APS209 Animal Behaviour
1. To present the two-stage ecological constraints model for the evolution of cooperative breeding
2. To explain the role of ecological and demographic factors in constraining reproduction
3. To examine the potential direct and indirect sources of inclusive fitness for helpers
1. To understand the principles underlying the ecological constraints hypothesis
2. To understand how predictions of the ecological constraints hypothesis have been tested
3. To understand the alternative routes to fitness for helpers
Cooperative breeding - Some individuals forego personal reproduction and spend all or part of their lives helping others to breed. The great majority of cooperative systems are kin-based and these breeding systems are therefore likely to provide examples of kin-selected behaviour. The social organisation of cooperative breeders varies widely, with a continuum of social organisation encompassing: 'helpers-at-the-nest' where breeding pairs are aided by one or more helpers, usually offspring of previous broods, and 'plural breeders' where communal nests have several breeders of one or both sexes.
Evolution of cooperative breeding - The ecological constraints hypothesis (aka the habitat saturation hypothesis) envisages the evolution of cooperative breeding as a 2-stage process:
Stage 1. Ecological factors (no suitable breeding territories available) and/or demographic factors (no breeding partners available) constrain independent breeding causing grown young to delay dispersal and 'stay at home' on their natal territory. Observational evidence provides some support, but the best evidence comes from removal experiments: e.g. superb fairy wren Pruett-Jones & Lewis 1990, Nature 348:541-542). However, reproductive constraints are widespread, comparative studies have been inconclusive, and other factors may be important, e.g. phylogeny.
Stage 2. Fitness benefits of helping exceed those of not helping, so young who have delayed dispersal help relatives to raise later broods.
What are the fitness benefits of helping?
(a) Direct fitness - fitness component resulting from personal reproduction.
- increased survival of helpers through group benefits – ‘group augmentation’, e.g. reduce predation risks, share resources
- increased probability of future breeding, e.g. territory or mate acquisition/ inheritance
- increased experience of parental care ('skills' hypothesis)
- direct reproduction
(b) Indirect fitness - fitness component from increased production of non-descendant kin.
- increased reproductive success of relatives (genetic benefits through raising siblings which share 50% of genes)
- increased survival of related breeders through reduced reproductive costs
The relative importance of indirect or direct fitness benefits in the evolution of helping behaviour is still debated
See Chapter 13 in Alcock’s Animal Behavior (2009). See also more detailed accounts in Cockburn A (1998) Evolution of helping behaviour in cooperatively breeding birds, Annual Review of Ecology & Systematics 29: 141-177. Emlen S.T. (1991). Evolution of cooperative breeding in birds and mammals; in J.R. Krebs & N.B. Davies (eds), Behavioural Ecology, 3rd edition.