Reproduction and Social Behaviour

Kin selection and helping behaviour

Kin selection explains how a helping trait can spread even though it lowers the helper's direct reproduction, because the helper's inclusive fitness includes the reproductive gains of relatives that share its genes. Hamilton's rule states that helping is favoured when rb > c, where r is the coefficient of relatedness, b the benefit to the recipient and c the cost to the helper. Several predictions follow. Helper effort should rise when the young are more closely related to the helper. A population with low extra-pair paternity has more full siblings among the young, so it should show more helping than a population with high extra-pair paternity. Likewise, a helper whose parents continue to breed rears full siblings and should give more care than a helper whose parents have been replaced, because replacement lowers relatedness. Explanations based on territory inheritance or on survival during dispersal are ecological or direct-benefit arguments. They do not test kin selection, because they change the payoffs without changing relatedness.

Altruism can also evolve when group members are closely related to one another, since relatedness raises the number of gene copies that recipients carry. Reciprocal altruism, in which individuals help each other at different times in repeated interactions, is a second route. In contrast, a benign environment with low risk reduces the need for cooperation, and equal distribution of mating opportunities gives no selective basis for helping.

The coefficient of relatedness is the probability that a gene in one individual is identical by descent with a gene at the same locus in another. Full siblings have r = \frac{1}{2}, and two offspring with the same mother but different fathers are maternal half-siblings with r = \frac{1}{2}\times\frac{1}{2} = \frac{1}{4}.

Haplodiploid relatedness

In haplodiploid organisms, such as the Hymenoptera, males develop from unfertilised eggs and are haploid, whereas females develop from fertilised eggs and are diploid. This system changes the expected relatedness between relatives. Full sisters share all the genes that their haploid father passes on, because he transmits an identical set to every daughter, and on average half of their maternal genes. Their average relatedness is therefore r = \frac{1}{2}(1) + \frac{1}{2}\left(\frac{1}{2}\right) = \frac{3}{4}, which is well above the value of \frac{1}{2} for full siblings in diploid species. Brothers carry only their mother's genes, so two brothers from the same mother share on average \frac{1}{2}, and a brother and a sister share on average \frac{1}{4}. Most ants and bees show this system, with diploid females from fertilised eggs and haploid males from unfertilised eggs.

Relatedness to more distant kin can be derived by combining these rules. The daughter of a full sister receives only half of her genome from her mother, and her relatedness to the aunt is therefore \frac{1}{2}\times\frac{3}{4} = \frac{3}{8}. A mated female and male from different colonies share almost no common ancestry, so their relatedness is close to zero.

Evolution of monogamy

Hypotheses for the evolution of monogamy are tested by asking whether a mating system increases the reproductive success of the individual, which is the logic of individual selection. Benefits such as joint provisioning of the young, or shared defence of the young against predators, raise the survival of a parent's own offspring, so these are individual-level explanations. When females are solitary and dispersed, a male cannot guard more than one female at a time, so monogamy may be favoured by a constraint on mate-guarding that acts on each individual male. An argument that monogamy regulates reproductive output to keep the species alive appeals to a group-level benefit, which individual selection does not support.

Expected reproductive success is the sum of each strategy's offspring number weighted by the probability of adopting it, E = \sum_i p_i\, w_i. For example, if a male is monogamous with probability 0.8 and produces 2 offspring, and polygynous with probability 0.2 and produces 3 offspring, then E = 0.8 \times 2 + 0.2 \times 3 = 2.2 offspring per season.

Bateman gradient

Bateman's principle states that, in most species, reproductive success rises more steeply with the number of mates in males than in females. The Bateman gradient is the slope of the relationship between the number of mates and the number of offspring. In males, additional matings can add many offspring, whereas in females reproduction is limited mainly by the supply of eggs or resources, so extra mates add little. As a result, males usually show greater variance in reproductive success, and sexual selection is typically stronger on males than on females.

The strength of sexual selection is measured by comparing the variance in mating success of each sex, V_m for males and V_f for females. With a 1:1 sex ratio, the ratio V_m/V_f is largest when a few males monopolise many females, as in polygyny. The ratio is close to one under random mating and falls below one under polyandry, where a few females mate with many males.

Mating systems and sexual dimorphism

Sexual dimorphism in body size often reflects sexual selection acting on one sex. Male-biased size dimorphism arises when large size gives males a competitive advantage in contests for mates, as in polygyny, where a male that secures several females gains most from size. Under monogamy, male competition is weak and dimorphism is usually small. Polyandry reverses the direction of competition, which tends to reduce male-biased dimorphism. Parthenogenesis involves no male contribution and therefore no male competition.

Any hypothesis for male-biased dimorphism must link body size to male reproductive success. A competitive advantage for large males alone selects for large males without requiring larger females. A fecundity advantage for large females would instead favour female-biased dimorphism. If body size were unrelated to mating success, sexual selection could not explain male-biased size.

Sex-biased dispersal

Sex-biased dispersal refers to whether individuals of one sex leave their natal site more often than the other. Dispersal reduces inbreeding and competition with relatives, so which sex disperses depends on the costs and benefits of staying in the natal area. In most birds, females disperse more than males, so dispersal is female-biased. In most mammals, males disperse while females remain near the natal range, so dispersal is male-biased. Students should memorise this direction, because the reverse pattern is a common distractor in examination options.

Sex-biased dispersal also shapes population genetic structure. The sex that stays at the natal site tends to show stronger local relatedness, which matters for kin-based social behaviour.

Sexual selection by female choice

Female choice is a form of sexual selection in which females prefer particular male traits. A sensory bias is a pre-existing perceptual or neural preference in females that is independent of male quality, and runaway selection is the process by which such a preference and a male trait reinforce each other. A standard test for sensory bias is to attach the trait to males of a species that naturally lacks it and see whether females are more attracted to them. If females prefer the artificial trait, their preference exists independently of the male's natural condition, which is the necessary evidence for the bias. Aggression among males reflects intrasexual competition, a separate mechanism that does not test female preference. Finding no female response to the artificial trait would count against the hypothesis.

Cooperator–cheater dynamics

In a cooperator-cheater interaction, one strain or individual pays a cost to supply a benefit, and another gains from that supply without paying the cost. When the two strains grow together, the cheater can increase while the cooperator declines, so the outcome depends on the frequency of each strain. Other interactions are distinguished by the effects on each partner. Amensalism inhibits one partner while leaving the other unaffected. Commensalism benefits one partner while leaving the other unaffected. Mutualism benefits both partners. A cooperator-cheater pattern is recognised when one partner gains at the expense of the other, and the gain is possible only because the cheater uses the cooperator's contribution.

Altricial vs precocial development

Altricial species hatch in a helpless state and depend on parental care for an extended period. Altricial hatchlings typically have closed eyes and are naked or have only sparse down, because feather development continues in the nest. Precocial young, by contrast, hatch with open eyes, a dense covering of down, and are mobile enough to leave the nest and feed themselves soon after hatching.