Coping with lots of fat: A marine mammal’s perspective

Imagine this scenario: You’re going for a jog outside, but seeing some snow on the ground, you decide to put on a thermal long-sleeve shirt underneath your sweatshirt. Right as you step out the door, you sure are glad you added that extra layer. After a few minutes into your jog, you notice you’re breathing heavy and your heart is beating faster (…especially if you’re out of shape). Your skin might get red, feel hot and flushed, and after a little while, you’ll probably also start sweating. These physiological responses keep your body fueled with oxygen during your aerobic workout while also preventing you from overheating. But you still start to feel slightly uncomfortably warm.



What do you do? Easy enough, just take off the extra layer!

Now imagine that ‘extra layer’ came in the form of thick blubber as it does for marine mammals. How do marine mammals cope with variable thermoregulatory demands—conserving heat while diving to cold depths but dissipating any excess heat when actively swimming? Whales, dolphins, seals, and sea lions are endothermic mammals, just like we are, and have to regulate their body temperature. But, they cannot just easily take off their blubber layer like we do with our clothes.

Living in a marine environment presents even more challenges for thermoregulating. Because sweating in water is pointless, many marine mammals don’t even produce sweat. Most marine mammals also spend a majority of their time underwater where they don’t have the leisure of breathing heavy like us to get more oxygen while actively swimming. To conserve oxygen while diving, marine mammals decrease their heart rate and restrict blood flow to only the most critical organs (brain, lungs, and heart). With no blood flowing to their skin, the heat generated from exercising stays in their core while their skin keeps cold, usually within a couple degrees of the water temperature.

Considering these adaptations, here’s my curiosity: if a marine mammal becomes overheated, how can they thermoregulate while balancing their physiological adaptations for diving? 

This is what I am studying as a graduate student—the seemingly paradoxical physiological adaptations for diving and thermoregulation of marine mammals—and I happen to be at the perfect place to study the physiology of freely diving marine mammals. Año Nuevo Reserve is home to a colony of northern elephant seals and is only 30 minutes north of the University of California, Santa Cruz campus. The Costa Lab at UCSC has been studying this population for over 40 years and has all the necessary research permits. As a new graduate student in the lab, I get to work with marine mammal experts and contribute to the lab’s body of research. My research will investigate the thermoregulatory response of diving marine mammals and what better species to study than one of the deepest diving marine mammals—northern elephant seals.

Northern elephant seals make routine dives down to 600 meters (2000 feet), but are capable of diving much deeper. The deepest recorded dive for a northern elephant seal was 1756 meters (5761 feet). This makes them the second deepest diving seal (after the closely related southern elephant seal). Of all other marine mammals, only sperm whales and Cuvier’s beaked whales surpass them in maximum dive depth. (Ponganis, Paul J. Diving Physiology of Marine Mammals and Seabirds. Cambridge University Press, 2015.) Infographic from

When planning a research project, finding the best model species is just the first step, and I happened to luck out by having elephant seals at our door step. The next step is to figure out what data are needed to address your question or test a specific hypothesis and how you will collect it.

This is where collaboration comes into my work—I am lucky to get to work with Alaska SeaLife Center’s Science Director, Dr. Markus Horning. In my previous 60° North Science blog, I described the morning of my first translocation study where I attached biologgers to two juvenile northern elephant seals. The biologgers that I used were custom-built by Wildlife Computers and designed by Dr. Horning and Dr. Kate Willis to specifically measure heat flux from sensors placed on an animal’s skin (Willis and Horning 2004). Heat flux is how much heat is transferred between the seal’s body surface and the surrounding water, which basically tells you when the animal is gaining or losing heat to its environment. This is exactly the kind of data I needed to begin to address my question. Since these heat flux biologgers were also designed for independent, long-term attachment, this made them perfect for collecting data from freely diving animals.

A juvenile elephant seal equipped with surface-mounted tags: a heat flux biologger and a VHF transmitter on its back and a satellite tag on its head. The heat flux sensors adhered to the skin to record heat flux and skin surface temperature at two locations on the body. The cables that connect the sensors to the biologgers are glued to the hair with adhesive wrap and tape. The tags collected data over 2-3 days as the juvenile swam back to Año Nuevo, performing natural dives along the way. Once the juvenile returned to the colony, the tags were carefully removed to retrieve the data. (Translocation performed under NMFS permit #19108 and IACUC approval)

Until these heat flux biologgers were invented, these measurements had only been possible on trained animals with human assistance, which prevented getting measurements from wild animals diving naturally. These heat flux biologgers have now been used on a few different species, including the ASLC’s Steller sea lions, wild Weddell seals, and, after my translocation study, juvenile elephant seals!

So, what did the data from the heat flux biologgers tell us about how marine mammals thermoregulate while diving? Read my next blog post to find out more about my first translocation study with elephant seals!

Written by: Arina Favilla, PhD Student, Ecology and Evolutionary Biology, University of California Santa Cruz. 

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