Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57272, diciembre 2023 (Publicado Nov. 01, 2023)

Heat loss or heat uptake? Skin temperature in Antillean manatees

(Trichechus manatus manatus, Sirenia: Trichechidae) in Belize

Nicola Erdsack 1*; https://orcid.org/0000-0002-8590-3609

Jamal A. Galves 2; https://orcid.org/0000-0003-0825-0744

James E. Powell 3; https://orcid.org/0000-0001-8757-0556

1. Mote Marine Laboratory, Manatee Research Program, Sarasota, FL, USA; erdsack@mote.org (*Correspondence)

2. Clearwater Marine Aquarium Research Institute, Belmopan City, Belize; jgalves@cmaquarium.org

3. Clearwater Marine Aquarium Research Institute, Clearwater, FL, USA; jpowell@cmaquarium.org

Received 13-VII-2022. Corrected 09-II-2023. Accepted 12-IV-2023.

ABSTRACT

Introduction: The two subspecies of the West Indian manatee (Trichechus manatus), Florida manatees (T. m.

latirostris) and Antillean manatees (T. m. manatus), face different environmental challenges. While Florida mana-

tees have to cope with winter water temperatures below their lower critical temperature of ~ 20 °C and air tem-

peratures below freezing, Antillean manatees live in year-round warm Caribbean waters. Sirenians lack effective

thermal insulation and have limited capability of controlling peripheral heat loss. Although severe cold related

health issues and mortality are primarily known in Florida manatees, it can be assumed that Antillean manatees

and other extant sirenians share the cold-sensitivity, but hardly ever experience it. Contrarily, during summer,

Antillean manatees may face the opposite form of thermal stress by being exposed to water temperatures close to

their body temperature. However, the upper critical temperature of manatees is not known.

Objective: To improve understanding of the impact of high ambient temperatures on manatee physiology.

Methods: We measured skin temperature in six Antillean manatees in two different habitats in Belize, and com-

pared the results to skin temperatures measured in two captive Florida manatees.

Results: We found a similar temperature distribution pattern over the body surface in both subspecies, but sig-

nificantly higher temperatures and larger temperature ranges among measuring points in Antillean manatees as

compared to Florida manatees. In one Antillean manatee, skin temperature was consistently lower than ambient

water temperature by up to 2.5 °C. This implies potential heat uptake from the environment, in contrast to the

heat loss experienced by Florida manatees at low water temperatures, apparent in skin temperatures above ambi-

ent water temperature.

Conclusions: Our findings suggest that heat stress may be a more likely risk for manatees in warm tropical

waters. Despite the small sample size, our results present important findings towards understanding thermal

tolerance and impact of high ambient temperatures on manatee physiology.

Key words: thermoregulation; peripheral heat loss; heat dissipation; heat retention; blubber lipid composition;

thermoregulatory adaptations; surface area to volume ratio SA:V.

RESUMEN

¿Pérdida o absorción de calor? Temperatura de la piel en manatíes antillanos (Trichechus manatus mana-

tus, Sirenia: Trichechidae) en Belice

Introducción: Las dos subespecies del manatí antillano (Trichechus manatus), los manatíes de Florida (T. m.

latirostris) y los manatíes antillanos (T. m. manatus), enfrentan diferentes desafíos ambientales. Mientras que

https://doi.org/10.15517/rev.biol.trop..v71iS4.57272

SUPPLEMENT • MANATEES

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57272, diciembre 2023 (Publicado Nov. 01, 2023)

INTRODUCTION

Florida manatees, Trichechus manatus lat-

irostris (Harlan, 1824) and Antillean manatees,

Trichechus manatus manatus (Linnaeus, 1758)

are the two subspecies of the West Indian man-

atee (T. manatus). While Antillean manatees

are found in the Caribbean and from Mexico

down south to the coast of Brazil (Marsh et

al., 2011), Florida manatees primarily inhabit

Florida coastal and inland waters, with a typical

range from Texas to North Carolina in summer

with occasional sightings as far north as Mas-

sachusetts (Marsh et al., 2011). Although their

habitats overlap to a small extent in the Carib-

bean, each subspecies faces different environ-

mental challenges.

