Stephanie is a PhD student at the University of California, Santa Cruz. She is a marine mammal ecologist with an interest in quantitative modelling approaches. Stephanie shares her recent work on trait evolution in pilot whales and the contrasts in ecogeographic rules between terrestrial and marine systems.
Stephanie conducting fieldwork off the coast of Long Island preparing to take photographs of humpback whales.
Institute. University of California, Santa Cruz.
Academic life stage. PhD.
Major research themes and interests. I study the ecology and biology of marine mammals using quantitative modelling methods.
Current study system. My current research examines harbor porpoise behaviour, reproductive success, and conservation using mathematical modelling. This modelling approach determines how human-induced disturbance alters harbor porpoise reproductive decisions, thereby influencing the population’s success. Although my current research differs from the work presented here and published in the Journal of Biogeography, the focus on better understanding the ecology and biology of marine mammals remains.
A pod of short-finned pilot whales travelling along Hudson Canyon in the Northwest Atlantic Ocean near New York. Drone imagery collected by Julia Stepanuk under National Marine Fisheries Service GA 21889 to L. Thorne.
Recent paper in Journal of Biogeography. Adamczak, SK, Pabst, DA, McLellan, WA, Thorne, LH. (2020) Do bigger bodies require bigger radiators? Insights into thermal ecology from closely related marine mammal species and implications for ecogeographic rules. Journal of Biogeography 47(5): 1193–1206. https://doi.org/10.1111/jbi.13796
Motivation for this paper. Ecogeographic rules, such as Bergmann’s rule and Allen’s rule, outline spatial variation in biological traits. Bergmann’s rule indicates that species in temperate climates will have larger bodies when compared to more tropical species. This is driven by thermoregulatory needs, as larger bodies typically have lower surface area to volume ratios which conserves more heat. Alternately, Allen’s rule states that species in temperate climates will have smaller appendages than more tropical species because smaller appendage surface area reduces heat loss. Although these rules have been well studied in terrestrial systems, little is known about how they impact the morphology of marine mammals. As a result, we set out to determine if these rules dictate morphological patterns observed in marine mammals as these animals already have very unique thermoregulatory adaptations. To test this, we compared the tropical short-finned pilot whale and the temperate long-finned pilot whale, two ecologically and phylogenetically similar species that occupy different thermal regimes.
Methodologies. To more accurately estimate surface area and volume of pilot whales, we employed a novel 3D modeling method to better account for the streamlined shape of marine mammals. After modelling surface area and volume for each individual, we compared the relationship of these two metrics between short- and long-finned pilot whales to examine differences in size and shape. We then compared overall surface area to volume ratios to directly test if pilot whales followed Bergmann’s rule. To test Allen’s rule we examined species-level differences in normalized appendage surface area (to account for differences in size between individuals) for the pectoral flippers, dorsal fin, and flukes individually and combined. This allowed us to determine the heat conservation/dissipation capacity of each appendage as well as the total thermoregulatory capacity of the combined appendages.
Example of the 3D model constructed with the program, Blender, used to estimate the surface area and volume of individual pilot whales in these analyses.
Major results. We expected the more temperate long-finned pilot whale to have a larger body size and lower surface to volume ratio when compared to short-finned pilot whales, as per Bergmann’s rule, and lower appendage surface area, as per Allen’s rule. Although Bergmann’s rule was upheld, Allen’s rule was reversed as we observed greater appendage surface area relative to body size in long-finned pilot whales when compared to short-finned pilot whales. This result was primarily driven by the very large pectoral flippers of long-finned pilot whales. My co-authors and I suggest that this reversal of Allen’s rule could be attributed to the use of appendages as heat dissipators in marine mammals. Marine mammal appendages act as thermal windows through which they can control heat loss and dissipate heat in times of thermal stress. It is possible that the large body size and effective insulation of long-finned pilot whales necessitates large thermal windows for effective heat dissipation when compared to short-finned pilot whales.
Unexpected outcomes. Our results suggest that Allen’s rule may not be applicable to marine mammals. We hypothesise that this may be a result of the different environmental pressures faced by marine mammals when compared to terrestrial mammals. Marine mammals are endotherms living in a highly conductive medium, and as such they are equipped with unique thermoregulatory adaptations such as thick blubber layers and large body size. These adaptations that offset the energetic costs of thermoregulation in the water may necessitate a greater need for rapid heat dissipation in times of thermoregulatory stress. Marine mammals have incredible control of the heat dissipated or conserved via their thermal windows, so we surmise that the improved capability to shut off or turn on thermal windows provides a unique advantage over terrestrial endotherms. We feel this research demonstrates that larger marine mammals inhabiting cold thermal regimes may need greater appendage surface area over which to rapidly dump heat, which contrasts the patterns seen in terrestrial species where larger, cold-climate species have small appendages to conserve heat.
Next steps? The next step in this research would definitely be to keep exploring biogeographical patterns in marine mammal species! Studies of Bergmann’s rule and Allen’s rule highlight the different evolutionary and environmental pressures faced by marine mammals when compared to terrestrial species, specifically when discussing thermoregulatory adaptations. Marine mammal thermoregulation is a fascinating field of study due to the unique adaptations of these animals, and as such, further analysis of Bergmann’s rule and Allen’s rule would highlight how these adaptations may lead to reversals or alterations to long-held ecological rules. Personally, I would love to see similar comparisons carried out in much larger marine mammal species occupying Arctic or Antarctic environments, as the patterns seen here may be even more prevalent in animals adapted for harsher environments.
If you could study any organism, what would it be? I find marine mammal species living in extreme, cold environments like Antarctica or the Arctic really interesting. The physiological adaptations that enable species like the Weddell seal or bowhead whale to persist in these environments fascinates me and I’d love to have the opportunity to study them some day.
Anything else you’d life to share? When I started this work as part of my Master’s thesis my intention was to examine the morphology of short- and long-finned pilot whales. It wasn’t until about half-way through my research that I realized I could apply my work to something broader and more interesting, such as biogeography. Thinking outside of the box can be extremely beneficial and lead you to some very interesting discoveries!