The functions of societies and the evolution of group living: spider societies as a test case

Abstract

Many models have been advanced to suggest how different expressions of sociality have evolved and are maintained. However these models ignore the function of groups for the particular species in question. Here we present a new perspective on sociality where the function of the group takes a central role. We argue that sociality may have primarily a reproductive, protective, or foraging function, depending on whether it enhances the reproductive, protective or foraging aspect of the animal's life (sociality may serve a mixture of these functions). Different functions can potentially cause the development of the same social behaviour. By identifying which function influences a particular social behaviour we can determine how that social behaviour will change with changing conditions, and which models are most pertinent. To test our approach we examined spider sociality, which has often been seen as the poor cousin to insect sociality. By using our approach we found that the group characteristics of eusocial insects is largely governed by the reproductive function of their groups, while the group characteristics of social spiders is largely governed by the foraging function of the group. This means that models relevant to insects may not be relevant to spiders. It also explains why eusocial insects have developed a strict caste system while spider societies are more egalitarian. We also used our approach to explain the differences between different types of spider groups. For example, differences in the characteristics of colonial and kleptoparasitic groups can be explained by differences in foraging methods, while differences between colonial and cooperative spiders can be explained by the role of the reproductive function in the formation of cooperative spider groups. Although the interactions within cooperative spider colonies are largely those of a foraging society, demographic traits and colony dynamics are strongly influenced by the reproductive function. We argue that functional explanations help to understand the social structure of spider groups and therefore the evolutionary potential for speciation in social spiders.

Maternal effects in cooperative breeders: from hymenopterans to humans

“…Such behaviour poses a paradox for current evolutionary thinking, namely: how can a strategy evolve which not only reduces personal fitness but also increases the fitness of competitors? Far from being an esoteric exception, cooperative breeding is found in multiple animal classes (Insecta, Arachnida, Crustacea, Pisces, Aves, Mammalia, including humans), has had multiple evolutionary origins within classes and is the hallmark of some of the most successful animals on the planet (ants, termites, humans) (Wilson 1971;Hö lldobler & Wilson 1990;Stacey & Koenig 1990;Taborsky 1994;Choe & Crespi 1997;Solomon & French 1997;Duffy et al 2000;Whitehouse & Lubin 2005). As a consequence, an impressive amount of work has been conducted on cooperative breeding systems since the publication of Hamilton's seminal work on inclusive fitness, over four decades ago (Hamilton 1964).…”

“…Helper effects refer to the effect of helper number on the reproductive output of the group: NR, no relationship; LR, linear; QR, quadratic; AR, accelerating relationship. (Sources : Wilson 1971;Taborsky 1984Taborsky , 1994Stacey & Koenig 1990;Duffy 1996;Crespi & Mound 1997;Solomon & French 1997;Stern & Foster 1997;Avilés & Tufiñ o 1998;Bourke 1999;Duffy & Macdonald 1999;Hatchwell 1999;Russell 2004;Dierkes et al 2005;Heg et al 2005;Kranz 2005;Whitehouse & Lubin 2005;Bono & Crespi 2006;Lubin & Bilde 2007;Salomon & Lubin 2007;Korb 2008 in each is the fact that substantial variation exists within the same mother over time, among mothers within a population and between species (table 1). First, in the majority of species, cooperative groups consist of primary reproductive(s) and their offspring, which have foregone dispersal (Emlen 1995;Bourke 1997;Choe & Crespi 1997;Thorne 1997).…”

“…First, in the majority of species, cooperative groups consist of primary reproductive(s) and their offspring, which have foregone dispersal (Emlen 1995;Bourke 1997;Choe & Crespi 1997;Thorne 1997). However, the genetic structure of groups varies owing to immigration, co-breeding, breeder replacement, extra-group mating and /or group amalgamation (Stern & Foster 1997;Bourke 1999;Cockburn 2004;Russell 2004;Dierkes et al 2005;Whitehouse & Lubin 2005;Bono & Crespi 2006;Ratnieks et al 2006;Korb 2008). Second, to varying degrees, helpers/workers can be associated with a reduction in maternal contributions to offspring care, an increase in maternal productivity within seasons and survival between seasons, and/or an increase in the condition, survival and future reproductive capacity of offspring (Wilson 1971;Bourke 1997;Choe & Crespi 1997;Keller & Genoud 1997;Thorne 1997;Dickinson & Hatchwell 2004;Russell 2004;Russell et al 2007a;Salomon & Lubin 2007).…”

“…In some species, offspring might have a very high chance of dispersing successfully, co-breeding with the dominant or replacing it, while in others the chances are low and determined by phenotype (Choe & Crespi 1997;Bernasconi & Strassmann 1999;Bourke 1999;Clutton-Brock et al 2006). Fourth, significant variation exists in the size and complexity of cooperative breeding units, again within and among species (Wilson 1971;Noirot 1991;Thorne 1997;Bourke 1999;Cockburn 2004;Miura 2004;Russell 2004;Kranz 2005;Whitehouse & Lubin 2005;Korb 2008;Hager & Jones in press). Among taxonomic groups, the number of female breeders varies from one to several, division of labour varies from absent to extreme and group sizes range from one to millions.…”

South African National Survey of Arachnida (SANSA): review of current knowledge, constraints and future needs for documenting spider diversity (Arachnida: Araneae)

“…Amongst spiders, there are varying degrees of sociality and aggregation, although the species involved in such behaviour represent a small proportion of the global spider diversity (Whitehouse & Lubin, 2005). Eresids are a group of considerable arachnological interest, primarily because of the evolution of subsocial and quasisocial species from solitary ancestors in the genus Stegodyphus, behaviour that is absent in all of the other genera of the family (Kraus & Kraus, 1988;Miller et al, 2012).…”