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Environmental factors exert strong effects on phenotypic expression. A particularly intriguing factor capable of inducing such plastic responses is the social environment experienced by a specific individual. Such social effects may alter the fitness of focal individuals if they affect the expression of reproductive traits and thus life-history strategies. To examine this question, we investigated the effects of individual density on morphology, reproduction, and behavior of male Bicyclus anynana butterflies. Increasing density significantly increased male body mass and the probability to succeed in aggressive interactions and tended to increase abdomen fat content. At the same time, increasing density significantly decreased courtship activity and tended to decrease sperm number. These results suggest that individual density seemed to induce differential strategic investment into survival and somatic maintenance versus reproduction in male butterflies. Males kept at high densities apparently favored high body mass and storage, which may enable longer survival during times of intense intraspecific competition. Moreover, their competitiveness was enhanced as suggested by a higher success in aggressive interactions. Males kept at low density, in contrast, favored reproduction through increased courtship activity and sperm production. Our study illustrates that the effects of density on the expression of morphological and behavioral traits are complex and difficult to predict, owing to resource-allocation trade-offs resulting in prudent strategic investment. [ABSTRACT FROM AUTHOR]

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