Chapter 12: The Evolution of Social Behavior

multilevel selection

A conceptulatization of adaptive evolution in which fitness is assigned to groups, and is a function of group composition. More broadly, an evolutionary process in which selection acts at multiple levels.

haplodiploidy

A reproductive system in which males are haploid and develop from unfertilized eggs, which females are diploid and haploid and all loci are linked.

Which of the following statements describe a "greenbeard" allele?

A theoretical evolutionary mechanism proposed by Richard Dawkins that may occur in slime molds. An allele that causes development of a trait, recognition of the trait in others, and altruistic behavior toward others that carry the trait. A theoretical evolutionary mechanism that could drive an allele for altruism to high frequency without the involvement of organism-level kin selection or reciprocal altruism.

reciprocity

An exchange of fitness benefits, separated in time, between two individuals resulting in a net gain for both.

inclusive fitness

An individual's total fitness; the sum of its indirect fitness, due to reproduction by relatives made possible by its actions, and direct fitness, due to its own unaided reproduction.

Hamilton's rule

An inequality that predicts when alleles for altruism, should increase in frequency.

mutually beneficial

An interaction between individuals resulting in increased fitness for both the actor and the recipient.

selfish

An interaction in which the actor gains fitness and the recipient loses fitness.

spiteful

An interaction resulting in a loss of fitness for both actor and recipient.

altruism

Behavior that decreases the fitness of the actor and increases the fitness of the recipient.

cooperative behavior

Behavior that is beneficial to both actor and recipient.

selfish behavior

Behavior that is beneficial to the actor, and costly to the recipient.

spiteful behavior

Behavior that is costly to both actor and recipient.

altruistic behavior

Behavior that is costly to the actor, and beneficial to the recipient.

All of the following describe real experimental results. However, one does not support the predictions of Hamilton's rule. Which one? A. Coots have evolved discriminatory mechanisms to detect and reject eggs that are not their own. B. Wood mouse sperm will sacrifice themselves to assist a "sibling" sperm to fertilize an egg. C. Prairie dogs are more likely to give alarm calls to a simulated badger attack if they have kin living nearby. D. Twenty percent of "in-law" bee-eaters that are not related to other birds at the nest will help raise the young anyway.

D

Heinsohn and Packer found that territory defense in female African lions defied the simple predictions of both reciprocity and kin selection theory based on which of the following? A. The willingness of pride females to assist the "leader" in pride defense was independent of the degree of relatedness to other pride members. B. Some females always led and some always lagged in defense. Although the behavior of leading females suggested that they recognized the laggards, they did nothing to threaten laggards or to withhold benefits. C. Some females adopted a conditional strategy, helping immediately in some cases and lagging in other cases. D. The first two choices are correct.

D

cooperative breeding

Describes a situation in which individuals go without reproducing themselves, and instead assist in the reproduction of others.

communal breeding

Describes a situation in which several adults cooperatively rear a shared brood to which all have contributed offspring.

eusocial

Describes a social system characterized by overlapping generations, cooperative brood care, and specialized reproductive and non reproductive castes.

identical by descent

Describes alleles, within a single individual or different individuals that have been inherited from the same ancestral copy of the allele.

cooperation

Describes behavior that is beneficial or costly to the actor, beneficial to the recipient, and selected for at least in part because the recipient benefits.

direct fitness

Fitness that an individual attains from his or her own reproduction, without help from relatives.

indirect fitness

Fitness that is due to increased reproduction by relatives made possible by the focal individual's actions.

Now consider weaning conflict in more mathematical terms. Choose the correct ratio in the following description of weaning conflict: When B and C are calculated from the mother's perspective, weaning conflict for families of full siblings should begin when B/C ratio is [1/4, 1/2, 1, 2, 4] and end when B/C ratio is [1/4, 1/2, 1, 2, 4]. Weaning conflict for families of half siblings should begin when B/C ratio is [1/4, 1/2, 1, 2, 4] and should end when B/C ratio is [1/4, 1/2, 1, 2, 4].