Florida manatees are the northern most

Sirenian population and have to face water

temperatures (Twater) as low as 13 °C in winter

over extended periods of time along with air

temperatures (Tair) below freezing in contrast

to summer water temperatures of up to 32 °C

and higher (National Oceanic and Atmospheric

Administration, 2022). Florida manatees are

known to be very sensitive to cold with low

metabolic rates (Scholander & Irving, 1941)

and a high lower critical temperature of ~ 20 °C

(Irvine, 1983) due to poor thermal insulation

and limited control of peripheral heat loss.

Therefore, they rely on behavioral thermoregu-

lation and migrate to warm water refuges when

temperatures drop (Marsh et al., 2011). Still,

cold stress is a major threat to Florida mana-

tees, and cold stress syndrome (CSS) affects

and kills manatees every winter (Bossart et al.,

2002; Hardy et al., 2019). Extant sirenians share

low metabolic rates (Gallivan & Best, 1980)

and lack of effective thermal insulation (Hor-

gan et al., 2014) and other thermoregulatory

adaptations to the aquatic life style (Bryden

et al., 1978; Fawcett, 1942; Gallivan et al.,

1983), present in other marine mammal spe-

cies. Therefore, Antillean manatees and other

extant sirenians are likely sensitive to cold as

well, which is furthermore supported by reports

los manatíes de Florida tienen que hacer frente a temperaturas invernales del agua por debajo de su temperatura

crítica ~20 °C y temperaturas del aire por debajo del punto de congelación, los manatíes antillanos viven en aguas

cálidas del Caribe durante todo el año. Los sirenios carecen de un aislamiento térmico efectivo y tienen una

capacidad limitada para controlar la pérdida de calor periférico. Aunque los problemas de salud y la mortalidad

relacionados con el frío se conocen principalmente en los manatíes de Florida, se puede suponer que los manatíes

antillanos y otros sirenios existentes comparten la sensibilidad al frío, pero casi nunca la experimentan. Por el

contrario, durante el verano, los manatíes antillanos pueden enfrentar la forma opuesta de estrés térmico al estar

expuestos a temperaturas del agua cercanas a la temperatura de su cuerpo. Sin embargo, se desconoce la tempe-

ratura crítica superior de los manatíes.

Objetivo: Mejorar la comprensión del impacto de las altas temperaturas ambientales en la fisiología del manatí.

Métodos: Medimos la temperatura de la piel en seis manatíes antillanos en dos hábitats diferentes en Belice y

comparamos los resultados con las temperaturas de la piel medidas en dos manatíes de Florida en cautiverio.

Resultados: Encontramos un patrón de distribución de temperatura similar sobre la superficie del cuerpo en

ambas subespecies, pero temperaturas significativamente más altas y rangos de temperatura más amplios entre

los puntos de medición en los manatíes antillanos en comparación con los manatíes de Florida. En un manatí

antillano, la temperatura de la piel fue consistentemente más baja que la temperatura ambiente del agua hasta en

2.5 °C. Esto implica una posible absorción de calor del medio ambiente, en contraste con la pérdida de calor que

experimentan los manatíes de Florida a bajas temperaturas del agua, lo cual se evidencio con temperaturas de la

piel por encima de la temperatura ambiente del agua.

Conclusiones: Nuestros hallazgos sugieren que el estrés por calor puede ser un riesgo más probable para los

manatíes en aguas cálidas tropicales. A pesar del pequeño tamaño de la muestra, nuestros resultados presentan

hallazgos importantes para comprender la tolerancia térmica y el impacto de las altas temperaturas ambientales

en la fisiología de estos mamíferos marinos.

Palabras clave: termorregulación; pérdida de calor periférica; disipación de calor; retención de calor; composi-

ción de lípidos de grasa; adaptaciones termorreguladoras; relación superficie-volumen SA:V.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57272, diciembre 2023 (Publicado Nov. 01, 2023)

of cold water avoidance behavior (Anderson,

1986; Zeh et al., 2018).

In contrast to Florida manatees, Antillean

manatees live in year-round warm Caribbean

waters, and knowledge about potential cold

sensitivity is only anecdotal. Contrarily, they

may experience Twater in the range of mamma-

lian body temperature (Tbody) during summer

months (Kaufman & Thompson, 2005), which

poses an entirely different thermoregulatory

challenge. To investigate and assess the poten-

tial risk of heat stress in manatees, knowledge

of their upper critical temperature is required.

