Dimensions and allometry of testes, epididymides and spermatozoa in the domestic dog (Canis familiaris).

Dimensions of the testes, epididymides and spermatozoa were measured from a sample of 104, mainly cross-bred, domestic dogs ranging in body mass from 5 to 48 kg. Allometric analyses in relation to body mass indicated isometric changes in testicular mass but epididymal mass and the mass of the cauda epididymidis and sperm numbers in the cauda showed significant positive allometry. Total sperm length, seminiferous tubule diameter and the relative proportion of tissues in the testis showed no significant correlation with body mass, but sperm principal piece length showed a low correlation with body mass. We suggest that the positive allometry of epididymal mass and sperm numbers is related to promiscuous mating and sperm competition in stray dogs.