Cooperative Breeding Flashcards
Cooperative breeding describes systems where more than two individuals in a group care for young. In most cooperative breeding systems, some individuals in such groups are non-breeders (termed ‘helpers’).
The great majority of cooperative systems are kin-based and are therefore likely to provide examples of kin-selected behaviour. There is a continuum of sociality across taxa from the relatively simple cooperative systems of some birds and fish through to the complex eusocial systems of social insects. In these lectures, we will focus on vertebrate cooperative breeding systems.
Evolution of cooperative breeding
The Ecological Constraints Hypothesis (ECH) envisages a 2-stage process:
Stage 1. Ecological factors (no territories), demographic factors (no partners ), or low chances of successful reproduction constrain independent breeding, causing grown young to delay dispersal and ‘stay at home’ on their natal territory.
Stage 2. Fitness benefits of helping exceed those of not helping, so helpers provide care for a brood. Helpers may gain inclusive fitness benefits either directly (by increasing their own current or future personal reproduction) or indirectly (by increasing the fitness of non-descendant kin; i.e. kin selection).
Intraspecific evidence for the Ecological Constraints Hypothesis. There is strong observational and experimental support for the ECH (see APS209 Animal Behaviour lectures; e.g. superb fairy wren, Seychelles warbler, red-cockaded woodpecker, sociable weaver) showing that when constraints are relaxed, cooperation decreases.
But, most species suffer constraints on reproduction (e.g. non-reproductive individuals who cannot defend a territory or obtain a mate) but are not cooperative. So, many researchers have sought to identify the critical differences between cooperatively breeding species and non-cooperative breeders. Such analyses have all identified a strong phylogenetic signal in cooperative breeding, and the multiple evolutionary transitions to and from cooperative breeding in the avian phylogeny, in particular, makes them very well suited to comparative analyses.
Interspecific studies of cooperative breeding
Hypothesis 1 – Ecological factors constrain independent breeding
Comparative analyses suggest that certain environments select for cooperative breeding. For example, among African starlings, Rubenstein & Lovette (2007) showed that cooperative breeding is associated with savannah habitats, which they argued are less predictable than non-savannah habitats. Jetz & Rubenstein (2011) tested this unpredictability hypothesis at a global scale, finding some support (cooperative breeding associated with greater variation in rainfall). The same pattern has also been shown in mammals (Lukas & Cluton-Brock 2017). However, the predictive strength of this hypothesis is weak in both taxa. Furthermore, other comparative studies have identified different ecological correlates of cooperation, including dense habitats, low temperature variation and stable/aseasonal environments (e.g. Arnold & Owens 1999; Covas 2012), so the evidence cannot be considered conclusive. A recent paper has also argued that rather than harsh environments selecting for sociality, sociality allows cooperative species to colonise harsh environments (Cornwallis et al. 2017). This argument relies on the reconstruction of ecological conditions at ancestral nodes in a phylogeny, which is not necessarily very reliable.
Hypothesis 2 – Benefits of philopatry select for delayed dispersal
The flip-side of the constraints hypothesis is the benefits of philopatry hypothesis. This argues that rather than dispersal being constrained by certain factors, philopatry (staying at home) is selected for by the benefits of living with family.
This is an old idea that is currently resurgent, e.g. Siberian jays Perisoreus infaustus exhibit nepotistic behaviour towards relatives that means kin survive the winter better than non-kin in the same social group. These potential benefits will depend on resource ‘wealth’ in the natal territory, as in western bluebirds Sialia mexicana (Dickinson & McGowan 2005). Therefore, dispersal and independent breeding may not always be the best option, as is assumed by the constraints hypothesis. Instead, staying at home may be a strategic life history decision that maximizes lifetime reproductive success (Covas & Griesser 2007), e.g. female green woodhoopoes Phoenicurus purpureus (Hawn et al. 2007). This idea has not been explicitly tested in comparative analyses, but Drobniak et al. (2015) argue that family-living is a transitional stage in the evolution of cooperative breeding.
Hypothesis 3 – Life history traits predispose some lineages to cooperate
The Life History Hypothesis (Arnold & Owens 1998) proposes that certain lineages are predisposed to be cooperative because they have ‘slow’ life histories (i.e. long-lived, small clutches) so the turnover in breeding vacancies is low.
Lineages with ‘fast’ life histories (short-lived, large clutches) have high turnover and tend not to be cooperative. The former are predicted to cooperate when ecological circumstances dictate, the latter are never expected to cooperate. Support for this idea is equivocal (Beauchamp 2014), and others have argued that this dichotomy between ecological and life history traits is false… it doesn’t matter what the source of constraint is so far as potential breeders are concerned (Hatchwell & Komdeur 2000).
Hypothesis 4 – Cooperative breeding is associated with brood parasitism
A recent analysis argues that brood parasites prefer cooperative breeders as hosts because of the additional food provided for their parasitic nestlings by helpers. On the other hand, brood parasitism should select for family formation because the extra vigilance provided by helpers would reduce the risk of being parasitized in the first place.
Detailed studies of the pair-wise interaction between Horsfield’s bronze cuckoo Chrysococcyx basalis and superb fairy-wrens Malurus cyaneus in Australia support this contention, and the global distribution of cooperative breeders and obligate brood parasites appear to coincide. Moreover, detailed analysis of southern African and Australian species show that hosts of brood parasites are more likely to be cooperative breeders than non-hosts (Feeney et al. 2013; but see Wells & Barker 2017).
Why is predicting sociality so difficult?
Given the interest in cooperative breeding systems, it is surprising that we don’t yet have a good understanding of the factors selecting for this behaviour. The answer may be that various factors pose problems in these comparative analyses.
Diverse constraints - Most studies have focused on species with spatial constraints on reproduction, e.g. a shortage of territories. However, experimental studies show that the proximate constraints on independent reproduction differ across studies, including food, nest site availability, territories and mates. Cornwallis The same argument applies to the potential benefits of staying at home. Furthermore, several authors have argues that there really is no fundamental difference between constraints on dispersal and benefits of philopatry – they differ only in the emphasis they place on either side of the disperse/stay trade-off.
Diverse social systems - Definitions of cooperative breeding vary, but whatever definition is used, the term encompasses a broad range of social organization, including monogamy + non-breeding helpers, polygamy (multiple reproductive males and/or females) + non-breeding helpers, and cooperative polygamy without helpers. Such diverse social systems may have evolved through different routes and so may not be explained by single factors.
Phylogeny and life history traits - There is a strong phylogenetic component to cooperative breeding in birds and mammals. Some families are composed entirely of cooperative breeders, some have none, and in some lineages cooperative breeding has evolved and then been lost (Cockburn 2006). Phylogenetic analyses are fraught with difficulty because some taxa are found only in particular environments, and because life history traits are highly conserved within families, so these traits will be confounded with phylogeny.
