Cool‐Water Dolphins Have Larger Appendages Than Warm‐Water Dolphins

Aim Endothermic marine mammals face significant thermoregulatory challenges due to heat loss in the highly conductive marine environment. Allen's rule states that endothermic animals in cold environments have smaller appendages than those in warmer environments. While body size adaptations in marine mammals are well‐studied, the role of appendage size as an adaptation to different thermal habitats remains unclear. Here, we aim to quantify how appendage size differs between warm‐ and cool‐water delphinids to explore how these species have adapted to different thermal regimes. Location Atlantic Ocean. Taxon Dusky dolphins ( Aethalodelphis obscurus ), Commerson's dolphins ( Cephalorhynchus commersonii ), white‐sided dolphins ( Leucopleurus acutus ), common dolphins ( Delphinus delphis ), Atlantic spotted dolphins ( Stenella frontalis ), and bottlenose dolphins ( Tursiops truncatus spp.). Methods We combined morphometric data from stranded delphinids with 3D modelling to generate 3D representations of individual animal body morphologies. From these models, we quantified surface area of both the body cores and appendages and volume of the body cores, and constructed standardized major axis regressions to evaluate how the relationships between body core volume and body core and appendage surface areas differ between cool‐ and warm‐water species. Results Cool‐water dolphins had larger appendages overall relative to body volume than warm‐water species, opposing predictions from Allen's rule. However, we also found that total surface area (body core and appendages) relative to body volume is similar between cool‐ and warm‐water species. Main Conclusions Our findings suggest that Allen's rule is not upheld in delphinids, and imply that heat conservation may not be a strong driver of appendage size. Rather, larger appendages may be more important for cool‐water delphinids to dissipate excess heat. This result, in combination with the finding that the total surface area to volume ratio was tightly conserved across cool‐ and warm‐water dolphins, suggests that factors other than temperature influence overall body morphology for these species. This study highlights a potential decoupling between appendage size and heat conservation in cetaceans, and provides insights into thermal adaptations in these species.