Full siblings: Start at 1, end at 1/2. Half siblings: Start at 1, end at 1/4. Note that mothers always initiate the period of weaning conflict when B/C (from their perspective) is 1; that is, benefit and cost are equal.

Choose the correct ratio in the following description of weaning conflict: When B and C are calculated from the offspring's perspective, weaning conflict for families of full siblings should begin when B/C ratio is [1/4, 1/2, 1, 2, 4] and end when B/C ratio is [1/4, 1/2, 1, 2, 4]. Weaning conflict for families of half siblings should begin when B/C ratio is [1/4, 1/2, 1, 2, 4] and should end when B/C ratio is [1/4, 1/2, 1, 2, 4].

Full siblings: Start at 2, end at 1. Half siblings: Start at 4, end at 1. Note that offspring should finally accept the end of weaning when B/C (from their perspective) is 1; that is, benefit and cost are equal.

Jabberwocks can produce a maximum of four surviving offspring over their lifetimes. A group of Jabberwock cousins are surprised in the woods by a hunter. One cousin has the opportunity to save the others, but only by sacrificing her life. Assume that none of the cousins (including the actor) has had children and that all cousins (actor and recipients) would have four children if they survived. According to Hamilton's rule, the actor should sacrifice her life to save the others if the number of cousins she can save is greater than _____.

Given the conditions of the problem, B and C are both 4; r=1/8. With 8 recipients, the total benefit to the recipients is 8*4*1/8 = 4 and Br-C = 0. More generally, the number of recipients should be equal to or greater than the reciprocal of the coefficient of relationship. This result was anticipated by J.B.S Haldane decades before Hamilton quantified the results. Haldane is reported to have claimed that he would sacrifice his life for two siblings or eight cousins.

What is Hamilton's rule? What are its three mathematical terms, and how are they calculated?

Hamilton's rule states that an allele for altruistic behavior should spread if Br - C > 0. B is the benefit to the recipient, and C is the cost to the actor, both measured as number of surviving offspring. r is the coefficient of relatedness; it is the probability that two homologous alleles in actor and recipient are identical by descent. (Equivalently, it is also the percentage of the genome that the actor and recipient are likely to share.) r is calculated by tracing each possible pathway of relatedness between two individuals; every step between parent and offspring represents a 0.5 probability of any allele being shared. The probability for a whole path is (0.5)<n where n is the number of steps. Finally, the probabilities for each separate path are summed. (See pp. 2-4 for a review of this topic.)

What is the evidence that whistling is selfish and trilling is altruistic in Belding's ground squirrels? Was similar evidence presented for the black-tailed prairie dogs? What patterns in the data from the black-tailed prairie dogs suggest that alarm-calling is altruistic in that species too? How could you verify whether alarm-calling truly is altruistic in the black-tailed prairie dogs?

In Belding's ground squirrels, observations of actual attacks by predators documented that trilling doubles the squirrel's risk of being killed by the predator, while whistling decreases the squirrel's risk. Therefore trilling is definitely altruistic, and whistling is not, in this species. (It is important to note that not all alarm-calling behavior is automatically altruistic.) Similar data was not presented for black-tailed prairie dogs, and thus we cannot say for sure that this behavior is altruistic in this species. However,the patterns of the data shown in Figures 12.2 and 12.3 show that squirrels are more likely to alarm-call when they have kin nearby. This strongly suggests (but does not prove) that alarm-calling is altruistic.

kin selection

Natural selection based on indirect fitness gains.

greenbeard effect

Occurs when an allele causes individuals carrying it to both recognize and be recognized by other carriers, and also to behave altruistically toward them.

