Questions About Social Behavior

Transcription

1 April 17: Altruism: Questions Questions About Social Behavior 1. Why live in groups? Costs: disease, competition, cannibalism, visibility to predators Benefits: more efficient foraging; defenses against predation These benefits are SELFISH benefits --i.e., individuals enjoy a net fitness benefit as a result of living in group 2. Altruism: given that you live in a group, why should you sacrifice yourself on behalf of others? 3. Social organization: for highly social species, in which the group functions as a cohesive unit, how should the society be organized so as to address the challenges to the survival and reproduction of the group?

2 April 17: Altruism: Cooperation Why Cooperate? How to account for evolution of apparently selfless behavior? (Can we do so in terms of selfish advantage?) Some examples: Allogrooming Helping at the nest (birds, social insects) Mate sharing (e.g., in polyandrous or polygynous mating systems) Alarm calls Cooperative defense against predator (e.g., mobbing) Rearing other animals young (e.g., cuckoo parasitism) Some terminology: donors versus recicipients Donor: individual that expressed helpful behavior, at some apparent cost to him/herself Recipient: individual that benefits from Donor s action Measuring costs and benefits: effects on mortality and reproduction

3 April 17: Altruism: Cooperation Hypotheses Concerning the Benefits of Cooperating Hypothesis Short-term Source of benefit to donor Bd/C d Manipulation < 1 None (recipient wins) Mutualism > 1 Short-term benefits from recipient's actions Reciprocity < 1 Direct long-term benefits from recipient's assistance in future Kin selection < 1 Indirect, genetic gains from helping relatives Brood parasitism, egg dumping

4 April 17: Altruism: Cooperation Hypotheses Concerning the Benefits of Cooperating Hypothesis Short-term Source of benefit to donor Bd/C d Manipulation < 1 None (recipient wins) Mutualism > 1 Short-term benefits from recipient's actions Reciprocity < 1 Direct long-term benefits from recipient's assistance in future Kin selection < 1 Indirect, genetic gains from helping relatives Mobbing, Selfish Herd effect, Dilution effect

5 April 17: Altruism: Cooperation Hypotheses Concerning the Benefits of Cooperating Hypothesis Short-term Source of benefit to donor Bd/C d Manipulation < 1 None (recipient wins) Mutualism > 1 Short-term benefits from recipient's actions Reciprocity < 1 Direct long-term benefits from recipient's assistance in future Kin selection < 1 Indirect, genetic gains from helping relatives Allogrooming in primates (not many other proven examples)

6 April 17: Altruism: Cooperation Hypotheses Concerning the Benefits of Cooperating Hypothesis Short-term Source of benefit to donor Bd/C d Manipulation < 1 None (recipient wins) Mutualism > 1 Short-term benefits from recipient's actions Reciprocity < 1 Direct long-term benefits from recipient's assistance in future Kin selection < 1 Indirect, genetic gains from helping relatives Alarm calls, helping at the nest, eusociality, wife sharing in Tasmanian Native Hens

7 April 17: Altruism: Kin Selection Kin Selection Kin selection--donors experience direct genetic costs, but INDIRECT genetic benefits, because their help is directed at close relatives Helps explain some of most puzzling examples of cooperation, in which individuals experience net fitness costs (but their genes, or copies of their genes in other individuals are benefited) Moves adaptive explanation from individual level to gene level Three behavioral traits in which kin selection has been studied Alarm calls (where caller has measurable risk of being attacked by predator) Helping at the nest: a temporary strategy prior to breeding (many birds) Eusociality: helping as a permanent strategy of non-reproducing members of a social group (bees, wasps, termites, naked mole rat)

8 April 17: Altruism: Eusociality Eusociality Definition: three criteria Overlapping generations Cooperative care of young Reproductive division of labor (with a permanently sterile work force providing for care of young and maintenance of nest) Darwin on the sterile workers of social insects [I will discuss here] one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind. Why was Darwin so concerned? How could sterility evolve and be maintained by natural selection? How could non-reproducing workers evolve traits not found in queen? His solution: Worker traits are adaptive because they promote success of relatives!

