Do bigger bodies require bigger radiators? Insights into thermal ecology from closely related marine mammal species and implications for ecogeographic rules

Abstract

AbstractAimThe aim of this study was to determine if marine mammals follow ecogeographic rules. We examined Bergmann's rule and Allen's rule in two pilot whale species with contrasting latitudinal distributions.LocationNorthwest Atlantic Ocean.TaxonGlobicephala spp.MethodsWe analysed morphometric data collected from strandings of short‐ and long‐finned pilot whales in the Northwest Atlantic Ocean to assess intraspecific and interspecific variation in surface area to volume ratios (SA:V) of the body core and appendage surface area relative to body core SA (normalized appendage SA) using a novel 3D modelling method.ResultsOur results suggest that ecogeographic variation in morphometrics between the two pilot whale species is consistent with morphological adaptations required to balance heat conservation and heat dissipation. Interspecific differences in morphology supported Bergmann's rule for fully grown individuals: the more temperate long‐finned pilot whale had a larger body size and lower body core SA:V than the short‐finned pilot whale, which has a more tropical distribution. Allen's rule was not supported; when all appendages were considered together, long‐finned pilot whales had larger normalized SA than short‐finned pilot whales. However, the pectoral flippers were the primary driver of this relationship; while long‐finned pilot whales had proportionally larger pectoral flippers, short‐finned pilot whales had proportionally larger dorsal fins and flukes. In addition, larger long‐finned pilot whales (i.e. males and mature individuals) had proportionally larger pectoral flippers than smaller long‐finned pilot whales.Main ConclusionsPilot whales follow Bergmann's rule but do not follow Allen's rule when fully mature. Thinly insulated appendages in marine mammals can be used to dissipate heat as the core warms, and larger and better insulated marine mammals may require relatively larger appendages in order to offload heat and thermoregulate effectively. Our results provide novel insight into ecogeographic rules and suggest that species in higher latitude climates towards the poles will demonstrate tradeoffs between core body heat conservation and appendage heat dissipation.