Despite the important role that population density plays in ecological and evolutionary processes, studies of solitary species that occur at low densities remain scarce. In the context of mating systems, density is expected to influence the ability of males to find and monopolize mates, in turn influencing variance in lifetime mating/reproductive success and the opportunity for selection. Herein we investigate variance in male lifetime mating success, lifetime reproductive success, and the mating system of a sexually dimorphic carnivore that occurs at low densities, the polar bear (Ursus maritimus). Across 17 cohorts, born from 1975 to 1991, male lifetime mating success ranged from 0 to10 mates and lifetime reproductive success from 0 to 14 cubs; 40% of known-age males were not known to have reproduced. The opportunity for sexual selection (Is = 1.66, range = 0.60-4.99) and selection (I = 1.76, range: 0.65-4.89) were low compared to species with similar levels of sexual size dimorphism. Skew in male lifetime reproductive success was also low but significant for most cohorts indicating non-random reproductive success. Age-specific reproductive success was biased toward males from 11-17 years of age, with variation in fecundity (54%) but not longevity (10%) playing an important role in male reproduction. Our results support a growing body of evidence that suggests that male-biased size dimorphism and polygynous mating systems need not be associated with high variance in male mating and/or reproductive success.
All methods are descriped in the publication.
Fitness measurements of adult male polar bears
Fitness measurements and year of birth are provided for all male bears in the study. Details on data collection and abbreviations can be found in the publication. (see data file Richardson et al. cohort data.xlsx)
Age frequency data
Age frequency data for paternal assignments (see Malenfant et al. 2016) and capture sample from the western Hudson Bay polar bear program. Data file (Richardson et al. age frequency data.xlsx) contains two work books. 'Paternity age frequency' contains the frequency of paternal assignments for each age class and 'Capture age frequency' contains the age frequency of male bears from 1980-2010.