Chapter 11. Kin Selection and Social Behavior • Interactions between individuals can have 4 possible outcomes in terms of fitness gains for the participants.
Kin Selection and Social Behavior • Cooperation (mutualism): fitness gains for both participants. • Altruism: instigator pays fitness cost, recipient benefits. • Selfishness: instigator gains benefit, other individual pays cost. • Spite: both individuals suffer a fitness cost.
Kin Selection and Social Behavior • No clear cut cases of spite documented. • Selfish and cooperative behaviors easily explained by selection theory because they benefit the instigator.
The puzzle of altruism • Altruism is the difficult one to explain because the instigator pays a cost and another individual benefits. • Hard to see how selection could favor an allele that produces behavior benefiting another individual at the expense of the individuals bearing the allele.
The puzzle of altruism • For Darwin altruism presented a “special difficulty, which at first appears to me insuperable, and actually fatal to my whole theory.” • Darwin suggested however that if a behavior benefited relatives, it might be favored by selection.
The puzzle of altruism • W.D. Hamilton (1964) developed a model that showed an allele that favored altruistic behavior could spread under certain conditions.
Coefficient of relatedness • Key parameter is the coefficient of relatedness: r. • r is the probability that the homologous alleles in two individuals are identical by descent.
Calculating r • Need a pedigree to calculate r that includes both the actor and recipient and that shows all possible direct routes of connection between the two. • Because parents contribute half their genes to each offspring, the probability that genes are identical by descent for each step is 50% or 0.5.
Calculating r • To calculate r one should trace each path between the two individuals and count the number of steps needed. Then for this path r = 0.5 (number of steps) • Thus, if two steps r for this path = 0.5 (2) = 0.25. • To calculate final value of r one adds together the r values calculated from each path.
Hamilton’s rule • Given r the coefficient of relatedness between the actor and the recipient, Hamilton’s rule states that an allele for altruistic behavior will spread if • Br - C >0 • Where B is benefit to recipient and C is the cost to the actor. Unit of measurement for B and C is surviving offspring.
Hamilton’s rule • Altruistic behaviors are most likely to spread when costs are low, benefits to recipient are high, and the participants are closely related.
Inclusive fitness • Hamilton invented the idea of inclusive fitness. Fitness can be divided into two components: • Direct fitness results from personal reproduction • Indirect fitness results from additional reproduction by relatives, that is made possible by an individual’s actions.
Kin selection • Natural selection favoring the spread of alleles that increase the indirect component of fitness is called kin selection.
Alarm calling in Belding’s Ground Squirrels • Giving alarm calls alerts other individuals but may attract a predator’s attention. • Belding’s Ground Squirrels give two different calls depending on whether predator is a predatory mammal (trill) or a hawk (whistle; Sherman 1985).
Is alarm calling altruistic? • Sherman and colleagues observed 256 natural predator attacks. • In hawk attacks whistling squirrel is killed 2% of the time whereas non-whistling squirrels are killed 28% of the time. • Calling squirrel appears to reduce its chance of being killed.
Belding’s Ground Squirrels • In predatory mammal attacks trilling squirrel is killed 8% of the time and a non-trilling squirrel is killed 4% of the time. • Calling squirrel thus appears to increase its risk of predation. • Whistling appears to be selfish, but trilling altruistic.
Belding’s Ground Squirrels • Belding’s Ground Squirrels breed in colonies in Alpine meadows. • Males disperse, but female offspring tend to remain and breed close by. Thus, females in colony tend to be related.
Belding’s Ground Squirrels • Sherman had marked animals and had pedigrees that showed relatedness among study animals. • Analysis of who called showed that females were much more likely to call than males.
Belding’s Ground Squirrels • In addition, females were more likely to call when they had relatives within earshot.
Belding’s Ground Squirrels • Relatives also cooperated in behaviors besides alarm calling. • Females were much more likely to join close relatives in chasing away trespassing ground squirrels than less closely related kin and non-kin.
