DOI: 10.5061/dryad.hmgqnk9j3
title: Larger body size leads to greater female beluga whale ovarian reproductive activity at the southern periphery of their range

description: 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.
description: The dataset included 172
 female reproductive tracts with at least one corpus: 41 from BB and 131
 from HB. To create a complete dataset required for robust statistical
 testing (Moritz & Bartz-Beielstein 2017), missing length and age
 data were replaced with the median value of all whales in each population.
 The five BB whales with missing age were assigned 20 years-of-age and the
 6 HB whales, 26 years-of-age. Similarly, the 6 BB whales with missing
 length were assigned 354 cm and the 17 HB whales with missing length, 327
 cm. We conducted post-mortem gross examinations of female reproductive
 tracts, collected from 17 northern communities within the Eastern Canadian
 Arctic from 1989 to 2014 (Fig. 1). Ageing was based on examination of
 dentine and cementum growth layer groups in teeth (Waugh et al. 2018).
 Whale standard length was measured in the field according to a standard
 protocol, measured from the middle of the fluke to the tip of the rostrum
 (American Society of Mammalogists, 1961). We combined reproductive
 morphology data for two northern populations (Cumberland Sound and high
 Arctic) into a BB region based on a similar growth-age-reproduction
 relationship (Ferguson et al. 2020). For consistency in terminology, we
 refer to BB and HB as populations while recognizing that each region
 likely comprises a number of sub-populations (Skovrind et al.
 2021). Sample processing is described in more detail in
 Ferguson et al. (2020); briefly, ovaries were excised, weighed, measured,
 and preserved in 10% neutral-buffered formalin. For each ovary, we
 recorded the number of CLs and CAs (Best, 1968). In cetaceans CLs and CAs
 form distinct and persistent features, accumulating within the ovary
 (Perrin et al. 1976) as a record of a female’s potential reproductive
 history (Slijper 1962; Collet and Harrison et al., 1972; but see Dabin et
 al. 2008). Corpora assessments were performed by one reader to minimize
 bias in the subjective determination of accessory corpora (Harrison,
 1977). As a measure of ORA, we counted all existing CLs and CAs within the
 beluga whales’ ovaries. For whales with only one ovary sampled (23 of 97
 whales from BB and 113 of 210 whales from HB), we doubled the corpora
 count since beluga whales do not appear to exhibit a side-dominance in
 ovarian function (Robeck et al. 2010; Sheldon et al. 2019).
description: Excel file: Whale ID, Group (northern or southern), Population
 (High Arctic, Cumberland Sound, Hudson Bay), Year, Age, Body length, Total
 corpora