Geographical variation in the body size of adult female sperm whales (Physeter macrocephalus) – an example of McNab’s resource rule?

Abstract

Abstract:This study investigates possible regional variations in size composition of adult female sperm whales (Physeter macrocephalus) using data from 3302 pregnant individuals taken on Soviet whaling expeditions to the Southern Hemisphere 1961Hemisphere /62-1974. A general linear model (GLM) was used to take the covariates of expedition, latitude and ocean basin into account. The average body size decreased from south to north in each ocean basin, with the biggest decrease (about 200 cm) in the Indian Ocean; followed by the Pacific Ocean (about 110 cm), and the Atlantic Ocean (about 80 cm). Independent data confirm the small size of female/immature sperm whales in some tropical areas of the Indian and Pacific Oceans. The mechanism responsible for this geographic variation in sperm whale growth could reflect culturally transmitted differences in foraging behaviour between clans of female/immature sperm whales in response to differing availabilities of prey resources by geographical regionMcNab's resource rule. However there is little available information for such a mechanism to be readily identifiable. Although data for oceanic squids (sperm whale's main source of food) are lacking, there is evidence that the individual sizes of neritic species are positively correlated with latitude. Hence feeding in equatorial regions may be energetically more demanding due to smaller individual prey size, with consequent effects on growth rate.

Growth in marine mammals: a review of growth patterns, composition and energy investment

“…Lower latitudes have lower prey densities, favoring smaller body sizes ( McNab, 2010 ). Latitudinal gradients in marine mammal body size attributed to both Bergmann’s and McNab’s rules have been identified at broad taxonomic levels and between closely related species ( Galatius and Gol’din, 2011 ; Oosthuizen et al, 2016 ; Torres-Romero et al, 2016 ; Best et al, 2017 ; Ferguson et al, 2018 ; van Aswegen et al, 2019 ; Adamczak et al, 2020 ). For example, closely related pilot whale (Globicephala spp.)…”

“…In support of McNab’s rule, harbor porpoise populations along the California coast are larger than other populations, which may be driven by the nutrient-rich upwelling in this region ( Galatius and Gol’din, 2011 ). Additionally, latitudinal gradients in sperm whale body size have been linked to changes in prey size along this gradient ( Best et al, 2017 ). Similar trends are seen when comparing the recently diverged California sea lion and Galápagos sea lion ( Zalophus wollebaeki ).…”

Life History Traits of Sperm Whales Physeter macrocephalus Linnaeus, 1758 Stranded along Italian Coasts (Cetartiodactyla: Physeteridae)

“…Best et al [42] shown that there is a strong geographical variation in body size of adult sperm whales and, as a rule, individuals collected in tropical waters are significantly smaller than those in temperate regions. The factors that could produce such diversity in size include a different prey availability, which directly influences energy intakes and growth rates, or even that different populations have differing prey preferences that they occupy different geographical regions.…”