Growth, allometry, and characteristics of a sexually selected structure in wolverine (Gulo gulo (Linnaeus, 1758)), northern river otter (Lontra canadensis (Linnaeus, 1758)), and sea otter (Enhydra lutris (Linnaeus, 1758))

Abstract

Allometric analyses of sexually selected structures have revealed many patterns of evolutionary and behavioural significance (e.g. in weapons, ornaments, genitalia). We investigated allometry of the baculum (penis bone) relative to body size in adults of three large mustelids: wolverine (Gulo gulo (Linnaeus, 1758)), northern river otter (Lontra canadensis (Schreber, 1776)), and sea otter (Enhydra lutris (Linnaeus, 1758)). Bacular growth took place over a longer period than body growth. Correlations among bacular variables were positive. No regression slopes expressed positive allometry (i.e. slope > 1 for linear variables). These trends point to the possibility that bacular size is adapted to average size of the reproductive tract of sexually mature female northern river otters and possibly sea otters, and that pre-ejaculatory (“pre-copulatory”) selection is highest in those species. Bacular size varied more than skull or limb-bone size; bacular shape also varied greatly. Species differed in size and complexity of the urethral groove and bacular apex, suggesting functional differences in intromission. Substantial variation in bacular shape resulted from healed fractures, especially in sea otter. Knowledge of copulatory behaviour, age of breeding, female reproductive anatomy, and genitalic interactions during intromission is needed for fuller understanding of bacular anatomy, allometry, and variation for these species.

Microarchitecture of the penis bone (baculum) of a seal: A 3D morphometric examination using synchrotron and laboratory micro‐computed tomography

“…Post sexual maturity, bacular development in harp seals is adapted to copulation involving prolonged intromission in an aquatic environment with opportunistic, competitive mating (Hammill & Stenson, 2022;Merdsoy et al, 1978) to sustain mechanical load, avoid urethral occlusion, and overcome sperm competition (Brassey et al, 2018); this is supported by our findings of increased cortical bone growth in the midshaft region from categories I to II, and in the proximal region from II to III. Furthermore, several studies examining bacular fractures have reported sites of fracture at the midshaft (Bartosiewicz, 2000;Friley, 1949;Kierdorf, 1996;Miller et al, 2023). This is supported by our finding that the proximal region can sustain torsion and bending better than the midshaft, which becomes increasingly predisposed to fractures with age (and reduced cortical area) due to asymmetric loading during intromission.…”