The influence of siblings on dispersal behavior is not easy to predict, particularly when siblings are capable of both cooperation and conflict. In most emigration models, the assumption that competition for resources is primary leads to the prediction that high sibling densities result in reduced fitness and early emigration. Most models do not consider the possibility that siblings may increase resource availability or fitness. In order to separate the effects of resource availability and sibling density on emigration behavior, we manipulated both factors in wood galleries of the passalid beetle, Odontotaenius disjunctus Illiger (Coleoptera: Passalidae). Passalid beetles are ideal for this study due to their cooperative sibling care and large numbers.

In the spring of 2007, beetles were placed as pairs on decayed logs that had been sliced to allow observation of galleries. In August all larvae were removed from their galleries and randomly assigned in densities ranging between 2 and 20 larvae. Food availability was altered by removing parents’ frass (larvae food) which was then redistributed to logs in treatments of low, medium, and high abundance. Logs were subsequently fitted with pitfall traps which were daily checked for emigrants. Every few weeks a subsample of logs from which emigrants had emerged was opened so that the number and condition of residents could be compared to that of emigrants.

Results/Conclusions

Parent survival and health were better predictors of offspring survival, length, and emigration behavior than was food availability. Early dispersal was most likely for those individuals with an injured father, especially if he was small. Sibling density had surprisingly little effect on survival and emigration behavior, negatively impacting survival only when a parent was missing. Delayed emigration was positively associated with sibling density, especially when the parents were naturally mated (wild-caught pairs were kept together when possible). Early dispersers tended to be large, but the opposite was true when a parent had died or a father was injured. Further research is required to determine if the relationship between offspring size and emigration timing is related to dominance or whether late dispersing offspring from poor habitats benefitted from the care of older siblings. These data emphasize the importance of parental and sibling interactions on offspring survival, body-size, and emigration timing.