Pied flycatchers are small birds that, like many small birds, often "mob" larger avian predators such as hawks, owls, and crows. During mobbing, several of the smaller birds harass the larger predator repeatedly until the predator leaves the area. Indrikis Krams and colleagues (2006) recently studied mobbing behavior of pied flycatchers presented with a stuffed Tawny Owl, a common predator of pied flycatcher nests. The stuffed owl was placed near and "looking at" one pied flycatcher nest (nest A), and within sight of another nest (nest B). In 17 control trials, the resident birds of nest A always began mobbing the owl, and birds from nearby nest B always assisted, joining the mobbing effort. The owl was later moved to nest B. Once again, the birds from nest B always mobbed the owl, and neighbor birds from nest A always assisted. Krams et al. then repeated this experiment on different pied flycatchers, except that this time, the first time the owl was presented, the neighbor birds in nest B were captured and held in a cage, preventing them from assisting. In 17 trials at different nests, the nest A birds always tried to mob the owl, but this time they were unassisted. The nest B birds were then released and the owl was moved to nest B. The nest B birds always mobbed the owl, but this time, only 4 of the 17 pairs of nest A birds assisted. Refer to the information above regarding pied flycatchers to answer these questions. Part I: Is assisting an altruistic behavior? How can you tell, or what further evidence would you need? How many hypotheses can you think of for why mob-assisting behavior has evolved in pied flycatchers? How could you test your ideas? Part II: Refer to the information above regarding pied flycatchers to answer these questions. Why did the nest A birds fail to assist their neighbors in the second experiment? Does this experimental result suggest which of your hypotheses might be correct? What else can you guess about pied flycatchers and their social systems?

Part I: Assisting may be altruistic or cooperative, depending on whether the cost outweighs the benefit for the assisting birds. To investigate this we would need to know whether the birds that assist are taking a risk (i.e., a cost), by observing many natural mobbing events and seeing if the smaller birds are ever killed. (It turns out that mobbing, and assisting, are indeed risky; the predator sometimes catches the mobbing individuals. It is riskiest to be the first bird to start mobbing.) If it is a cooperative behavior (benefit greater than cost for the assisting bird), it could have evolved through simple natural selection. If it is an altruistic behavior (cost greater than benefit for the assisting bird), it might have evolved through kin selection or reciprocal altruism. A third possibility for evolution of altruism, the greenbeard effect, is extremely rare in nature. Therefore, the first question to ask would be whether the birds are related. (It turns out they are usually not.) Part II: The fact that the nest A birds failed to help nest B birds in the second trial, but always helped them in the first trial, indicates that this is probably a case of reciprocal altruism. A hallmark of reciprocal altruism is that "cheaters" (birds that don't assist when their help is needed) are remembered and punished - that is, help is withheld later. Knowing the results of this one experiment, we can also predict that pied flycatchers probably have other traits typical of species that have reciprocal altruism. They probably have a fairly complex social system in which they interact with the same individuals repeatedly. They also probably have good memories, and should be capable of recognizing and remembering individuals. (All of these turn out to be true.)

Menopause is an unusual feature seen in humans and some whales that have matrilineal social systems (family groups led by older females), such as pilot whales and orcas. Menopause involves a complete and sudden cessation of reproduction function relatively early in a healthy female's life. In humans and in these whales, females completely cease reproduction about one-half to two-thirds of the way through their life spans, but continue to assist siblings, offspring, and grand-offspring for the rest of their lives. (In most other animals, females continue to reproduce as long as they are alive.) Part I: Refer to the information above regarding menopause to answer these questions. What must B and C be for an older female to cease reproduction and instead care for her grand-offspring? Part II: Refer to the information above regarding menopause to answer this question. Can menopause in these two species be considered a form of eusociality - i.e. can the older females be considered a caste of nonreproductive workers? Part III: Refer to the information above regarding menopause to answer this question. What traits of a species might favor evolution of menopause?

