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.

Methods
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).

鉴定繁殖成功个体的表型特征，可为解析环境变化引发物种分布范围变迁的进化过程提供重要见解。来自加拿大北极巴芬湾（Baffin Bay, BB）区域——该区域为白鲸（Delphinapterus leucas）地理分布的核心范围——的雌性个体，其体型较分布范围南部边缘的哈德逊湾（Hudson Bay, HB）同种个体更大。我们探究了南北种群的这一体型差异机制，该机制与卵巢生殖活动（ovarian reproductive activity, ORA=总黄体数）相关，该指标整合了形态测量数据与从雌性生殖道中计数得到的卵巢黄体数据。本研究的目的为评估雌性白鲸的年龄与体型对两个种群卵巢生殖活动的相对影响程度。

雌性白鲸的卵巢生殖活动随年龄增长的提升速率，巴芬湾种群显著快于哈德逊湾种群（前者解释了63%的偏变异，后者仅为41%）。与之相对，相较于巴芬湾种群（仅1%），哈德逊湾种群雌性白鲸的体长对卵巢生殖活动总变异的解释度显著更高（12%）。我们推测，哈德逊湾种群雌性白鲸的卵巢生殖活动与体长关联更强，或因种群密度更高引发食物竞争，在季节性食物匮乏期更青睐体型较大个体的能量优势。解析卵巢生殖活动随物种分布范围产生变化的进化机制，将有助于保护工作预判并缓解全球变暖引发的未来生存挑战。

研究方法
本研究数据集共纳入172份至少含1个黄体的雌性生殖道样本：其中巴芬湾种群41份，哈德逊湾种群131份。为构建可支撑稳健统计检验的完整数据集（Moritz & Bartz-Beielstein, 2017），我们将每个种群中缺失的体长与年龄数据替换为该种群所有个体的中位数。其中5份巴芬湾种群样本缺失年龄数据，统一赋值为20龄；6份哈德逊湾种群样本缺失年龄数据，统一赋值为26龄。同理，6份巴芬湾种群样本缺失体长数据，统一赋值为354 cm；17份哈德逊湾种群样本缺失体长数据，统一赋值为327 cm。

本研究的雌性生殖道样本采集于1989年至2014年间，来自加拿大北极东部的17个北部社区，均通过尸检完成大体检查（图1）。年龄鉴定基于牙齿的牙本质与牙骨质生长层组分析（Waugh et al., 2018）。鲸类标准体长按照标准规程在野外进行测量，测量起点为尾叶中点，终点为吻部尖端（美国哺乳动物学会, 1961）。基于相似的生长-年龄-繁殖关系，我们将两个北部种群（坎伯兰湾种群与高北极种群）的繁殖形态学数据合并至巴芬湾区域种群（Ferguson et al., 2020）。为统一术语表述，尽管每个区域可能包含多个亚种群（Skovrind et al., 2021），本研究仍将巴芬湾与哈德逊湾统称为种群。

样本处理的详细流程参见Ferguson等（2020）的研究；简要而言，卵巢被摘除后称重、测量，并以10%中性缓冲福尔马林固定保存。针对每个卵巢，我们记录了黄体（corpus luteum, CL）与白体（corpus albicans, CA）的数量（Best, 1968）。在鲸类中，黄体与白体可形成独特且持久的卵巢结构，作为雌性潜在繁殖历史的记录不断在卵巢内累积（Perrin et al., 1976; Slijper, 1962; Collet & Harrison et al., 1972，但详见Dabin et al., 2008）。为减少主观判定副黄体的偏倚，所有黄体计数均由同一名研究者完成（Harrison, 1977）。本研究以卵巢内所有现存黄体与白体的总数作为卵巢生殖活动（ORA）的衡量指标。对于仅采集到单侧卵巢的个体（巴芬湾种群97头个体中的23头，哈德逊湾种群210头个体中的113头），我们将黄体计数翻倍，因为白鲸未表现出卵巢功能的侧偏好性（Robeck et al., 2010; Sheldon et al., 2019）。