However, manatee upper critical temperature is

currently not known.

Due to manatees’ poor thermal insulation

and limited ability to control peripheral heat

loss, their skin temperature (Tskin) is ideally

suited to indicate the animal’s thermal state by

assessing heat exchange between body surface

and environment (Erdsack et al., 2018; Worthy

et al., 2000). Long-term studies of Tskin and heat

flux in two captive Florida manatees indicated

areas of increased heat exchange on the body

surface, temperature distribution patterns and

potential impact of ambient temperature on

manatee Tskin (Erdsack et al., 2018; N. Erd-

sack unpublished).

Besides the unknown upper critical tem-

perature, knowledge about manatee thermo-

regulation, in particular the impact of high

ambient temperature is incomplete. In order

to fill some of these gaps, we measured Tskin

in wild Antillean manatees captured for health

assessments in two different habitats in Belize.

Here, we present these preliminary data in

comparison to Tskin measured in captive Flori-

da manatees between 2016 and 2021.

Tskin was measured in six adult Antillean

manatees captured for health assessments in

Belize in May 2019: three females in Southern

Lagoon, around Gales Point, and a female and

two males in the waters around Placencia. The

manatees were captured using a circle net and

hefted onto the capture boat for processing.

Tskin measurements took approximately 10 min

and started as soon as the net was removed and

the animal was in a safe and stable position

on the boat. If necessary, mud was rinsed off

with sea water. Animals were shaded by a tarp

during processing. Tskin was measured using a

wireless thermometer with an attached K-Type

surface thermocouple (TMD-55W, Amprobe,

Everett, WA, USA), which was also used in

a long term study of Tskin measurements in

captive Florida manatees (N. Erdsack unpub-

lished). The 14 defined measuring spots on

the manatees’ dorsal body surface (Fig. 1A)

were selected in accordance with this study

(N. Erdsack unpublished). The two trained

manatees (“Hugh”, “Buffett”) are held at Mote

Marine Laboratory, Sarasota, FL, U.S. in an

outdoor tank at a constant water temperature

of 26.3±0.4 °C. For measurements, the mana-

tees were stationing at the water surface and

the respective body part was lifted above the

water surface. For comparison with Antillean

manatees, only measurements at average Tair

comparable to average Tair during measure-

ments in Belize were considered (Hugh: n = 7;

Buffett: n = 6). Temperature differences were

tested for statistical significance using a two-

tailed paired or homoscedastic t-test, respec-

tively, in MS-Excel 2019. Level of significance

was α = 0.5. Relations between Tskin and Twater,

Tair were reported using Pearson correlation

coefficient (r).

Average Tskin per measuring point in Antil-

lean and Florida manatees are displayed in Fig.

1B. Both subspecies exhibited similar tempera-

ture distribution patterns over the body surface.

Except for the almost identical temperature

on the top of the head, manatees in Southern

Lagoon and Placencia exhibited almost congru-

ent temperature distribution patterns. However,

average Tskin in manatees in Southern Lagoon

was significantly higher than in manatees cap-

tured around Placencia (p << 0.0001), and

average Tskin in Antillean manatees was signifi-

cantly higher than in Florida manatees (p <<

0.0001). Moreover, average temperature range

amongst measuring points was significantly

larger in Antillean manatees (2.92 ± 1.08 °C,

p = 0.0011) as compared to Florida mana-

tees (1.28 ± 0.74 °C). One animal captured in

Southern Lagoon at Twater = 33 °C had average

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57272, diciembre 2023 (Publicado Nov. 01, 2023)

Tskin = 31.6 °C and Tskin < Twater in all but one

of the measuring points (center of the tail,

P12 = 33.1 °C). In Southern Lagoon, average

Twater during measurements was 31 ± 1.7 °C,

in Placencia 28.9 ± 0.6 °C, and 26.4 ± 0.3 °C in

the Florida manatee tank. Average Tair during

measurements was 30.2 ± 0.8 °C in Belize, and

30.7 ± 1.3 °C in Florida. In Antillean manatees,

Tskin was weakly correlated to Twater (r = 0.60),

but no correlation to Tair (r = 0.29) was found.

