Unrelated helpers will not fully compensate for costs imposed on breeders when they pay to stay

Abstract

Unrelated subordinates may invest in costly help to avoid being evicted from groups (the 'pay-to-stay' hypothesis). However, the effectiveness of eviction to enforce help should depend on its being applied accurately and on the costs it imposes on both dominants and subordinates. The relative cost of being evicted is a function of the population frequency of eviction when population growth is limited by density-dependent factors. We describe a stage-structured pay-to-stay model incorporating density-dependent population growth, costly eviction and occasional errors. Breeders demand some amount of help and evict subordinates that do not provide it. Helpers decide on the amount of help they will provide. The threat of eviction alone is sufficient to enforce helping. However, helping will not be favoured if helpers do not impose costs on breeders. The amount of help provided is less than the cost that subordinates impose upon breeders, when any help is provided. Thus, the net fitness effect of a helper under pay-to-stay alone is negative, even if it is investing in cooperative behaviour. Constraints on dispersal have no effect on the amount of help, although they may influence the tolerance threshold of breeders and group stability, depending on the mechanism of density dependence.

Kinship reduces alloparental care in cooperative cichlids where helpers pay-to-stay

“…If in contrast, dominants force subordinates to provide help, such as assumed by pay-to-stay models, unrelated subordinate females should provide more alloparental brood care than related ones, and unrelated subordinates should be evicted more readily by dominants if they do not compensate for the lack of kinship by increased helping levels (that is, payment). If such compensation occurs, however, similar eviction rates of related and unrelated subordinates might be expected in case helping depends on coercion 28 . Furthermore, if subordinates breed in the territory of dominants 34 , alloparental brood care should not be reciprocated by the dominant female 35 in a pay-to-stay scenario, and dominant female fitness should be independent of relatedness to subordinates, as helper payment may only compensate for imposed costs and not provide net fitness benefits to dominants 28 .…”

“…If subordinates consequently 'pay-to-stay' in the group, this resembles the exchange of services in different commodities 26 . Such helping by demand, or through coercion, may apply particularly when relatedness between dominants and subordinates is low, the presence of subordinates inflicts costs on dominants, dispersal and survival outside of groups and independent breeding by subordinates are constrained, and individuals are generally long-lived 27,28 . However, if pay-to-stay is the ultimate cause of helping behaviour, subordinates should only compensate for the costs they inflict on dominants, and dominants will gain no net fitness benefits from the presence and help of subordinates 28 .…”

“…Such helping by demand, or through coercion, may apply particularly when relatedness between dominants and subordinates is low, the presence of subordinates inflicts costs on dominants, dispersal and survival outside of groups and independent breeding by subordinates are constrained, and individuals are generally long-lived 27,28 . However, if pay-to-stay is the ultimate cause of helping behaviour, subordinates should only compensate for the costs they inflict on dominants, and dominants will gain no net fitness benefits from the presence and help of subordinates 28 . Furthermore, if subordinates help because they are coerced to do so by dominants, the conventional predictions of kin-selection theory are reversed: in this case related individuals are expected to provide less help than unrelated ones 27 .…”

“…Furthermore, if subordinates help because they are coerced to do so by dominants, the conventional predictions of kin-selection theory are reversed: in this case related individuals are expected to provide less help than unrelated ones 27 . Hence, the pay-to-stay hypothesis takes kinship effects into account [26][27][28] and generates testable predictions about the effect of relatedness on helping behaviour. Hitherto, these predictions have been rarely scrutinized empirically, despite evidence for this form of reciprocity in a wide range of vertebrates 15,[29][30][31][32] .…”

On the further integration of cooperative breeding and cooperation theory

“…involving a negative control mechanism (restraint models of RS), or it may not be cooperative but a result of competition between helpers and breeders (tug of war models) (Johnstone, 2000;Magrath et al, 2004). Restraint models of reproductive skew show obvious parallels to pay-to-stay based explanations for helping (Kokko et al, 2002;Bergmüller and Taborsky, 2005;Hamilton and Taborsky, 2005b). Snowdon and Cronin (2007) and Sachs and Rubenstein (2007) highlight that 'gain in experience' may be an important factor promoting helping (Skutch, 1961;Brown, 1987;Komdeur, 1996).…”