Size, Life-History Traits, and Social Organization in the Canidae: A Reevaluation

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Any selective advantages of large body size may be counteracted by an increase in energetic costs associated with reproduction. In canids, larger female body size has been sug- gested to be associated with increasingly altricial young, larger litter sizes, and an increase in female pre- and postpartum energetic investment in offspring. It is hypothesized that the chang- ing costs of reproduction with increasing body size result in a size-related diversity of canid reproductive life histories and social organization. Smaller canid species require less postpartum investment and thus tend toward polygyny and have a skewed dispersal ratio toward males. Larger species, with greater prepartum investment, require greater male investment in the rear- ing of offspring and thus tend to be group living. Using data on canid life histories, a phylogeny of the Canidae based on 383 base pairs of mitochondrial DNA, and an autoregressive compara- tive method, we found this hypothesis was not supported. Strong isometric relationships be- tween neonate weight and female weight indicate that neonate size is constrained by female size directly or by parameters co-varying with female body size. Female weight accounted for only 26% of the variance in litter size, and no correlation was found between litter size and neonate weight. This result implies that female prenatal investment can be adjusted only by litter size (and not by neonate or litter weight), which in turn may be adjusted according to resource availability, an explanation supported by field and laboratory studies. In general, we hypothesize that much of canid interspecific and intraspecific variation in social structure may be explained by focusing on proximate enivironmental mechanisms, specifically resource avail- ability.