Chapter 11 Kin Selection and Social Behavior Types of Social Interactions z Cooperation = mutualism {Fitness gains for both participants z Altruism {Fitness gain for recipient {Cost for actor z Selfishness {Actor gains {Recipient loses z Spite {Fitness loss for both participants 1 Prevalence of Altruism z Appears to be common in nature {Young macaws help parents raise their siblings instead of reproducing themselves {Human runs into a burning house to save a child z Darwin mentioned that altruism was a “special difficulty” for his theory z How can an allele that codes for altruism survive in the face of natural selection? Kin Selection and the Evolution of Altruism z William Hamilton developed a genetic model showing how an allele for altruistic behavior could persist z Coefficient of relationship, r {Probability that homozygous alleles in two individuals are identical by descent z Hamilton’s Rule {Altruistic behavior will spread if z Br – C > 0 • B = benefit to recipient • C = cost to actor Kin Selection and the Evolution of Altruism z Altruism will spread when benefits to recipient are great, cost to actor is small, and participants are closely related z Inclusive Fitness {Direct fitness results from personal reproduction {Indirect fitness results from reproduction by relatives 2 Kin Selection and the Evolution of Altruism z Indirect fitness accrues when relatives reproduce more than they would have without aid by actor z When natural selection favors the spread of alleles that increase indirect fitness, Kin Selection occurs z Kin selection explains many cases of apparent altruism {True altruism does not exist in nature Calculating Coefficients of Relatedness z Haldane was quoted in a bar: “I would lay down my life for the sake of two brothers or eight cousins” z Perform path analysis to assess relatedness {Parents related to offspring 1/2 {Full siblings related 1/2 {Half siblings related 1/4 {Cousins related 1/8 3 Alarm Calling in Belding’s Ground Squirrels z When predators approach individuals give an alarm call. z A trill in response to mammals and a whistle in response to hawks z 2% the whistler is captured while nonnonwhistlers are captured 28% z The squirrel raising the alarm reduces its own chances of capture by calling z Whereas 8% are killed when trill is giving compared to 4% nonnon-trill. z Whistles are selfish, trills are genuinely altruistic Alarm Calling in Belding’s Ground Squirrels z Females are more likely to give alarm calls than males z Mothers, daughters, and sisters were more likely to assist each other chasing trespassers off their territories than unrelated individuals 4 WhiteWhite-Fronted Bee Eaters z Young adults forgo breeding to help their parents raise their siblings {Nest building, nest defense, food delivery, incubation z Helping at the nest usually found in species where breeding opportunities are limited {Best of a bad job strategy z Bee eater coefficient of relatedness determines if they will help WhiteWhite-Fronted Bee Eaters WhiteWhite-Fronted Bee Eaters z Presence of helpers increases parental success by 0.47 fledglings 5 Kin Selection in Other Contexts z Cannibalistic tadpoles (Spadefoots (Spadefoots toads) {2 morphs zNormal (omnivorous, generally eat plant mat. zCannibalistic (enlarged jaw muscles) {28 Can in sep. containers zShared with 2 omniv. omniv. Never seen before (1 full sib, 1 not related. (should eat randomly) z6 out of 28 ate siblings Kin Selection in Other Contexts Kin Selection in Other Contexts 6 Altruistic Sperm Evolution of Eusociality z True sociality describes social systems with three characteristics {Overlap in generations {Cooperative brood care {Specialized castes of nonreproductive individuals z Will examine two groups {Hymenoptera {Naked mole rats 7 Haplodiploidy and Eusocial Hymenoptera z Hymenopterans exhibit most extreme form of eusociality z Millions of individuals per colony {Very few reproduce z How can this persist? z Hamilton proposed that haplodiploidy may be the reason {Females grow from fertilized eggs {Males grow from unfertilized eggs Haplodiploidy and Eusocial Hymenoptera z Because of disparity in chromosome number, sisters share 75% of genes {Parents and offspring share 50% {Females are better off rearing sisters than offspring z Testing haplodiploidy hypothesis {Workers should prefer to invest in sisters over brothers zRelated to sisters 3/4 zRelated to brothers 1/4 8 Haplodiploidy and Eusocial Hymenoptera z Testing haplodiploidy hypothesis {Workers should favor 3:1 sex ratio {Queens are equally related to sons and daughters and should favor 1:1 sex ratio {Conflict