Altered aggression in different sized groups of crayfish supports a dynamic social behaviour model

Living in groups with conspecifics can increase an animal's fitness in the wild. A social environment may also be imposed by commercial farming industries. One important measure of competition and group dynamics is the level of aggressive interaction. This can also influence the level of damage or injury in cultured populations, a commercial issue at point of sale. There is considerable research into this issue in commercial species such as pigs, cattle and chickens but less is known about aquatic communal species such as decapod crustaceans. Here we manipulated group size in the freshwater crayfish Cherax destructor, a species that forms social groups in the wild and one that is also commercially farmed. Aggressive behaviour was scored during 1 h of observation in replicates of groups of 4, 16 and 36 animals to analyse 11 variables of fight dynamics that ensued. The number of fights per crayfish (4.0 ± 0.8 to 1.9 ± 0.2 fights, P = 0.017) and the time each crayfish was involved in a fight (113.9 ± 32.6 to 21.6 ± 2.6 s, P = 0.011) decreased as group size increased. Conversely, the number of failed tailflips elicited per crayfish increased from 0 to 0.08 ± 0.03 tailflips in the largest groups (P = 0.011). Together, the data suggest that despite C. destructor's different biology and habitat, compared to prior work that manipulates group size, the crayfish adjusts its fighting strategy when social circumstances change. Theory has proposed aggressive behaviour could change in groups of animals and our data indicates that this applies more broadly across species and more dynamically than previously demonstrated.