Environmental enrichment reduces aggression of pearl cichlid, Geophagus brasiliensis, during resident-intruder interactions

Abstract

Among fishes, when residents and intruders fight, residents usually win, most likely because they value the residence more than intruders. We hypothesized that enriched environments increase the value of an area in dispute, causing a resident to more fiercely defend a resource-rich environment than a poor one. However, in the present study, intruder-resident tests with the pearl cichlid, Geophagus brasiliensis, showed environmental enrichment actually reduces aggression and can even lead to co-habitation without fighting. Additionally, in our experiments, the prior residence effect occurred irrespective of enrichment condition. Decreased visibility from increased habitat complexity reduces interactions between fish and consequently might explain the lower aggression observed herein.Em lutas entre peixes residentes e intrusos, residentes geralmente vencem, provavelmente porque valorizam o território mais do que os intrusos. Nós aventamos a hipótese de que ambientes enriquecidos aumentam o valor da área em disputa, levando os residentes a defenderem mais violentamente ambientes enriquecidos de recursos do que ambientes empobrecidos, pois possuiriam mais motivação para mantê-lo. No entanto, no presente estudo, ao testarmos as interações entre intrusos e residentes em acarás, Geophagus brasiliensis, observamos que o enriquecimento ambiental reduz a agressividade e pode levar a cohabitação entre os peixes, sem luta. Adicionalmente, em nossos experimentos, o efeito da residência prévia ocorreu independente da condição de enriquecimento. A diminuição das interações entre os peixes e, consequentemente, a diminuição do nível de agressividade é aqui atribuída aos efeitos da diminuição da visibilidade entre os peixes devido ao aumento de complexidade do ambiente.

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

“…These proportions were statistically different (Goodman test, between multinomials; G observed = 2.83 > G expected = 1.96, P <0.05 and two-tailed Fisher's exact test, P = 0.029). Furthermore, as shown in our article (Kadry & Barreto, 2010), the frequency of aggressive interactions was significantly lower in the enriched environment compared to the nonenriched environment for both resident (Z = 3.27, p < 0.01) and intruder fish (Z = 2.17, p < 0.05). Moreover, in the nonenriched environment, the frequency of attacks directed by the resident animal was statistically higher than for the frequency of attacks directed by the intruder animals (Z = 2.07, p < 0.05).…”

“…Other possibilities could also be suggested. In the case of our study (Kadry & Barreto, 2010), how can we explain our data differently if EE reduced aggression or led to cohabitation without confrontation in pearl cichlids? We assume that the data should be above expectations, irrespective of how nice expectations should be.…”

“…Here, I offer five aspects of counter-arguments that maintain the validity of our study. Furthermore, as the research coordinator of the Kadry & Barreto (2010) publication and as the former supervisor of the undergraduate student V. O. Kadry, I assume any liability in relation thereto.…”

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

“…higher tenure length, more complex environment) and the more valuable the environment is the stronger the prior-residency effect should be (Nijman & Heuts, 2000;Johnsson & Forster, 2002). Kadry & Barreto (2010) recently reported that in intruderresident tests with pearl cichlid, Geophagus brasiliensis, pairs in environmentally enriched test conditions [an aquarium with two pebbles and a plastic kelp model] showed lower frequencies of aggressive interactions than pairs in nonenriched test conditions [a barren aquarium]. Furthermore, they found that in non-enriched test conditions the frequency of directed attacks by resident fish was higher than that of intruders.…”

“…Working with a cichlid, Kadry & Barreto (2010) made reference to our work on another cichlid (Burton's mouthbreeder, Haplochromis burtoni) in intruder-resident tests in enriched and non-enriched conditions (Nijman & Heuts, 2000). They reported that for this species we found the residents to have a greater dominance advantage in environmentally enriched conditions.…”