Part I: Females are related to grandoffspring by r = 1/4 and to offspring by r= 1/2. For menopause to evolve, the benefit in helping grandoffspring must be twice as high as the cost in foregoing the chance to produce more offspring. This can occur if (1) an older female's chances of raising offspring to independence are fairly low, and (2) the female can offer valuable care to her grand-offspring that substantially increases their chances of survival. If menopause evolves, the age at which it should occur is: average female lifepsan minus the average age for young to attain independence. Part II: Many answers are possible. Both species do have the other two requirements for eusociality - shared care of young, and overlapping generations. Menopause has not traditionally been considered as eusociality. However, evolutionary biologists differ about whether the sterile caste must be sterile for their whole lives. Some consider that being reproductively sterile for part of ones' life is good enough to be called eusociality, and that humans should be included in the list of eusocial vertebrates. Part III: Many answers are possible. Evolution may favor evolution of menopause in other long-lived species in which (a) the young have an extremely long period of care, and (b) older individuals can make a large difference in survival of younger kin, which seems to be the case in certain intelligent species with complex social systems. Other species with some of these traits include African elephants and certain desert antelopes in which only the older individuals know the locations of very distant food and water sources.

Should sibling rivalry be more or less common in species that are monogamous or polygamous? Why?

Sibling rivalry should, in theory, be more common in species that are polygamous, since most litter-mates or nest-mates will be only half-siblings instead of full siblings.

Simpson's paradox

The appearance of a trend within subsets of a population that is reversed when the subsets are combined.

relatedness

The genetic similarity between individuals. May be calculated as the probability that gene copies in the two individuals are identical by descent or as a function of the allele frequencies in the two individuals relative to that in the population as a whole.

Summarize the logic behind the haplodiploidy hypothesis of eusociality. What is the evidence in favor of and against this hypothesis? Describe two other hypotheses for the evolution of eusociality.

The haplodiploidy hypothesis is based on the proposition that females are more closely related to sisters than to their own potential offspring. In this case, natural selection should favor the evolution of sterile female workers who raise sisters rather than raising their own young. In favor of this hypothesis, eusociality is particularly common in haplodiploid taxa, and female workers do manipulate colonies toward a female-biased sex ratio. However, not all haplodiploid species are eusocial, and many eusocial species are not haplodiploid. The major evidence against the hypothesis, however, is experimental evidence that relatedness among females is actually much lower than the hypothesis assumes. Because most colonies have multiple fathers and often multiple queens, most worker females are not more closely related to sisters than to their own offspring. Two alternative hypotheses are (1) eusociality may represent the best option for females in taxa that build complex nests and in which young require extended care. If solitary females have very little chance of establishing a viable nest on their own, raising siblings is probably the next best option. (2) Eusociality may evolve in taxa that live in extremely inbred colonies, such as naked mole-rats. (Note that this is similar to the haplodiploid hypothesis in its essential logic, i.e., r among colony members is proposed to be very high.)

List the three features of true eusociality. Name two types of insects, and two types of non-insects, that have true eusociality.

True eusociality includes: (1) overlap in generations between parents and offspring, (2) cooperative care of offspring, and (3) specialized castes of nonreproductive individuals. Eusociality is found the Hymenoptera (bees, ants, etc.), termites, and a few other orders such as plant bugs and one family of beetles. It also occurs in at least two non-insects: snapping shrimp and naked mole-rats.

eusociality

a social system involving overlap in generations, cooperative care of young, and nonreproductive individuals.

greenbeard effect

a theoretical model of a type of allele for altruistic behavior that could spread through a population via recognition of similar traits, without the involvement of kin selection or reciprocal altruism.

direct fitness

fitness resulting from personal reproduction.

indirect fitness

fitness resulting from reproduction of kin.

Which of the following correctly complete the statement, "Reciprocal altruism directed toward non-kin may evolve when _______________"?

individuals can recognize and remember other individuals

kin selection

natural selection that favors the spread of alleles that increase reproduction by related individuals.

inclusive fitness

total reproductive success of an individual, including reproduction by the individual and also reproduction by related kin.