Animals fight over resources such as mating partners, territory, food or shelter and repeated contests lead to stable social hierarchies in different phyla. The group dynamics of hierarchy formation are not characterized in the Norway lobster (). Lobsters spend most of the day in burrows and forage outside of them according to a diel (i.e. 24 h-based) activity rhythm. Here, we use a linear and generalized mixed model approach to analyse, in seven groups of four male lobsters, the formation of dominance hierarchies and rank-related changes in burrowing behaviour. We show that hierarchies emerge within 1-3 days and increase in steepness over a period of 5 days, while rank changes and number of fights gradually decrease over a 5-day period. The rank position determined by open area fights predicts the outcome of fights over burrows, the time spent in burrows, and the locomotor activity levels. Dominant lobsters are more likely to evict subordinate lobsters from their burrows and are more successful in defending their own burrows. They spend more time in burrows and display lower levels of locomotor activity outside the burrow. Lobsters do not change their diel activity rhythms as a result of a change in rank, and all tested individuals showed higher activity at night and dusk compared with dawn and daytime. We discuss how behavioural changes in burrowing behaviour could lead to rank-related benefits such as reduced exposure to predators and energy savings.
In this study we aim at disentangling the causes and consequences of diel feeding rhythms in marine microzooplankton. We focused on the diel feeding activity of two heterotrophic dinoflagellate species, Gyrodinium dominans (one laboratory strain) and Oxyrrhis marina (laboratory cultivated and wild strains). We observed higher ingestion during the day in both dinoflagellate species. Feeding rhythms appeared to be independent of circadian changes in prey biochemical composition. Grazers fed with prey under stationary phase, with equivalent stoichiometric composition between day and night, showed 5 (G. dominans) and 10 (O. marina) times higher ingestion rates during the day. Previous grazer feeding history (starved vs well-fed) did not affect the feeding rhythm. However, prey concentration altered the rhythm; food limiting conditions reduced the amplitude of the rhythms. Our results establish a resource dependence of diel periodicity in microzooplankton grazing, which can have unanticipated consequences for standard field dilution grazing experiments.
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