Our framework emphasizes the need to address the evolution of family living at an interspecific level. While some questions can only be addressed at the intraspecific level (see below), more interspecific studies are needed to compare how different species respond to variation in different factors. Below we develop some ideas on how to test the framework proposed (see also ).
One of the basic issues of our hypothesis is that delayed reproduction and dispersal in long-lived species might be adaptive and not just a result of constraints. In other words, it might be a better strategy for offspring to remain at home than to disperse and breed under suboptimal conditions. Thus, a key prediction of our model is that the delayed onset of reproduction is only a beneficial strategy in long-lived species but not in short-lived ones. This is an important point since it could explain the link between longevity and propensity for sociality. We therefore predict that longer-lived species are more likely to avoid breeding in suboptimal conditions to avoid incurring a higher reproductive cost and compromising their lifetime reproductive success. In contrast, short-lived species cannot afford to postpone the onset of reproduction and thus should breed under poorer conditions, since they have fewer chances of additional breeding opportunities during their short lives. This hypothesis can be tested by manipulating breeding conditions to create less favourable conditions, or territories, in species with contrasting survival prospects. Similar mechanisms should influence dispersal decisions within species and individuals are expected to choose dispersal strategies according to an interaction of individual factors (e.g. age, sex, phenotypic quality) and external factors that affect fitness (e.g. quality of the breeding opening, population density, mate quality). Hence, within species, we expect EC to play a role in limiting independent breeding. However, the two levels of analysis should not be confounded.
We also expect parental investment strategies in offspring and parent/offspring survival ratio to influence investment and hence offspring dispersal decisions. This can be tested through comparative studies by analysing parental and offspring survival in relation to dispersal and reproductive decisions of offspring. These studies can be field based or literature based, as better quality data from long-term studies become available. In some systems it should also be possible to manipulate (improve) parental or offspring survival prospects to provide an experimental test of this hypothesis, although it might be difficult to achieve this without manipulating resource levels, and hence it could be complicated to distinguish the effects of the two.
Resource levels and predictability (after the breeding season) and low within-family competition are also crucial factors to allow family living. Studies that manipulated resources to modify dispersal behaviour have already been conducted successfully (e.g. Dickinson & McGowan 2005
; Baglione et al. 2006
) and provided support to the idea that an experimental change in available resources affects the offspring dispersal decisions. At this point, a decisive experimental test would be to cause offspring in a non-social species (from a family social clade) to delay dispersal and form family groups through continuous food supplementation or improvement of other crucial resource. Such a change of behaviour, if obtained in a situation where nearby breeding vacancies remained available, would be in striking contrast to what is expected under EC hypotheses.
Previous comparative analyses have failed to find an effect of ‘ecology’ on family formation among species (Cockburn 2003
). However, we propose that ecological factors which affect the survival of individuals and the predictability of access to resources might reveal significant associations. This could be investigated by manipulating and comparing different populations or through broader literature-based comparative analyses.
Finally, when offspring decide to postpone independent reproduction and the parents tolerate the presence of young, remaining in a family group should be the best strategy, in terms of lifetime reproductive success, for offspring to follow until becoming breeders. Tests of this hypothesis might only be possible intraspecifically, where fitness of offspring with contrasting strategies is compared. Good evidence in support of this hypothesis is already available from field studies (Stacey & Ligon 1987
; Ekman et al. 1999
; Griesser et al. 2006
), but more studies in other systems comparing lifetime reproductive success of individuals with delayed versus prompt reproduction are needed to substantiate or reject this hypothesis.
More generally, studies on family living have normally focused on costs and benefits of delayed dispersal and helping behaviour in cooperative breeders. We suggest that we should look to alternative systems in order to progress. Novel and important insights might now be achieved by studying non-family-living species in family-living clades (e.g. Green & Cockburn 2001
) or species from non-family-living clades that have cooperative behaviours. In addition, the overwhelming majority of studies have focused on what happens during the breeding season, but to reproduce individuals must survive the non-breeding season, and thus the factors affecting the survival of individuals outside the breeding season should be crucial. Finally, it would also be important to understand why delayed dispersal and family living are not found in many species (both short- and long-lived) living in saturated habitats. In fact, most situations where kin associations should be found but are not might provide the best opportunities to falsify hypotheses and obtain novel insights into the evolution of family living.