DOI: 10.7291/d1m09m
title: Advances in thermal physiology of diving marine mammals: The dual role of peripheral perfusion

description: The ability to maintain a high core body temperature is a defining
 characteristic of all mammals, yet their diverse habitats present
 disparate thermal challenges that have led to specialized adaptations.
 Marine mammals inhabit a highly conductive environment. Their
 thermoregulatory capabilities far exceed our own despite having limited
 avenues of heat transfer. Additionally, marine mammals must balance their
 thermoregulatory demands with those associated with diving (i.e., oxygen
 conservation), both of which rely on cardiovascular adjustments. This
 review presents the progress and novel efforts in investigating marine
 mammal thermoregulation, with a particular focus on the role of peripheral
 perfusion. Early studies in marine mammal thermal physiology were
 primarily performed in the laboratory and provided foundational knowledge
 through in vivo experiments and ex vivo measurements. However, the
 ecological relevance of these findings remains unknown because comparable
 efforts on free-ranging animals have been limited. We demonstrate the
 utility of biologgers for studying their thermal adaptations in the
 context in which they evolved. Our preliminary results from freely diving
 northern elephant seals (Mirounga angustirostris) reveal blubber’s dynamic
 nature and the complex interaction between thermoregulation and the dive
 response due to the dual role of peripheral perfusion. Further exploring
 the potential use of biologgers for measuring physiological variables
 relevant to thermal physiology in other marine mammal species will enhance
 our understanding of the relative importance of morphology, physiology,
 and behavior for thermoregulation and overall homeostasis.
description: As one of the deepest
 diving pinnipeds, northern elephant seals (<i>Mirounga
 angustirostris</i>) are an ideal model species to study the thermal
 physiology of diving marine mammals. Translocating juvenile elephant seals
 from Año Nuevo State Park (San Mateo County, CA) has proven valuable for
 short at-sea studies using physiological biologgers that must be recovered
 (Oliver et al. 1998). Año Nuevo is ideally situated north of Monterey Bay,
 where the bathymetry allows for deep dives, similar to the average dives
 of adults (e.g., Costa et al. 2002, Maresh et al. 2014). Juvenile elephant seals (1-2
 years old) were sedated following standard protocols (Le Boeuf et al.
 2000), instrumented, and transported to Monterey where they were released
 into the bay. The instruments were recovered to access the data stored
 onboard once the seals returned to land generally within 1 week.
 In addition to a satellite
 tag and VHF transmitter to facilitate tracking and relocating the seal
 upon its return, the seals were equipped with two heat flux biologgers
 that measure diving depth, acceleration, water temperature and the
 following physiological variables: heat flux at four locations (neck,
 axilla, flank, swimming muscle), skin temperature, and the temperature
 gradient within the blubber layer with three subcutaneous thermistors. The
 heat flux biologgers were custom-built by Wildlife Computers (Redmond, WA)
 for a prior study on Weddell seals in Antarctica (Hindle et al. 2015)
 using heat flux sensors first validated for use in diving animals on
 Steller sea lions (Willis and Horning 2005). They were refurbished and
 reconfigured to integrate the subcutaneous thermistors for this project,
 which was necessary to investigate the underlying physiological mechanisms
 of their thermal responses while diving. Heat flux was measured using
 small disc sensors (24.5 mm diameter, Concept Engineering, Old Saybrook,
 CT) that were attached to the seal’s shaved skin using custom-made PVC
 ring holders. A thin layer (1 mm) of a two-part thermally conductive epoxy
 (1.8 Wm<sup>-1</sup>K<sup>-1</sup>, Epo-Tek®
 T905BN, Epoxy Technology, Billerica, MA) was applied on the sensor which
 provides some adhesive quality after curing so that minimal superglue
 (Loctite Super Glue Gel Control) was used around the periphery of the PVC
 ring to ensure proper attachment. The sensor has directionality and was
 oriented on the seal such that positive heat flux indicates the animal is
 gaining heat and negative heat flux indicates the animal is losing heat.
 One of the four heat flux sensors also has an embedded thermistor to
 measure skin temperature at the flank. To account for the additional
 thermal resistance provided by the sensor itself and attachment mechanism,
 a correction factor was experimentally determined following Willis and
 Horning (2005) and used to multiply the raw voltage output. The
 manufacturer’s calibration constant was used to convert from mV to W
 m<sup>-2</sup>, and further corrections will be applied
 pending post-deployment calibrations. Three thermistors (NTC 2.252kΩ at
 25°C) were encapsulated in epoxy (Epo-Tek® 302-3M, Epoxy Technology,
 Billerica, MA) within stainless steel capillary tubes and connected to the
 biologgers with Kevlar® cable. To place the thermistors in the blubber
 layer, a small incision was made in the flank. A muscle biopsy cannula was
 used to create a channel where the thermistor was then inserted. A
 portable ultrasound (Philips Lumify L12-4 linear array transducer) was
 used to measure blubber depth prior to insertion and provided guidance
 while inserting the thermistors to the appropriate depths. The thermistor
 cable was secured using a box-stitch suture at the incision site.
 Antibiotics (1 g Cefazolin) were administered IV at deployment and
 recovery as a preventative measure, but there was no sign of infection in
 any of the seals.
description: The files contain data from
 one juvenile northern elephant seal (male, 172 kg) whose trip at-sea was
 ~7 days. Two sections of data are provided with the variables listed below
 that were used to create the figures in Favilla et al. 2021
 <em>Temperature</em>. All variables listed were sampled at 1
 Hz. <b>Data-0502204000_0503083000.csv & Data-0508013000_0509013000.csv </b> Columns and relevant descriptions: DateTime: GMT date and time Depth: meters WaterTemp: degrees Celsius, temperatures during surface interval (where the tag may have been out of the water) was corrected by replacing surface ‘air’ values with average value within top 2 meters HeatFlux: W m<sup>2</sup>, corrected for additional thermal resistance from sensor and attachment mechanism and manufacturer’s calibration constant, measured at flank BlubberDeep: degrees Celsius, thermistor placed near muscleblubber interface BlubberMid: degrees Celsius, thermistor placed near the middle of the blubber layer BlubberShallow: degrees Celsius, thermistor placed subcutaneously in blubber layer <b>DiveStat-No530_601.csv </b> Columns and relevant descriptions: DiveNumber StartTime: GMT date and time of dive start MaxDepth: meters DiveDuration: H:M:S, time elapsed between beginning of dive descent and end of ascent SurfaceDuration: H:M:S, time elapsed between end of dive ascent and beginning of subsequent dive SolarEl: degrees, solar elevation determined using geolocation and date and time TimeOfDay: defined by SolarEl, where elevation &gt; 0° is ‘Day’ and elevation ≤ 0° is ‘Night’