commit infanticide, too. Male giant water bugs take care of egg masses, protecting them against predators (Fig. 4.3). Females sometimes attack the egg masses and stab them with her mouthparts. If enough eggs are destroyed, Figure 4.3 Male giant water bugs, Belostomatidae, protect egg masses laid by the female from predators by carrying them on their backs. -'1 I 4.2 Altruism Although natural selection favors individual rather than group selection, it is still common to see apparent cooperation. Animals of the same species groom one another, hunt communally, and give warning signals to each other in the presence of danger. How can this altruistic behavior be explained by natural selection? All offspring have copies of their parents' genes, so parents taking care of their young are, in the process, caring for copies of their own genes. Genes for altruism toward one's young will therefore become more numerous, because offspring have copies of those same genes. In meiosis, any given gene has a 50-percent chance of entering an egg or sperm. Thus, each parent contributes 50 percent of its genes to its offspring. The probability that a parent and offspring will share a copy of a particular gene is a quantity r called the coefficient of relatedness. By similar reasoning, brothers or sisters are related by an amount r = 0.5, grandchildren to grandparents by 0.25, and cousins to each other by (Fig. 4.4). The famed British evolutionary ecologist W. D. Hamilton (1964) was the first to realize the implication of relatedness for the evolution of altruism. If an organism can pass on its genes through parental care, it can also pass them on by caring for siblings, nieces, nephews, and cousins. So even if you have no offspring of your own, you can pass on your genes via nieces, nephews, or cousins. This means you have a vested interest in protecting the offspring of your brothers and sisters. The term inclusive fitness is used to designate the total copies of genes passed on through all relatives, nieces, nephews, and cousins, as well as sons and daughters. Selection for behavior that lowers an indi-

4 CHAPTER4 Group Selectionand IndividualSelection 45 grandparents grandparents 0.251, father I I mother 0.25 '--/ halfsib cousin granddaughter or grandson niece or nephew Figure 4.4 Degree of genetic relatedness to oneself in a diploid organism. Open circles represent completely unrelated individuals. vidual's own chance of reproduction, but raises that of a relative, is known as kin selection. Hamilton proposed that the conditions under which a gene for altruism will spread by kin selection can be quantified as follows: If the donor sacrifices C offspring, for which the recipient gains B offspring, then the gene causing the donor to act in this way will spread if rb - C > 0, where r is the coefficient of relatedness of donor to recipient. Therefore, your own life is equivalent in genetic value to two sisters, two brothers, or eight cousins. Kin selection. Many insect larvae-especially caterpillars-are soft-bodied creatures. They rely on having a bad taste or poison to deter predators and advertise this condition with bright warning colors. Caterpillars with such colors are known as aposematic and are in stark contrast to other caterpillars that match the color of the leaves they feed on. (We say these camouflaged caterpillars are cryptic.) Individuals may display altruism toward relatives because doing so maximizes their inclusive fitness. Applied Ecology The University of Santa Barbara ecologist Garrett Hardin wrote many famous ecological and environmental pieces, but perhaps none as famous as a 1968essay in Sciencecalledthe ''Tragedy of the Commons." In his article, Hardin invited us to imagine a "commons": a piece of pastoralland owned by no one, but used by all to graze their cattle. Each person seeks to maximize the profit from his or her herd of cattle. Say there are 10 ranchers, each with 100 cattle, on a piece of land that can support 1,000cattle. We can call1,000cattlethecarryingcapacityofthe land.oncethereare more than 1,000 cattle on the land, it deteriorates: The grass is overgrazed, and the pasture becomes of little value for grazing. To maximize profit, a rancher increases her herd by one animal. The profit to that individual is clear: a positive value of + 1for the one extra cow.but the herd is now over the limit that the land can sustain: The number of animals is now 1,001, not 1,000.However, the negative value of -1 cow is shared by all the ranchers. Each one of their cows suffers a little, an amount probably not noticeable. The logic is clear: Adding one cow is a good strategy for one individual rancher, with a positive value of + 1and a negative value of -0.1, since the negative is shared by all 10 ranchers. The problem is that soon all the ranchers realize the value of this strategy, so that all add cows to the commons, which then becomes overgrazed and useless. This is the tragedy of the commons. The upshot of this tale is that the benefits of the environment often accrue to single individuals, but the cost of using the environment is borne by the whole population. This disparity holds true for commons in agriculture, grazing land, and other habitats such as forests and the open sea. Exploitation of the land for the good of a few at the expense of the majority must stop if we are to prevent degradation of the environment.

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