Variance in lifetime reproductive success of male polar bears

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.