Larger body size leads to greater female beluga whale ovarian reproductive activity at the southern periphery of their range

Identification of phenotypic characteristics in reproductively successful
individuals provides important insights into the evolutionary processes
that cause range shifts due to environmental change. Female beluga whales
(Delphinapterus leucas) from the Baffin Bay region (BB) of the Canadian
Arctic in the core area of the species’ geographic range have larger body
size than their conspecifics at the southern range periphery in Hudson Bay
(HB). We investigated the mechanism for this north and south divergence as
it relates to ovarian reproductive activity (ORA = total corpora) that
combines morphometric data with ovarian corpora counted from female
reproductive tracts. Our study aim was to assess the relative influence of
age and body size of female beluga whale on ovarian reproductive activity
in the two populations. Female beluga whale ORA increased more quickly
with age (63% partial variation explained) in BB than in HB (41%). In
contrast, body length in HB female beluga whales accounted for
considerably more of the total variation (12 vs 1%) in ORA compared to BB
whales. We speculate that female HB beluga whale ORA was more strongly
linked with body length due to higher population density resulting in food
competition that favors the energetic advantages of larger body size
during seasonal food limitations. Understanding the evolutionary mechanism
of how ORA varies across a species’ range will assist conservation efforts
in anticipating and mitigating future challenges associated with a warming
planet.