9 April 17: Altruism: Analysis of kin selection Kin Selection From A Quantitative Perspective Kin selection works in the following way Heritable altruistic trait can spread in population if it enhances the fitness of individuals carrying copies of the gene(s) that produce the trait This can work even if the trait lowers the fitness of the individual expressing the trait Two factors determine whether kin selection can favor altruistic traits Ecological predispositions: high benefit relative to cost, where: Benefit, B r, is BENEFIT TO RECIPIENT Cost, C d, is COST TO DONOR Genetic predispositions: high degree of relationship between donor and recipient, where: Coefficient of relatedness, (r), is probability of two individuals carrying a particular allele that is identical by virtue of descent from a common ancestor

10 April 17: Altruism: relatedness Quantifying Relatedness (in Diploid Species) We are interested in quantifying the probability that two individuals share an allele that is identical by virtue of descent from a common ancestor Some facts to remember In diploid organisms, an individual receives two copies of each gene, one from each parent Thus, if you have a particular allele, there is a 50% probability that you got it from your mom, and a 50% probability that you got it from your dad Mom Dad r you-mom = Probability of sharing allele with mom=0.5 You r you-dad = 0.5

11 April 17: Altruism: relatedness Quantifying Relatedness (in Diploid Species)-cont d Relatedness among kin other than parent-offspring Find all pathways by which two individuals might have inherited copies of genes For each link, probability of sharing gene is discounted by 0.5 Total probability is found by summing over all pathways Mom Dad 0.5 r you-sis = P(via mom) + P(via dad) 0.5 = (0.5 x 0.5) + (0.5 x 0.5) = = 0.5 You Is red allele present in sis? Sis

12 April 17: Altruism: relatedness Quantifying Relatedness (in Diploid Species)-cont d Relatedness among kin other than parent-offspring Find all pathways by which two individuals might have inherited copies of genes For each link, probability of sharing gene is discounted by 0.5 Total probability is found by summing over all pathways Grampa Grandma Mom Aunt r you-cousin = P(via Grampa) + P(via Grandma) = (= 1/8) You Cousin In general, the more distant the genealogical connection between two individuals, the lower the relatedness

13 April 17: Altruism: haplodiploidy Haplodiploidy Basic definition Females are diploid and arise from fertilized egg Males are haploid and arise from unfertilized egg (males have no father!) Mom Dad r you-sis = P(via mom) + P(via dad) = = 0.75 r you-bro = P(via mom) = 0.25 You Sis Bro Thus, in haplodiploid species, relatedness between sisters is higher than relatedness between mother and offspring!!

14 April 17: Altruism: Hamilton s rule Putting it together: Hamilton s rule William Hamilton In ground-breaking papers in 1964, outlined the conditions under which altruistic traits could evolve by kin selection Hamilton s rule: An allele coding for altruistic tendency should increase in frequency in a population if: Br / C d > 1/r rb r > C d Here, Benefits and Costs are measured in terms of lives saved or sacrificed. Consider (e.g., alarm call) with cost = 1 to donor: How high does benefit have to be to repay the cost? Depends on relatedness of donor to recipient For siblings or offspring (r=0.5), benefit must be >2 For grandchildren (r=0.25), benefit must be >4 For cousins (r=0.125), benefit must be >8

15 April 17: Altruism: Hamilton s rule Hamilton s rule--cont d Another way of expressing Hamilton s rule: An allele coding for altruistic tendency should increase in frequency in a population if: Br / C d > r donor-offspring /r donor-recipient Br x r donor-recipient s offspring > C d x r donor-own offspring Here, Benefits and Costs are measured in terms of number of offspring produced (or forgone) Hamilton argued that we have to consider an animal s INCLUSIVE FITNESS = Direct fitness (via production of own offspring) + Indirect fitness (via effects of behavior on reproduction of relatives)

16 April 17: Altruism: Helping Can Kin Selection Account for Evolution of Cooperation? Helping At The Nest in birds (e.g., Florida Scrub Jay) Males, upon fledging, often do not go off to establish their own territories, but instead stay on parental territory, helping parents to rear more offspring. Question: can this altruistic behavior be explained as result of kin selection? Do helpers really help? They appear to help: feeding, defending offspring Correlational evidence: territories with helpers produce more fledglings than those without Experimental evidence: remove helpers from some territories, not others

17 April 17: Altruism: Helping Can Kin Selection Account for Evolution of Cooperation?-cont d Helping At The Nest in birds (e.g., Florida Scrub Jay) Do helpers help relatives? In Florida Scrub Jay, yes: helpers are typically rearing full siblings (r=0.5) Because of EPCs by female, siblings are sometimes half siblings (r=0.25), but average relatedness is still pretty high (r=0.43) NOTE: in other species with helpers (e.g., Pied kingfisher), the helpers may be unrelated to breeding pair, so we would need to invoke another explanation

