Animals, whether they are sessile or mobile, must also 'self-thin', insofar as growing individuals within a cohort increasingly compete with one another and reduce their own density. There is nothing linking all animals quite like the shared need for light interception that links all plants, so there is even less likelihood of a general self-thinning 'law' for animals. On the other hand, crowded sessile animals can, like plants, be seen as needing to pack 'volumes' beneath an approximately constant area, and mussels, for example, have been found to follow a thinning line with a slope of -1.4, and barnacles a line with a slope of -1.6 (Hughes & Griffiths, 1988). Moreover, self-thinning in the gregarious tunicate, Pyura praeputialis, on the coast of Chile was found to follow a slope of only -1.2; but when the analysis was modified to acknowledge that rocky shore invertebrates are more 'three-dimensional' than plants, and may fit more than one layer into a fully occupied area (as opposed to the constant leaf area index of plants), then the estimated slope was -1.5 (Figure 5.34a).
For mobile animals, it has been suggested that the relationship between metabolic rate and body size could generate thinning lines with slopes of -4/3 (Begon et al., 1986). However, the generality of this is probably even more questionable than the 'rules' in plants, given variations in resource supply, variations in the coefficients in the underlying relationships, and the possibilities of self-thinning depending on, say, territorial behavior rather than simply food availability (Steingrimsson & Grant, 1999). Nonetheless, evidence of self-thinning in animals is increasingly reported, especially in fish, even if the basis for it remains uncertain (e.g. Figure 5.34b).
Plants are not so consistent in their pattern of self-thinning as was once thought. It may be that animals are not much less bound than plants by 'general' self-thinning rules.
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