Evolutionary allometry of the canid baculum (Carnivora: Mammalia)

While the mammalian baculum shows enormous morphological variability, the baculum of canids is highly conserved, with most variation restricted to size. Here, we explore the allometric relationship between baculum length and body size in extant and extinct canids. Examination of 26 species in the extant subfamily Caninae using standard linear regression revealed isometry. Phylogenetic regression also revealed an allometric slope indistinguishable from isometry. This pattern differs from the substantially negative slopes seen in other mammalian clades. The strength of the canid allometric relationship (r2) is also greater than in other clades, suggesting functional constraints on their baculum size. The constraints may be related to the copulatory tie that is characteristic of canids, and/or their monogamous mating system. Complete bacula are known from just four extinct species. The two complete bacula from the extinct subfamily Borophaginae (Aelurodon ferox and Aelurodon stirtoni) fall on the same allometric relationship as the living canids. However, the baculum of the extinct dire wolf (Aenocyon dirus, from the extant subfamily Caninae) and from the extinct subfamily Herperocyoninae, Hesperocyon gregarius, are significantly longer than expected based on their body sizes, suggesting that they may have had a different reproductive biology from that of extant canines.