Effects of competitor-to-resource ratio on aggression and size variation within groups of convict cichlids

doi:10.1016/j.anbehav.2004.07.019

Animal Behaviour

Volume 69, Issue 5, May 2005, Pages 1157-1163

https://doi.org/10.1016/j.anbehav.2004.07.019 Get rights and content

Resource defence theory predicts that the intensity of competitive aggression, degree of resource monopolization, and variation in fitness will be highest at intermediate levels of the spatial clumping of resources. We tested for this predicted dome-shaped relationship by manipulating the spatial clumping of food via competitor-to-resource ratio (CRR), the number of potential competitors divided by the number of resource units. Groups of 10 convict cichlids, Archocentrus nigrofasciatus, were allowed to compete for a fixed amount of food in one of five CRR treatments (1, 1.43, 2, 5 and 10), created by varying the number of patches in which the food appeared (i.e. 10, 7, 5, 2 and 1 patch, respectively). As predicted, both the frequency of aggression and the coefficient of variation of body mass (a measure of the consequences of food monopolization and variation in a component of fitness within groups) increased significantly as CRR increased from 1 to 2, and then decreased significantly at CRRs of 5 and 10. In addition, mean growth rate decreased in groups with high rates of aggression, suggesting an important cost of aggression. Our study provides the first quantitative support for the predicted dome-shaped relationship and suggests that CRR is a useful measure of the degree of spatial clumping of resources.

Section snippets

Experimental subjects

We chose juvenile convict cichlids as test subjects because they grow quickly and readily defend food patches in laboratory conditions (Grant and Guha, 1993, Praw and Grant, 1999). In the wild, adult convict cichlids are aggressive when defending offspring and nest sites, but neither adults nor juveniles have been observed to defend food resources (Wisenden 1995). Food defence in the laboratory occurs presumably because food is distributed in an economically defensible manner, unlike in the

Results

If fish distribute themselves equally among patches, as predicted by an ideal free distribution (Fretwell & Lucas 1970), then the average number of fish per patch should increase in direct proportion to CRR (Fig. 2). Consistent with this assumption, the average number of fish within one body length of a given patch increased with increasing CRR (ANOVA: F_4,20 = 262.3, P < 0.0001; Fig. 2). However, more fish than expected (i.e. the one-to-one line was below the 95% confidence limit) were observed per

Discussion

The distribution of fish across patches was similar to what was expected based on an ideal free distribution. Not surprisingly, some fish were more than one body length from any patch during the scan samples. Some of these individuals were probably moving between patches, whereas others were excluded from patches by aggressive behaviour. The higher than expected number of fish per patch at CRRs of 1 and 1.43, when patches were close together, was probably caused by some dominant fish attempting

Acknowledgments

We thank Cindy Breau for help in the laboratory and Stefan Steingrímsson, Diana Hews and two anonymous referees for comments on the manuscript. This research was financially supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada to J.W.A.G.

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View full text

Effects of competitor-to-resource ratio on aggression and size variation within groups of convict cichlids

Section snippets

Experimental subjects

Results

Discussion

Acknowledgments

Animal Behaviour

Aquaculture

Animal Behaviour

Animal Behaviour

Animal Behaviour

Animal Behaviour

Animal Behaviour

The relative growth rate of dominant and subordinate juvenile steelhead trout (Salmo gairdneri) fed equal rations

Behaviour

The effect of group size on mean food intake rate in birds

Biological Reviews

Fitness consequences of foraging success in water striders (Gerris remigis; Heteroptera: Gerridae)

Behavioral Ecology

The evolution of diversity in avian territorial systems

Wilson Bulletin

Food abundance and territoriality: to defend or not to defend?

American Zoologist

Mating systems

Food distribution and a variable mating system in the dunnocks, Prunella modularis

Journal of Animal Ecology

Resource defense in a group-foraging context

Behavioral Ecology

Ecology, sexual selection, and the evolution of mating systems

Science

On territorial behavior and other factors influencing habitat distribution in birds

Acta Biotheoretica

Whether or not to defend? The influence of resource distribution

Marine Behaviour and Physiology