of interest between queens and workers {Sudstrom found that wood ant queens lays equal numbers of male and female eggs {Workers selectively killed male eggs prior to hatching {Workers win sex ratio battle Haplodiploidy and Eusocial Hymenoptera z Does haplodiploidy explain eusociality? eusociality? {Workers should favor production of sisters if all have the same father zHoneybee queens mate over 17 times when founding a colony zAverage r for workers is 1/3 zIn this case, workers are not more closely related to sisters than offspring zSometimes more than one queen founds nest zSome workers may not be related at all zMany eusocial species are not haplodiploid Using Phylogenies to Analyze Social Evolution z Hunt reconstructed a phylogeny of hymenopterans {All are haplodiploid {Few families are eusocial {Eusocial families not closely related {Eusociality must have evolved multiple times {Evolved in groups that build complex nests and have extended care for larvae 9 Using Phylogenies to Analyze Social Evolution z Phylogeny suggests that the primary agent favoring eusociality is not genetic z Best of a bad job hypothesis {Building a complex nest and caring for many larvae would be impossible for a female to do by herself {Must examine factors that affect B and C as well as r Facultative Strategies in Paper Wasps z Polistes paper wasps are not completely eusocial z Workers are not sterile z Females may reproduce on their own z Females pursue one of three strategies {Initiate own nest {Join a nest as a helper {Wait for a breeding opportunity 10 Facultative Strategies in Paper Wasps z Examine costs and benefits of each strategy z Nests founded by single females or multifemale groups {Single foundress nests less successful {Multifoundress nests more likely to be rebuilt if destroyed {Fights among foundresses determined by body size {Multifoundress nests grew fastest if large size difference of dominant female and subordinate helpers Facultative Strategies in Paper Wasps z Why would females join a coalition and help rear offspring that are not theirs? {Indirect fitness gains because usually related to foundress {Direct fitness gains if foundress dies and a subordinate inherits the nest {Costs and benefits of this strategy depend on female’s body size and coefficient of relatedness 11 Facultative Strategies in Paper Wasps z If females do not help found nests and wait, they may be able to adopt an alreadyalready-constructed nest {SitSit-andand-wait tactic {They leave their nest in spring and enter a dormant state until the following season to try to take over a new nest Facultative Strategies in Paper Wasps z Sociality is facultative in Polistes z Adaptive response to environmental conditions z Important conditions are female body size relative to competitors, coefficient of relatedness, and availability of nest sites z Genetic, social, and ecological factors important Naked Mole Rats z Live underground in huge nests in Africa z Colonies of 7070-80 members z Hairless, ectothermic, digest cellulose z All species are eusocial {Single queen {2-3 reproductive males {Workers are males and females z Castes change with age • First they tend young • Later they excavate tunnels • The oldest defend the nest 12 Naked Mole Rats z Not haplodiploid z Why are they eusocial? eusocial? {Highly inbred zAverage r for siblings is 0.81 zHighest coefficient of relatedness ever recorded in mammals {Still conflict among group members zWorkers more closely related to offspring than halfhalf-siblings Naked Mole Rats z Conflict among group members {Queens maintain control by physical dominance {If the workers slow their work pace the queen shoves them with her head {Afterwards the workers increase their work pace {Shoves are directed more often towards more distant relatives {Queen maintains eusociality by intimidation 13 ParentParent-Offspring Conflict z Parental care is a special case of kin selection z Even parents and offspring can have conflicts in costs and benefits z Weaning Conflict {Mothers begin to ignore or push young away near end of weaning period {Offspring will scream or attack mother {Fitness interests are not symmetrical ParentParent-Offspring Conflict z Weaning Conflict {Offspring are related to themselves r = 1 {Parents are related to offspring r = 0.5 {Parents are equally related to all offspring and should optimize their investment in each {Offspring demand unequal amount of parental investment 14 ParentParent-Offspring Conflict z Weaning Conflict {At start of nursing benefit of offspring relative to cost of parent {Ratio declines with time {Young demand