18 April 17: Altruism: Helping Can Kin Selection Account for Evolution of Cooperation?-cont d Helping At The Nest in birds (e.g., Florida Scrub Jay) Do helpers help enough (to repay costs of being helper)? The cost: lost reproductive opportunity = C d x r donor-own offspring = # offspring produced by first-time breeders x r parent-offspring = 1.24 x 0.5 = 0.62 The benefit: To break even by helping, helpers would have to: ADD TO THE PRODUCTIVITY OF THEIR PARENTS This added productivity is discounted by relatedness to these extra fledglings Net payoff due to helping should be 0.62 or greater Actual payoff to helpers (average): Avg. extra fledglings per helpers = B r = 0.33 Avg. relatedness of helpers to fledglings = r donor-recipient s offspring = 0.43 Net payoff = B r x r donor-recipient s offspring = 0.33 x 0.43 = 0.14 NOT ENOUGH!

19 April 17: Altruism: Helping Can Kin Selection Account for Evolution of Cooperation?-cont d Helping At The Nest in birds (e.g., Florida Scrub Jay) Bottom line Helpers help They help relatives They don t help enough to equal what they might have produced if they bred on their own So, why help? In Florida Scrub Jay and in other species with helping, there are often limits on breeding opportunities for first-time breeders Being helper has two selfish benefits Helpers often inherit all or part of breeding male s territory (long-term payoff) Helpers may get short-term genetic benefit from rearing siblings For species in which helpers help nonrelatives, only the long-term payoffs apply

20 April 17: Altruism: Hamilton s rule Can Kin Selection Account for Evolution of Cooperation?-cont d Other examples (see book) Alarm Calls in Belding s Ground Squirrels Mating cooperation among males (lions) Cooperative courtship in manakins Sensory basis of kin-biased behavior: Animals should be expected to have evolved mechanisms for ensuring that only kin benefit from altruism What cues might enable animals to decide whether kin are likely to benefit from their help? Spatial cues (animals encountered near home are likelier to be related) Imprinting (animals you grow up with are likelier to be related to you) Phenotype matching (if she resembles me, she must be related)-- doesn t require prior experience with related individual Alcock Fig. 4.10

21 April 17: Altruism: Hamilton s rule Eusociality and Kin Selection Eusociality is just an extreme form of helping at the nest Workers are helpers They clearly help, and they help close relatives Why should helping evolve to such an extreme form, where helping is a permanent condition? Two hypotheses (not mutually exclusive): Extraordinarily high relatedness between donor and recipient Extraordinarily high ratio of Benefits to Costs (owing to ecological pressures) Ant bomb --soldier explodes on being grasped by predator

22 April 17: Altruism: Haplodiploidy Haplodiploidy: A genetic predisposition toward eusociality? Haplodiploidy creates incentive to rear sisters (r=0.75) rather than daughters (r=0.5) or brothers (r=0.25) Evidence for role of haplodiploidy: Eusociality has evolved at least 12 times in haplodiploid insects, but only twice in diploid species (once in termites, once in naked mole rats) In haplodiploid eusocial species: workers are only females (males don t benefit by being helpers)

23 April 17: Altruism: Haplodiploidy Haplodiploidy: A genetic predisposition toward eusociality? But there must be more to it than the genetic asymmetry due to haplodiploidy Haplodiploidy is not necessary for evolution of eusociality (termites and naked mole rats are diploid) Haplodiploidy is not sufficient for evolution of eusociality (many haplodiploid wasps and bees are solitary) Relatedness among colony members in haplodiploid species is often not particularly high Multiple mating by queen dilutes average relatedness among sisters In some species, presence of multiple queens means workers may be helping non-sisters

24 April 17: Altruism: Haplodiploidy Ecological predispositions toward eusociality? Limits on breeding opportunities for solitary individuals Hymenoptera: parental care Vulnerability of offspring to predators, climate and disease Perhaps it is not coincidental that eusociality has evolved so often only among the non-parastic hymenoptera, which exhibit parental care Termites and naked mole rats: nutritional constraints Both organisms use a food that is difficult to digest: cellulose Processing this food is easier in a group Thus, as in birds that help at the nest, eusociality is explained as a combination of genetic and ecological factors