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.

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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.

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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 https://www.behance.net/gallery/20387023/Animated-Infographic

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.

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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. 

Gif from:  https://giphy.com/gifs/run-VFCiY7qk1uFYQ)

10 thoughts on “Coping with lots of fat: A marine mammal’s perspective

  1. I enjoyed your article.
    I was directed to this site by Patrick Robinson.
    The Corona Virus scare has lots of old folks like me very much restricted
    and, I guess, Patrick felt getting us to pay attention to papers like yours might keep
    us out of trouble.
    After reading the article I have a couple of questions:
    1. Why do these animals dive so deeply? Do they need to go that deep for food?
    2. Doesn’t the restriction of circulation suppress other functions like tail and flipper
    agility and maybe even mental activity. (My few experiences with severe cold
    have impaired my ability to function as well as required hours to recuperate.)
    Are there consequences for diving so deeply?
    Can’t wait for the next chapter!

    Jeff Breen (BA, MA, RA )

    1. Hi Jeff—thank you for taking the time to read and follow up with questions!

      1-Food is definitely a motivator to dive as deep as they do! Biologging tools, such as animal-borne cameras, stomach temperature telemetry, and jaw accelerometers, provide evidence that foraging activity generally occurs between 400-600 meters, but some elephant seals also feed at depths >800 m. (Two neat papers by our collaborators about their foraging activity—Naito et al. 2013, 2017). It’s certainly possible that there are other reasons for diving that deep. For example, elephant seals are negatively buoyant for the most part (depending on pregnancy status), which means it takes them less energy to descend than ascend. So, hypothetically, as long as they have the oxygen stores to make those deep dives, they may just passively drift down while ‘sleeping/resting’—a topic that’s actively being researched in our lab group!

      2-Great question! The degree to which circulation is restricted will depend on the particular conditions of the dive. While the brain generally continues to receive some blood flow, their peripheral tissues like their swimming muscle can become quite ischemic. One adaptation that allows their muscles to still function while receiving little to no blood flow is their higher levels of the oxygen-binding protein myoglobin (i.e. muscle’s equivalent of hemoglobin in the blood). With larger stores of oxygen in their muscle, they can continue to metabolize aerobically without using oxygen from the blood, which is saved for more critical organs, like the brain and heart. These organs are also adapted by having larger stores of glycogen compared to terrestrial mammals to allow for continued aerobic metabolism despite reduced blood flow. However, if the dive lasts longer than the oxygen stores in their muscles, they could resort to anaerobic metabolism, which has consequences for their diving efficiency since more time at the surface is required to ‘washout’ the lactate produced by anaerobic metabolism. There are many other consequences of diving so deeply because of the immense pressure they face at depth, but their adaptations are key to avoiding irreparable damage. For example, to prevent pressure-related injuries, such as decompression sickness and nitrogen narcosis, marine mammals allow their lungs to fully collapse at depth so as to cease gas exchange across the alveoli and reduce absorption of nitrogen into the blood.

      I hope these answers helped and I welcome more questions at any time!

      Stay happy and healthy!

      Arina

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