Our measurements revealed similarities

but also significant differences in average Tskin

and Tskin distribution between Antillean and

Florida manatees. Distribution of Tskin over

the body surface, indicating body locations

with higher and lower heat exchange with the

environment, was similar in both subspecies.

However, in Antillean manatees, this pattern

was much more pronounced, that is, tempera-

ture differences between measuring points were

Fig. 1. Location of measuring spots on the manatees’ dorsal body surface including ventral (P2) and dorsal (P3) side of the

pectoral flipper (A) and average Tskin+SD per measuring point (B) in Antillean manatees captured in Southern Lagoon (black

diamonds, n = 3), and Placencia (white diamonds, n = 3), and in two captive Florida manatees (Hugh: black dots, n = 7;

Buffett: black circles, n = 6). Average Twater during measurements was 31±1.7 °C in Southern Lagoon (black dotted line),

28.9±0.6 °C in Placencia (gray dotted line), and 26.3±0.4 °C in the Florida manatee tank (dashed line). Average Tair during

measurements was 30.2±0.8 °C in Belize, and 30.7±1.3 °C in Florida.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 71(S4): e57272, diciembre 2023 (Publicado Nov. 01, 2023)

significantly larger in Antillean manatees (up to

4.8 °C in individual manatees) than in Florida

manatees (up to 2.7 °C). The overall higher Tskin

in Antillean manatees can likely be attributed to

the higher Twater in Belizean waters, as indicated

by the positive correlation between Tskin and

Twater found in Antillean manatees (r = 0.6).

A potential impact of netting and handling on

metabolic rate and thermal state of individual

manatees cannot be excluded. However, Twater

cannot explain the significantly larger tempera-

ture ranges on the body surfaces of Antillean

manatees in comparison to Florida manatees.

The most obvious difference between

the two subspecies is their body size. Antil-

lean manatees are on average smaller than their

Florida conspecifics (Castelblanco-Martínez et

al., 2021; Wong et al., 2012), which are the

largest extant Sirenian (sub)species. Since heat

exchange with the environment occurs primar-

ily via the body surface, a reduced surface-area-

to-volume ratio (SA:V) is favorable in terms of

heat retention in the cold (Schmidt-Nielsen,

1997). Florida manatees’ larger body size along

with the small pectoral flippers found in all

sirenians result in a reduced SA:V, constitut-

ing an adaptation to the colder climate they

inhabit in comparison to other extant Sirenians.

This was even more pronounced in the extinct

Steller’s sea cow, Hydrodamalis gigas (Zimmer-

mann, 1780), which inhabited the cold waters

of the Bering Sea with estimated body lengths

and masses up to 10 m and 10 000 kg (Marsh et

al., 2011). In comparison to Florida manatees,

Antillean manatees have larger SA:Vs due to

their smaller body size (Castelblanco-Martínez

et al., 2021). Heat retention is likely not essen-

tial in this subspecies, considering that they

rarely experience Twater < 18 °C. Contrarily,

during summer, Antillean manatees may expe-

rience Twater in the range of mammalian Tbody.

Data on manatee core Tbody is scarce. Irvine

(1983) reported rectal temperatures of 27-32

°C measured in three captive Florida manatees,

but simultaneously measured stomach temper-

atures of 35-36.8 °C, which, moreover, could be

significantly impacted by food intake. Recent

measurements of pharyngeal temperature in

20 captive Florida manatees resulted in 35.1-

35.9 °C (Martony et al., 2020). Average oral

temperature measured in wild manatees during

health assessments was significantly higher in

Antillean (34.6 ± 0.9 °C) than in Florida (32.6 ±

1.8 °C) manatees (Wong et al., 2012). It is likely

that during summer months, Twater in shallow

Caribbean lagoons can reach and exceed these

values, regarding that we measured Twater up

to 33 °C in Southern Lagoon as early as May.

