Baculum and testes of the hooded seal (Cystophora cristata): growth and size-scaling and their relationships to sexual selection

Abstract

Growth and size-scaling of the baculum and testes in the moderately polygynous hooded seal (Cystophora cristata) were studied using 107 specimens of known age (1 month to 28 years) from the northwestern Atlantic. Bacular growth was rapid between 2 and 5 years of age: length increased 150% and "density" (i.e., mass/length) increased 8-fold and mass 20-fold. Growth continued throughout life. In large, old (>14 years) males, the baculum averaged 20.7 cm in length, 2.1 g/cm in density, and 44.4 g in mass. Bacular length increased relative to body length until seals were about 5 years of age, after which it averaged 8.2%. Testicular growth continued until the seals were about 12 years of age. Testes from breeding males >12 years old averaged 11.2 cm in length, 4.6 cm in width, and 138 g in mass; length averaged 4.9% of body length. In males 2-5 years of age, bacular and testicular sizes were positively allometric relative to body length; in older males, bacular mass and density were positively allometric, and bacular length and testicular size isometric, relative to body length. Bacular size was mostly positively allometric relative to testicular size (bacular length exhibited some isometry). Compared with that of the related and ecologically similar harp seal (Pagophilus groenlandicus), which is presumed to have a promiscuous mating system, the baculum of the hooded seal was structurally simpler and grew more quickly but reached a relatively smaller size in adults (8.2 vs. 9.9% of body length). Relative testicular length was also smaller (4.9 vs. 5.7% of body length) and bacular density lower (2.1 vs. 2.8 g/cm) than in the harp seal. These observations suggest that intra-or inter-sexual competition via copulation is weaker in the hooded seal.Résumé : La croissance et la taille relative du baculum et des testicules ont été étudiées chez le Phoque à capuchon (Cystophora cristata), un animal modérément polygyne, par examen de 107 spécimens d'âge connu (1 mois à 28 ans) de l'Atlantique du nord-ouest. La croissance du baculum est rapide de 2 à 5 ans et accuse une augmentation de longueur de 150%, une augmentation de la « densité » par un facteur de 8 (i.e., masse/longueur) et une augmentation de la masse par un facteur de 20. La croissance se poursuit durant toute la vie. Chez les vieux mâles (>14 ans), le baculum est en moyenne de 20,7 cm de longueur, de 2,1 g/cm de densité et de 44,4 g de masse. La longueur du baculum augmente relativement à la longueur du corps jusqu'à l'âge de 5 ans environ, après quoi elle fait 8,2% de la longueur du corps. La croissance des testicules se poursuit jusqu'à l'âge de 12 ans environ. Les testicules des mâles reproducteurs de plus de 12 ans ont en moyenne 11,2 cm de longueur, 4,6 cm de largeur et 138 g de masse; leur longueur est égale en moyenne à 4,9% de la longueur du corps. Chez les mâles de 2-5 ans, la taille du baculum et celle des testicules ont une relation allométrique positive avec la longueur du corps; chez les mâles âgés, la masse et la densité du baculum sont également en rel...

Morphological variability of baculum (os penis) in the polecat Mustela putorius

“…One source of such differences between baculum was age: juveniles had smaller os penis than adults in all three analysed traits. This is typical for the polecat (WALTON 1968;BUCHALCZYK & RUPRECHT 1977;WOLSAN 1993), as well as for other mammals (REINWALDT 1961;SUMIÑSKI 1968;MILLER et al 1999;MILLER & BURTON 2001). The obtained measurements were similar to those presented in other studies (WALTON 1968;BUCHALCZYK & RUPRECHT 1977).…”

“…that baculum shape and size is related to the testosterone level of an individual (MILLER et al 1999;KELLY 2000;MILLER & BURTON 2001;DYCK et al 2004). …”

Morphological variation and allometry of the baculum in stoats, Mustela erminea (Carnivora, Mustelidae) from Western Carpathians

“…In contrast, for two species (Martes americana and M. pennanti) with male-biased sexual size dimorphism was showed different results depending on the regression model used (SchulteHostedde et al 2011). These results were not consistent with other studies of mammals MORPHOLOGICAL VARIATION AND ALLOMETRY OF THE BACULUM IN STOATS that implicate sexual selection as an important factor in explaining variation in baculum morphology (Miller et al 1999, Miller & Burton 2001, Lüpold et al 2004, Kinahan et al 2007, Tasikas et al 2007, Yurkowski et al 2011). Authors assumed that stabilizing selection rather than sexual selection was the evolutionary force shaping variation in baculum length because allometric slopes were less than one (using the OLS regression model).…”

“…Demuth et al (2009) and Krawczyk et al (2011) confirmed that the males in better condition have bigger bacula which confirms that this bone is potentially a good indicator of viability and quality in males. On the other hand, reproductive behaviour and mating system are very important for the evolution of penis size (Miller et al 1998, 1999, Oosthuizen & Miller 2000, Miller & Burton 2001, Ferguson & Lariviére 2004, Ramm 2007. In contrast, according to Kinahan et al (2008) and Schulte-Hostedde et al (2011), baculum allometry may be isometric or exhibit negative allometry in species with pre-copulatory selection.…”

“…A multi-male mating system occurs in many species of Mustelidae (Lariviére & Jennings 2009), so the baculum may be under strong sexual selection. If so, there may be a limit to baculum growth (Miller et al 1998, 1999, Oosthuizen & Miller 2000, Miller & Burton 2001, Miller & Nagorsen 2008, Kinahan et al 2008.…”