more milk which increases parental cost {Young can start finding own food which decreases benefit {Mothers should stop producing milk when benefit to cost ratio reaches 1 ParentParent-Offspring Conflict z Weaning Conflict {By continuing to nurse, offspring devalue mother’s cost of care {Offspring should continue to try to nurse until benefitbenefit-cost ratio is 1/2 {Period between these stages is weaning conflict {Avoidance and aggressive behavior throughout this period {If halfhalf-siblings are produced then ratio should be extended to 1/4 15 ParentParent-Offspring Conflict z WhiteWhite-fronted Bee Eaters {Sons may set up territory or may help at their parental nest {Fathers coerce sons into helping by harassing sons as they attempt to set up territories zFathers prevent courtship feeding zHarassment is preferentially directed at sons to prevent them from breeding and coerce them to help at the nest ParentParent-Offspring Conflict z WhiteWhite-fronted Bee Eaters {16 of 47 observed harassment events resulted in successful recruitment to help {Why don’t sons resist more effectively? zSons are equally related to siblings and own offspring zParents are more closely related to each offspring than to grandchildren zHelpers add 0.47 offspring to parental success zNearly same as own offspring • Worth it to save harassment from father ParentParent-Offspring Conflict z Siblicide {In birds and mammals it is common to kill siblings {Seems maladaptive since r = 1/2 {Lougheed and Anderson studied boobies in Galá Galápagos Islands {Lay clutch of two eggs separated by 22-10 days {First chick often pushes younger from nest 16 ParentParent-Offspring Conflict z Siblicide {Masked boobies push second egg from nest immediately {BlueBlue-footed booby older chicks may reduce food intake during short food shortages to provide extra for sibling zDuring long food shortages they kill their siblings {What role do parents play? zParents should intervene to prevent death of any of their chicks ParentParent-Offspring Conflict z Siblicide {Reciprocal transplant experiment {Chicks more likely to die with masked booby nestmate {Chicks more likely to die with masked booby parents (surrogate or real) {BlueBlue-footed booby parents intervene but masked do not {Why there is a difference among species is not known 17 Reciprocal Altruism z What about cooperation among unrelated individuals? z Trivers proposed individuals will act altruistically if favor is later returned z Two conditions for reciprocal alturism to evolve: {Cost to actor must be smaller than or equal to benefit to recipient {Individuals that fail to reciprocate must be punished Reciprocal Altruism z Most likely to evolve when: {Each individual repeatedly interacts with same set of individuals {Many opportunities for altruism in an individual’s lifetime {Individuals have good memories {Potential altruists interact in symmetrical situations z Roughly equal benefits and costs 18 Reciprocal Altruism z Will evolve in longlong-lived, intelligent, social species with small group size, low dispersal rates, and mutual dependence in activities z Less likely to evolve in species with dominance hierarchies z Difficult to observe and quantify in nature Reciprocal Altruism z BloodBlood-Sharing in Vampire Bats {Social group of 88-12 females and their dependent offspring {Roost together and associate with each other daily {Average r between individuals in study group in Costa Rica was 0.11 {Vampire bats share blood meals {Hunting is difficult and individuals are only successful 6767-93% of time zPrey are wary 19 Reciprocal Altruism z BloodBlood-Sharing in Vampire Bats {Without eating for three nights, a bat will starve to death {Both degree of relatedness and degree of association were significantly related to probability of regurgitating blood {BloodBlood-sharing is not random but based on relatedness and hope of future reciprocity Reciprocal Altruism z Territory Defense in Lions {Females cooperate in defending young against infanticidal males, hunting prey, and defending pride’s territory {When females hear the roar of a female of another territory, they head to the area to attack {Some individuals always tend to lead the attack while others always seem to lag behind 20 Reciprocal Altruism z Territory Defense in Lions {Why are laggards tolerated? {It is not known but perhaps the laggards make up later by being exceptional hunters or good milk producers z Social interactions are very complex and much needs to be learned about them to fully understand their natural selection 21
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