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

Abstract

Aim:The 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. Location: Northwest Atlantic Ocean.Taxon: Globicephala spp. Methods:We 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. Results:Our 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 longfinned pilot whales. Main Conclusions:Pilot 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.

Why Baja? A bioenergetic model for comparing metabolic rates and thermoregulatory costs of gray whale calves (Eschrichtius robustus)

“…These were used to derive Equation 2. Flippers and flukes account for another 11% of the total body surface area, but are equipped with counter‐current vascular heat exchange systems capable of reducing heat losses to negligible levels (Adamczak et al, 2020; Elsner et al, 1974; Scholander & Schevill, 1955), so the surface areas of these appendages are not included in estimates of the area insulated by blubber. Body mass (M b ; equivalent to body volume) is a function of both L b and maximum body girth (G max ) and is described by Equation 3 (Sumich et al, 2013).…”

“…For most mammals, the use of ventilatory characteristics to estimate metabolic rates can be confounded by the dual roles of their respiratory surfaces that function simultaneously as sites for gas exchange while providing extensive surfaces for evaporative cooling during periods of heat stress. Unlike terrestrial mammals, however, cetaceans regulate heat losses with vascular radiators in flukes, flippers, and other specialized body surfaces (Adamczak et al, 2020; Elsner et al, 1974; Scholander & Schevill, 1955), and the relative simplicity of their ventilation mechanics is expected to more closely reflect their rates of gas exchange and O 2 utilization (Piscitelli et al, 2013).…”

Body Composition of Common Bottlenose Dolphins in Sarasota Bay, Florida

“…Ontogenetic patterns in body composition revealed that mature dolphins were leaner, and body composition was less variable as dolphins reached maturity (Figure 5). Mature dolphins are larger with lower surface area to volume ratios, which would improve their capacity to retain heat, reducing the need for thicker blubber (Worthy and Edwards, 1990;Noren and Wells, 2009;Adamczak et al, 2020). In contrast, as young dolphins are smaller, they have thicker, more insulative blubber to compensate for the additional thermoregulatory costs of being small (McLellan et al, 2002;Dunkin et al, 2005;Koopman, 2007;Montie et al, 2008;Noren and Wells, 2009).…”

Trait-mediated shifts and climate velocity decouple an endothermic marine predator and its ectothermic prey

“…Our analysis of NEFOP and ASM data agreed with this assessment; any pilot whale bycatch identified to the species level consisted exclusively of long-finned pilot whales, and morphological measurements and habitat assessments indicated that observed pilot whales were long-finned pilot whales. For example, all adult female pilot whales that were taken as bycatch were greater than 400 cm in length, and a recent study of pilot whale morphometrics from the eastern seaboard of the United States indicated that mature female short-finned pilot whales are considerably less than 400 cm in length (mean 358.48 ± 4.26 cm for mature female short-finned pilot whales and 432.81 ± 5.82 cm for mature female long-finned pilot whales, respectively 32 . Further, stock assessment reports delineate short- and long-finned pilot whales based on sea surface temperature, with the probability of a pilot whale being a long-finned pilot whale of near 1 at water temperatures < 22 °C 49 ; see also results of this study), and all observations of pilot whale bycatch occurred in temperatures below 22 °C.…”

“…For example, cooler water species typically have lower surface area to volume ratios in their body core, and blubber with a higher lipid content, lower conductivity, and increased fatty acid stratification 30 , 31 . The larger appendages of long-finned pilot whales in comparison to short-finned pilot whales may reflect the need for enhanced mechanisms of heat dissipation in the large, well-insulated long-finned species, which occurs in cooler waters 32 .…”