Aggression and dominance in cichlids in resident-intruder tests: the role of environmental enrichment

When establishing dominance residents have a greater likelihood to dominate intruders than vice versa, partially because the resident has more to loose that the intruder has to win. This is known as the prior residency effect. In environmentally rich environments this effect should be stronger than in poor environments. Recently Kadry & Barreto (2010, Neotrop Ichthyol 8: 329-332) tested this in the pearl cichlid Geophagus brasiliensis (17 test pairs) and reported that environmental enrichment led to a reduction of aggression. We here present data on four other cichlids (332 test pairs) showing a stronger prior residency effect in enriched conditions, and, for two species, an increase in aggression. We discuss possible reasons for the differences between studies, focussing on the relationship between aggression and dominance and sample size effects.

Prior residency does not always pay off – co-infections inDaphnia

SUMMARYThe epidemiological and ecological processes which govern the success of multiple-species co-infections are as yet unresolved. Here we investigated prior versus late residency within hosts, meaning which parasite contacts the host first, to determine if the outcomes of intra-host competition are altered. We infected a single genotype of the waterfleaDaphnia galeatawith both the intestinal protozoanCaullerya mesniliand the haemolymph fungusMetschnikowiasp. (single genotype of each parasite species), as single infections, simultaneous co-infections and as sequential co-infections, with each parasite given 4 days prior residency. Simultaneous co-infections were significantly more virulent than both single infections and sequential co-infections, as measured by a decreased host life span and fecundity. Further, in addition to theDaphniahost, the parasites also suffered fitness decreases in simultaneous co-infections, as measured by spore production. The sequential co-infections, however, had mixed effects:C. mesnilibenefited from prior residency, whereasMetschnikowiasp. experienced a decline in fitness. Our results show that multiple-species co-infections ofDaphniamay be more virulent than single infections, and that prior residency does not always provide a competitive advantage.

Density-dependent effects of prior residence and behavioural strategy on growth of stocked brown trout (Salmo trutta)

When animals face temporally periods of strong intraspecific competition, prior residency, available resources, and their competitive abilities in terms of size and behavioural strategy affect their chances of survival. Density, individual size, and behavioural strategy had the strongest effect on growth. The mean growth rate of both resident and intruding brown trout (Salmo trutta L., 1758) decreased with increasing density, and the largest individuals were the most successful ones independently of the density of prior residency. An aggressive behavioural factor was beneficial at the lowest and intermediate densities, whereas a nonaggressive behavioural factor was beneficial at higher densities. Prior residency had no overall significant effect on growth, as the effect was highly density-dependent. The difference in growth rate between introduced and resident individuals was significant only at high density and low food abundance per individual. The intruders had a significantly lower growth rate at high densities and fewer individuals had a high growth rate. These results suggest that stocking fish at densities exceeding the carrying capacity of the habitat results in fewer individuals that are able to compete for resources than if fewer individuals were stocked. Brood stock can then be used in a more efficient way.

Determinants of social rank in female black-capped chickadees (Poecile atricapilla)

Black-capped chickadees (Poecile atricapilla) mate assortatively by social rank. Previous field studies suggest that intrinsic characteristics of females may influence success at pairing with dominant males. Here we examined factors leading to dominance using dyads of captive unfamiliar females. The owner–intruder hypothesis predicts that prior residency determines dominant–subordinate relationships. The resource-value hypothesis suggests that social status is initially determined by need and the relationship persists through familiarity of the interactants. The resource holding potential hypothesis suggests that individuals win in dyadic contests because of intrinsic characteristics such as size or age. We tested the owner–intruder and resource-value hypotheses by allowing females prior residency in aviaries where dominance interactions subsequently occurred and by food depriving the intruders. Post-hoc comparisons of dominant–subordinate attributes tested the resource holding potential hypothesis. We found that owners were more likely to win interactions. Hungry individuals showed no competitive advantage. Dominants and subordinates did not differ in morphology or age. Our results agree with data from willow tits (Parus montanus) which show that captive females establish dominance independent of males and that prior residence plays a key role. These findings, together with field studies, suggest that assortative mating in chickadees results, at least in part, from intrasexual interactions among females.