Belding’s Ground Squirrels • Overall, data show that altruistic behavior is not randomly directed. It is focused on close relatives and should result in indirect fitness gains.
Helping behavior in birds:White-fronted Bee-eaters • In a large number of birds young that are old enough to breed on their own instead help their parents rear siblings. • Helpers assist in nest building, nest defense and food delivery.
Helping behavior in birds:White-fronted Bee-eaters • Helping usually occurs in species where breeding opportunities are limited: territories or nest sites are hard to acquire. • Young make the best of a bad job by remaining home to assist their parents.
Helping behavior in birds:White-fronted Bee-eaters • Steve Emlen et al. studied white-fronted bee-eaters intensively in Kenya. • Nest in colonies of 40-450 individuals. Groups of relatives (clans) defend feeding territories in vicinity of colony.
Helping behavior in birds:White-fronted Bee-eaters • First year birds that opt to help can choose among many relatives when deciding whom to help. • Bee-eaters conform to predictions of Hamilton’s rule.
Coefficient of relatedness determines whether a bee-eater helps or not. • Also, bee-eaters choose to help their closest relatives.
Nonbreeders in clan that are not relatives (birds that have paired with members of the clan) are not related to offspring being reared and are much less likely to help than relatives.
Assistance of helpers is of enormous benefit to parents. More than 50% of bee-eater young starve before leaving the nest. • On average, presence of each helper increases number of offspring successfully reared to fledging by 0.47. Thus, there is a clear inclusive fitness benefit.
Kin selection and cannibalism in tadpoles • Spadefoot toad tadpoles come in two morphs. • Typical morph is omnivorous mainly eats decaying plant material. • Cannibalistic morph has bigger jaws and catches prey including other spadefoot tadpoles.
Kin selection and cannibalism in tadpoles • Pfennig (1999) tested whether cannibals discriminate between kin and non-kin. • Placed 28 cannibalistic tadpoles in individual containers. Added two omnivorous tadpoles (tadpole had never seen before) to each container. One was a sibling, the other non-kin.
Kin selection and cannibalism in tadpoles • Pfenning waited until cannibal ate one tadpole, then determined which had been eaten. • Found that kin were significantly less likely to be eaten. Only 6 of 28 kin were eaten, but 22 of 28 non-kin.
Kin selection and cannibalism in tadpoles • Pfennig also studied tiger salamanders whose tadpoles also develop into cannibalistic morphs. • Kept 18 cannibals in separate enclosures in natural pond. To each enclosure added 6 siblings and 18 non-kin typical morph tadpoles.
Kin selection and cannibalism in tadpoles • Some cannibals discriminated between kin and non-kin. Others did not. • Degree of relatedness to siblings = 1/2
Kin selection and cannibalism in tadpoles • Thus, by Hamilton’s rule discrimination in favor of kin favored if B(r) - C > 0 • Benefit estimated by counting number of siblings that survived. Siblings of discriminating cannibals twice as likely to survive as siblings of non-discriminating cannibals.
Kin selection and cannibalism in tadpoles • Benefit thus approximately 2. • Cost assessed by evaluating effect of not eating siblings by comparing growth of discriminating and non-discriminating cannibals. No difference in growth rates. Cost then estimated as close to 0.
Kin selection and cannibalism in tadpoles • By Hamilton’s rule discrimination should be favored because 2(1/2) - 0 = 1 which is >0.
Altruistic sperm in wood mice • Moore et al. have demonstrated altruistic behavior by sperm of European wood mice. • Females highly promiscuous. Males have large testes and engage in intense sperm competition with other males.
Altruistic sperm in wood mice • Wood mice sperm have hooks on their heads. And connect together to form long trains of sperm that can include thousands of sperm. • Swimming together sperm travel twice as fast as if they swam separately.
Altruistic sperm in wood mice • To fertilize egg, train must break up. • To break up train many sperm have to undergo acrosome reaction releasing enzymes that usually help fertilize an egg.