Animalioinfo

Introduction

Social organization is among the most fundamental traits in animal societies, characterizing the ecological and social selective pressures of a species (Ross & Keller 1995). Social organization can lead to extremely complex societies (de Waal & Tyack 2003). One of the most basic structural components of any society is the formation of groups. According to Wilson (1975), we define a group as a collection of individuals of the same species who stay together and interact more with each other than with others of the same species. Although the process by which individuals decide to aggregate has been of interest to scientists for over a century (Galton 1871), there are still many unanswered questions about how this decision occurs (Krause & Ruxton 2002).

Social structure (ie, the formation of groups through connections between individuals) can have a significant effect on selective pressure. Grouping can provide benefits such as protection from predators, mate finding assistance, thermoregulation assistance, increased movement efficiency and foraging benefits (Krause & Ruxton 2002). However, for group life to be adaptive, the benefits must generally outweigh the costs in the group (Alexander 1974).

Materials and methods

Study organism and laboratory conditions
We collected male shore crabs c. 2 km north of Dublin, Ireland and immediately transported them to the laboratory, where we marked each crab (1 × 1 cm2 tag attached to the central region of the dorsal carapace) and measured its carapace width. We then placed individuals in isolated containers of seawater in an environmental chamber (14 °C, 12 h light:dark) for 7 days before starting the experiment ( Sneddon & Swaddle 1999).

Experiments and observations
To start the experiment, crabs were placed in tanks (nine crabs per 0.4 m2 tank, approximately 22 crabs m−2) filled with 25