Territorial behavior and breeding frequency of male Weddell seals (Leptonychotes weddelli) in relation to age, size, and concentrations of serum testosterone and cortisol

The onset of sexual maturity and changes in weight and serum testosterone and cortisol concentrations were studied in male Weddell seals during October–December, 1986, at a breeding colony in McMurdo Sound, Antarctica. Ages were estimated from length or known from tagging history. Underwater copulatory and territorial activities were monitored by colored grease transfer and radiotelemetry, respectively. Hormone concentrations were measured by radioimmunoassay. Of 37 male seals visiting the colony, 22 were resident for ~5 weeks. About one-third of the male residents defended territories into estrus and were called territorial (T). Another one-third appeared unsuccessful at defending a territory and were called transitional (TN). The remaining one-third were nonterritorial (N). Males were closely matched in size (coefficient of variation ≤ 15% for length, girth, and weight). Most N males were 5–7 years old. T males (≥ 7 years old), being older (P < 0.05) than N males, attained 19 of 20 observed copulations. T males were heavier initially (P < 0.10) than TN or N males, and they lost more weight during the breeding season (P = 0.08, 3.2 vs. 2.1 kg/day) than N males. In all males, serum testosterone and cortisol concentrations declined, approaching nadir as estrus and the breeding season ended. Mean (±SE) daily concentrations ranged from 6.8 ± 2.4 ng/mL to nondetectable concentrations for testosterone and from 104.8 ± 13.2 to 54.7 ± 4.5 μg/dL for cortisol. Concentrations of both hormones were higher in T males than in N males. Hormone profiles of TN males initially resembled those of T males, but at estrus resembled those of N males. Coincident with a change in competitive behavior was a transient rise in cortisol accompanied by a drop in testosterone.