Hypothesis: Flocking behavior confers antipredator benefits.

METHOD Release hawks near pigeon flocks of different sizes. Observe whether a hawk captures a pigeon.

METHOD Release hawks near pigeon flocks of different sizes. Observe whether a hawk captures a pigeon.

Pegeon Size Hawk
Conclusion: Flocking behavior provides protection against predation.

53.10 Groups Provide Protection from Predators The larger a flock of pigeons, the greater the distance at which they detect an approaching hawk, and the less likely the hawk is to succeed in capturing a pigeon.

written records. The diseases of wild animals are not as well known, but they too are spread mostly by close contact.

In some species, parents care for their offspring

The most widespread form of social system is the family, an association of one or more adults and their dependent offspring. If parental care lasts a long time, or if the breeding season is longer than the time it takes for offspring to mature, adults may still be caring for younger offspring when older offspring reach parenting age. These older offspring may help their parents care for their younger siblings. Among birds, many communal breeding systems probably evolved by this route. Florida scrub jays live all year on territories, each of which contains a breeding pair and up to six helpers that bring food to the nest. Nearly all helpers are offspring from the previous breeding season that remain with their parents.

Most mammals also evolved social systems via an extended family. In simple mammalian social systems, solitary females or male-female pairs care for their young. As the period of parental care increases, older offspring are still present when the next generation is born, and they often help rear their younger siblings. In most social mammal species, female offspring remain in the group in which they were born, but males tend to leave, or are driven out, and must seek other social groups. Therefore, among mammals, most helpers are females.

Raising a family involves tremendous costs for parents and helpers. Animals who provide food for their young may sacrifice food for themselves, and protecting the young may involve the animal putting itself in danger. Acts that benefit another individual at a cost to the performer are altruistic acts. How can behavior that inflicts a cost on the performer evolve?

Altruism can evolve by means of natural selection

Altruistic behaviors exhibited by parents toward their offspring are easily understood in terms of close genetic relat-edness. Genetic relatedness extends beyond the parent-offspring relationship, allowing an individual to influence its fitness in two different ways. First, it may produce its own offspring, contributing to its own individual fitness. Second, it may help relatives (who bear some of the same genes) in ways that increase their fitness.

Because relatives are descended from a common ancestor, they are likely to bear some of the same alleles. In diploid organisms, two offspring of the same parents share on average 50 percent of the same alleles; an individual is likely to share 25 percent of its alleles with its sibling's offspring. Therefore, by helping its relatives, an individual can increase the repre sentation of some of its own alleles in the population. This process is called kin selection. Together, individual fitness and fitness gained through helping non-descendent kin determine the inclusive fitness of an individual. Occasional altruistic acts may eventually evolve into altruistic behavior patterns if the benefits of increasing the reproductive success of relatives exceed the costs of decreasing the altruist's own reproductive success.

Many social groups consist of some individuals that are close relatives and others that are unrelated or distantly related. Individuals of some species recognize their relatives and adjust their behavior accordingly. White-fronted bee-eaters are African birds that nest colonially. Most breeding pairs are assisted by nonbreeding adults that help incubate eggs and feed nestlings. Nearly all of these helpers assist close relatives (Figure 53.11). When helpers have a choice of two nests at which to help, about 95 percent of the time they choose the nest with the young more closely related to them.

Several other pieces of evidence suggest that the helping behavior of white-fronted bee-eaters evolved through kin selection. First, both males and females help to care for nestlings, but males help more often than females. Males remain in the social group in which they were born, but females join other social groups when they mature. Therefore, females typically live in social groups composed primarily of nonrelatives.

White Fronted Bee Eaters Are Altruists

Relationship to nestlings

Relationship to nestlings

53.11 White-Fronted Bee-Eaters are Altruists Bee-eaters that help to care for nestlings preferentially help close relatives.

Second, individual bee-eaters do not appear to gain anything in addition to inclusive fitness by helping—helpers do not gain experience that improves their performances when they become breeders. Finally, nests with helpers produce more fledglings than do nests without helpers, showing that helpers do increase the number of fledglings produced by their close relatives. Notice that all these patterns are consistent with the principle that bee-eaters behave in ways that improve their individual fitness, not in ways that benefit the species.

Eusociality is extreme social behavior

Species whose social groups include sterile individuals are said to be eusocial. This extreme form of social behavior has evolved in termites and many hymenopterans (ants, bees, and wasps). In these species, most females are workers that forage for the colony and/or defend it against predators, but do not reproduce. Workers may include soldiers with large, specialized defensive weapons (Figure 53.12), which may be killed while defending the colony. Only a few females, known as queens, are fertile, and they produce all the offspring of the colony.

Both genetic and environmental factors facilitate the evolution of eusociality. The British evolutionist W. D. Hamilton first suggested that eusociality evolved among the Hymen-optera because its members have an unusual sex determination system in which males are haploid but females are diploid. Among the Hymenoptera, a fertilized (diploid) egg hatches into a female; an unfertilized (haploid) egg hatches into a male.

If a female copulates with only one male, all the sperm she receives are identical because a haploid male has only one set

Eciton burchelli

53.12 Sterile Workers are Extreme Altruists Eusocial insect species contain classes of sterile worker individuals.These soldier army ants from Panama protect their colonies with their large, powerful jaws.

Eciton burchelli

53.12 Sterile Workers are Extreme Altruists Eusocial insect species contain classes of sterile worker individuals.These soldier army ants from Panama protect their colonies with their large, powerful jaws.

of chromosomes, all of which are transmitted to every sperm cell. Therefore, a female's daughters share all of their father's genes. They also share, on average, half of the genes they receive from their mother. As a result, they share 75 percent of their alleles on average, rather than the 50 percent they would share if both parents were diploid. Since workers are more genetically similar to their sisters than they would be to their own offspring, they can increase their fitness more by caring for their sisters than by producing and caring for their own offspring.

Eusociality may also be favored if establishment of new colonies is difficult and dangerous. Nearly all eusocial animals construct elaborate nests or burrow systems within which their offspring are reared. Naked mole-rats—the most eusocial mammals—live in underground colonies containing 70 to 80 individuals. The colony's tunnel systems are maintained by sterile workers. Breeding is restricted to a single queen and several kings that live in a nest chamber in the center of the colony. Individuals attempting to found new colonies are at high risk of being captured by predators, and most founding events fail. Thus, high predation rates, which favor cooperation among founding individuals, may facilitate the evolution of eusociality.

Inbreeding—the mating of individuals who are genetically related—can generate increased genetic relatedness within a group. Even if two parents are unrelated, but each is the product of generations of intense inbreeding, all of their offspring may be genetically nearly identical. Such offspring would increase their fitness by helping to rear siblings. Genetic similarity generated by inbreeding could explain the evolution of eusociality among the many hymenopteran species in which queens mate with many males and among termites and naked mole-rats, in which both sexes are diploid.

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