Heat flux in manatees: an individual matter and a novel approach to assess and monitor the thermal state of Florida manatees (Trichechus manatus latirostris)

Abstract

Florida manatees (Trichechus manatus latirostris) possess an unusual suite of adaptations to accommodate both a fully aquatic lifestyle and an herbivorous diet, including a low metabolic rate and a very limited thermoneutral zone. Their relatively high lower critical temperature of around 20 °C suggests strong sensitivity to cold, thereby limiting their distribution to tropical and subtropical waters. "Cold stress syndrome" affects and kills Florida manatees every year during intense or prolonged cold weather, posing one of the major threats to manatees. However, knowledge regarding manatee thermoregulation is sparse, but essential for effective conservation and management of this threatened species. We measured heat flux in two captive Florida manatees at multiple times of the year, at 41 sites distributed across the entire body surface of each manatee. Heat flux differed significantly between individuals, and among body sites and times of the year. The pectoral flippers and axillae were identified as areas with highest heat exchange. Despite exposure to constant water temperature throughout the year, the manatees in this study had significantly lower heat flux in winter than in summer. We used the measured heat flux values to calculate total heat dissipation in individual manatees. The values estimated this way correspond well with the low metabolic rates estimated in previous studies, confirming the reliability of our novel approach. Our method provides simple and useful options for enhancing manatee welfare by monitoring the animals' thermal state during potentially stressful activities such as during medical treatment, capture restraints and transportation.

Spatial and seasonal variations in surface water temperature and salinity in the Mexico‐Belize riverine estuary: Possible comfort conditions for manatees?

“…The water conditions (T > 28°C) determined in this study are similar to those described for the Amazonian (Gallivan et al, 1983; Rosas, 1994) and Caribbean manatees due to their low metabolic rate and limited ability to generate heat (Davis, 2019; Irvine, 1983). Manatee thermoregulation requires further research, as it is recognized as being important for recognizing the interaction of manatee species with their habitats (Erdsack et al, 2018). Salinity is also an important factor since low salinity relates to freshwater drinking sources, that could explain the manatee's distant travel upstream during the RaS, when the riverine estuary extent is at its maximum.…”

Advances in thermal physiology of diving marine mammals: The dual role of peripheral perfusion

“…Therefore, it is critical to determine a correction factor for the sensor and the attachment mechanism [ 38 , 41 , 145–148 ]. Additionally, observations of high variability in heat flux values both among and within individuals are common, which should caution the quantitative interpretation of heat flux values [ 41 , 149 ].…”

Thermoregulatory Strategies of Diving Air-Breathing Marine Vertebrates: A Review

“…When Florida winter water temperatures reach below their lower critical temperature of 20 • C, manatees may migrate to warm-water thermal refuges in inland natural springs or near power plants (Laist et al, 2013). There have been up to 282 manatee deaths due to cold-stress in a single year, and those most vulnerable are juveniles and sub-adults due to their inexperience of finding thermal refuges and high SA:V relative to adults (Erdsack et al, 2018). Still, even those that use these thermal refuges may face energetic trade-offs with foraging as it requires traveling further distances to feeding grounds, resulting in reduced foraging time (Haase et al, 2019).…”

“…Most heat flux studies have been performed on captive animals using handheld devices, which measure the amount of heat transferred per unit area per unit time (Hampton et al, 1971;McGinnis et al, 1972;Hampton and Whittow, 1976;Heath and Ridgway, 1999;Noren et al, 1999;Williams et al, 1999b;Erdsack et al, 2018). and Westgate et al (2007) measured heat flux in free-ranging Weddell seals and dolphins using packages specifically designed for their study species.…”

“…Heat flux measurements have improved our understanding of the spatial variability of heat transfer across the body surface and how it is affected by changes in insulation, peripheral perfusion, behavior, and water temperature. By comparing heat flux at multiple body sites in relation to water temperature and activity level in the lab, thermal windows have been identified in various species: dorsal fin and flippers in dolphins (Hampton et al, 1971;McGinnis et al, 1972;Heath and Ridgway, 1999;Noren et al, 1999;Williams et al, 1999b;Meagher et al, 2008), fluke and flippers in manatees (Erdsack et al, 2018), flippers in seals , and soft white skin areas in turtles (Heath and McGinnis, 1980;Standora et al, 1982). Unlike the dorsal fin of dolphins, heat flux sensors have not been attached to the highly maneuverable flippers of sea lions or fur seals and the wings of penguins to confirm the role of these appendages as thermal windows (Goldsmith and Sladen, 1961;Hindle et al, 2015).…”