An evolutionary paradox? This week in Animal Behaviour Lecture 19: Parental care cont. Lecture 20: Kinship Lecture 21: Social behaviour LAB: Intro to project 2 Text chapters 8-9 An evolutionary paradox? When to help? When to cooperate? A range of social interactions... Kin selection can explain many behaviours that appear to be altruistic. change in recipient fitness inclusive fitness = direct fitness + indirect fitness + – direct fitness: genes contributed by an individual via personal reproduction change cooperation selfishness + indirect fitness: genes contributed by in actor an individual by helping kin that are not fitness direct descendants – altruism spite When to help? When to cooperate? Calculating relatedness Relatedness (r): the probability that two alleles at a particular locus are identical by descent. By extension, this equals the proportion of W. D. Hamilton (1964) genes shared between individuals over the entire genome. Hamilton’s rule: help if r b – c > 0 “I would give up my life for 2 brothers or 8 cousins...” Kinship, cooperation and social behaviour Potential costs of social living 1. Conspicuousness to Costs and benefits of social living fieldfares predators Cooperation among kin 2. Competition for food • Parental care e.g. fieldfares 3. Reproductive interference • Cooperative breeding nestling survival e.g. acorn woodpeckers • Case study: eusociality colony size 4. Misdirected parental care Evolution of cooperative behaviour (adoption) • Four theoretical paths to cooperation 5. Parasite/disease transmission /nestling • Cooperation between species e.g. cliff swallows swallow bugs • Case study: cooperation during courtship cliff swallows colony size Potential benefits of social living Yellow-saddled goatfish collaborate 1. Anti-predator defence e.g. dilution effect, confusion effect, “many eyes” effect, selfish herd effect, mobbing, and other group defences... leader % attack formation 2. Foraging efficiency e.g. African wild dogs, goatfish 2 fish 3 fish 3. Reproductive opportunities e.g. cuckoldry 4. Opportunity to receive care net kJ / dog day pack size Sociality can be good, to a point Co-operative breeding As group size increases, how do the costs and benefits change? e.g. social spiders probability of helping probability coefficient of relatedness (r) female lifetime RS colony size Helping in dwarf mongooses The influence of ecological factors females males acorn woodpecker 40.00 inclusive fitness payoff (# offspring) payoff 30.00 20.00 % dispersing 10.00 delay dispersal age age % of yearlings that 0 low medium high natal territory quality Case study: eusociality Taking cooperation to an extreme Eusociality: a complex social organization with: 1. group living 2. overlap of generations 3. reproductive division of labour (with or without sterile castes) 4. cooperative care of young Eusociality is prevalent in Hymenoptera (wasps, bees and ants) and Isoptera (termites). Hymenoptera are haplodiploid Hymenoptera are haplodiploid Consequences of haplodiploidy Haplodiploidy and Hamilton’s rule Links between haplodiploidy and eusociality • across species, workers provision sisters over brothers in accordance with the predicted 3:1 investment ratio (predicted from r = 0.75 for sisters vs. 0.25 brothers) • ...and experimentally removing the queen can change this • other haplodiploid insects such as thrips have also evolved a sterile non-reproductive caste in some species • in Hymenoptera (haplodiploid), only females are workers, whereas in Isoptera (diploid), males are too queen and soldier of an Australian thrips species Haplodiploidy and Hamilton’s rule Haplodiploidy and Hamilton’s rule Problems with the haplodiploidy/eusociality connection Problems with the haplodiploidy/eusociality connection • queens of some species mate multiple times, so that • queens of some species mate multiple times, so that sisters are no longer related with r = 0.75 sisters are no longer related with r = 0.75 0.8 BUT: 0.7 changes in investment ratios fit Hamilton’s rule predictions 0.6 mother and offspring (e.g. Formica ants) 0.5 0.4 sister and sister 0.3 sister and brother 0.2 average relatedness 0.1 number of matings by mother Haplodiploidy and Hamilton’s rule Haplodiploidy and Hamilton’s rule Problems with the haplodiploidy/eusociality connection Problems with the haplodiploidy/eusociality connection • queens of some species mate multiple times, so that • queens of some species mate multiple times, so that sisters are no longer related with r = 0.75 sisters are no longer related with r = 0.75 BUT: • many haplodiploid insects are NOT eusocial (e.g. many changes in investment ratios fit Hamilton’s rule predictions wasps and bees are solitary) (e.g. Formica ants) • not all eusocial organisms are haplodiploid (e.g. termites, workers “police” nephews at least one beetle, some mammals...) (e.g. honeybees) Queen mates once: workers–nephews, r = 0.375 workers–brothers, r = 0.25 Queen mates multiple times: workers more closely related to brothers than nephews eggs remaining percent hours Haplodiploidy and Hamilton’s rule Naked mole rats Problems with the haplodiploidy/eusociality connection • Found in hot, dry regions of Africa • queens of some species mate multiple times, so that • Spend entire life underground feeding on tubers sisters are no longer related with r = 0.75 • Long-lived (16 years) and prolific (12-27 in a litter) • many haplodiploid insects are NOT eusocial (e.g. many wasps and bees are solitary) • not all eusocial organisms are haplodiploid (e.g. termites, at least one beetle, some mammals...) BUT • some of these non-haplodiploid eusocial organisms have very high levels of relatedness (e.g. clonal aphids with soldier castes) Naked mole rats Why are mole rats eusocial? Naked mole rats are eusocial: 1. High relatedness as a result of inbreeding (r ~ 0.8) • 1 reproductive “queen” and “king” per colony • however, the Damaraland mole-rat is eusocial and not • other females do not ovulate inbred (r ~ 0.2) • small non-breeders build tunnels and collect food 2. Environmental factors may have led to colonial living • large non-breeders defend colony and transport dirt • high cost to dispersal, risk of predation outside burrow Reproductive suppression maintained via behavioural and • grouping to exploit patchy food resources chemical cues; when queen dies, violent fighting breaks out • reproductive success correlated with number of helpers Conclusions on eusociality Social living as a continuum • Genetic factors (haplodiploidy, inbreeding) alone are neither necessary nor sufficient to account for eusociality. eusociality • They may make it easier for eusociality to evolve (e.g. as preconditions). 0 1 • Ecological constraints on independent breeding may also be important in driving the evolution of eusociality..