Marsh et al. (2011) even mentioned Twater as

high as 41 °C in an area frequented by Florida

manatees in summer. Without a thermal gradi-

ent from the body surface to the surrounding

water, an animal is not capable of dissipating

excess heat, which will eventually result in heat

stress. Average Tskin measured in one Antillean

manatee in Southern Lagoon with Tskin < Twater

did not differ from average Tskin measured

in the other manatees in Southern Lagoon;

however, Twater was higher. We did not observe

physiological abnormalities in this manatee,

suggesting the presence of physiological and/

or behavioral adaptations that help them deal

with these extreme thermal conditions, at least

temporarily. These findings also leave room for

speculations about upper critical temperature

and thermal tolerance in Antillean manatees,

which may be relatively high in the water.

As the differing body sizes, the observed

temperature distribution pattern along with the

differences in Tskin and Tskin ranges over the

body surface may indicate anatomical adapta-

tions to differing thermal environments. Sire-

nians lack arteriovenous anastomoses (AVAs)

in the skin (Bryden et al., 1978; Fawcett, 1942),

which are essential structures for the regulation

of peripheral heat dissipation and retention

(Hales, 1985). Thus, the observed temperature

distribution pattern on the manatees’ body

surfaces likely displays underlying anatomical

structures and conditions, such as distribution

of blood vessels, differences in skin thickness,

and variations in blubber distribution. Since

manatees, in contrast to other marine mam-

mals, do not have thick insulating blubber

layers (Reynolds & Lynch, 2017), potential

differences in blubber distribution are more

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 71(S4): e57272, diciembre 2023 (Publicado Nov. 01, 2023)

likely in lipid composition rather than thick-

ness. Blubber lipid composition varies between

species and individuals (Iverson, 2009), as well

as intra-individually between body locations

and seasons (Neises et al., 2021). Blubber lipid

composition in Antillean manatees may have

evolved to facilitate higher heat transfer as

opposed to thermal insulation and heat reten-

tion. Despite the analogous distribution of

blood vessels in the subspecies, apparent in

the similar temperature distribution pattern,

in Antillean manatees, heat dissipation from

underlying blood vessels would be more appar-

ent in increased temperatures in the overlying

skin. A further possible cause for the higher

Tskin range measured in Antillean manatees

could be increased heat dissipation at the less

insulated body parts at high Twater. This is indi-

cated by significantly higher Tskin at head, flip-

pers and in particular the tail (p = 0.0001) than

on the trunk, found in Antillean manatees, but

not in the Florida manatee data presented here.

The absence of a correlation between Tair

and Tskin in the presented data can likely be

attributed to the preliminary state of the data,

with small sample sizes and similar Tair dur-

ing measurements. Heat flux measurements in

Florida manatees indicated a potential impact

of Tair on heat flux (Erdsack et al., 2018). In any

case, more temperature measurements in Antil-

lean and in particular wild Florida manatees

under varying environmental conditions are

required. Comparative analyses of blubber lipid

composition will help identify potential dif-

ferences in thermal properties of blubber and

their role in manatee thermoregulation. Despite

limitations due to small sample size, our find-

ings provide valuable new information and an

important step towards a better understand-

ing of thermal tolerance in manatees and the

impact of environmental temperature on mana-

tee physiology, thermoregulation and health.

This knowledge is essential for the prevention

and treatment of thermal stress in the threat-

ened West Indian manatee.

Ethical statement: the authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that they fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are fully

and clearly stated in the acknowledgments sec-

tion. A signed document has been filed in the

journal archives.

Author Contribution: NE was responsible

for data collection, data analysis, and prepara-

tion of the initial, revised, and final manuscript,

JG assisted in data collection, provided back-

ground and environmental information, and

JP provided supervision, infrastructure, and

technical information for this project. Both JG

and JP contributed to various versions of the

manuscript.

ACKNOWLEDGEMENTS

Manatee research in Belize was conduct-

ed under a scientific research permit (#FD/

WL/1/19 (20)) issued by the Belize Forest

Department and was part of a Clearwater

Marine Aquarium Research Institute long-term

manatee research and conservation project.

We would like to thank Dr. Robert Bonde and

Cathy Beck of USGS as well as the Government

of Belize and local partners for their expertise

and support. Manatee captures were facilitated

by our experienced local crew from Gales Point

and many volunteers from Belize and abroad.

The skin temperature study in Florida mana-

tees was funded by a Research Grant of the

German Research Foundation to NE (DFG, ER

800/1-1) and performed under USFWS permits

MA837923-8 and MA100361-4.

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