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

Abstract

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.No estabelecimento de dominancia, residentes têm uma maior probabilidade de dominar os intrusos que vice-versa, em parte porque o residente tem mais a perder que o invasor tem a ganhar. Isto é conhecido como o efeito de residência prévia. Em ambientes ecologicamente ricos esse efeito deve ser mais forte do que em ambientes pobres. Recentemente Kadry & Barreto (2010, Neotrop. Ichthyol. 8: 329-332) testaram isso no cará Geophagus brasiliensis (17 pares de teste) e relataram que o enriquecimento ambiental levou a uma redução de agressão. Aqui apresentamos dados de quatro outros ciclídeos (332 pares de teste), mostrando um efeito mais forte de residência prévia em condições enriquecidas, e, para duas espécies, o aumento da agressão. Discutimos possíveis razões para as diferenças entre os estudos, concentrando-nos sobre a relação entre agressão e dominação e os efeitos do tamanho da amostra.

What tunes aggression in fish during fighting in environments that vary in enrichment?

“…First, Heuts & Nijman (2011) argue that our data do not show a significant effect of prior residence. We tested 9 pairs in a non-enriched environment (control) in the context of the intruder-resident animal relationship.…”

“…Moreover, we also concluded that, regardless of the condition of enrichment (pebbles and kelp or no enrichment), there is a greater probability that the resident pearl cichlid will win a fight. In volume 9, issue 3, Jul-Sep 2011 of Neotropical Ichthyology, however, our study was criticized by Nijman & Heuts (2011). Here, I offer five aspects of counter-arguments that maintain the validity of our study.…”

“…In 10 of these 13 pairs, the resident animal became dominant. By using the binomial test, Heuts & Nijman (2011) found that the proportions of dominant residents and intruder animals are statistically equivalent (P = 0.092) and concluded that we were wrong to suggest that there was effect of prior residence regardless of the condition of enrichment. But when we analyzed the same data with Goodman's test, we found that the proportion of dominant resident fish is statistically higher (P <0.05) than the proportion of dominant intruder fish.…”

“…Of these 9 pairs, 7 residents and 2 intruders became dominant. These authors performed a new statistical analysis (binomial test) and found that the frequencies of resident and intruder fish as dominant were statistically indistinguishable (P = 0.18); however, this analysis by Heuts & Nijman (2011) used an incorrect sample size. While the correct sample size is 9 pairs, they used 10 pairs and compared 7 resident winners vs. 3 intruder winners.…”

“…The fourth point concerns a false dichotomy made by Nijman & Heuts (2011). They said that as the resident animals tend to value their territory, they tend to defend that territory more forcefully, increasing the likelihood of winning a fight against an intruder animal.…”