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Journal of Experimental Biology partnership with Dryad

Heat storage in Asian elephants during submaximal exercise: behavioral regulation of thermoregulatory constraints on activity in endothermic gigantotherms
M. F. Rowe, G. S. Bakken, J. J. Ratliff, V. A. Langman

SUMMARY

Gigantic size presents both opportunities and challenges in thermoregulation. Allometric scaling relationships suggest that gigantic animals have difficulty dissipating metabolic heat. Large body size permits the maintenance of fairly constant core body temperatures in ectothermic animals by means of gigantothermy. Conversely, gigantothermy combined with endothermic metabolic rate and activity likely results in heat production rates that exceed heat loss rates. In tropical environments, it has been suggested that a substantial rate of heat storage might result in a potentially lethal rise in core body temperature in both elephants and endothermic dinosaurs. However, the behavioral choice of nocturnal activity might reduce heat storage. We sought to test the hypothesis that there is a functionally significant relationship between heat storage and locomotion in Asian elephants (Elephas maximus), and model the thermoregulatory constraints on activity in elephants and a similarly sized migratory dinosaur, Edmontosaurus. Pre- and post-exercise (N=37 trials) measurements of core body temperature and skin temperature, using thermography were made in two adult female Asian elephants at the Audubon Zoo in New Orleans, LA, USA. Over ambient air temperatures ranging from 8 to 34.5°C, when elephants exercised in full sun, ~56 to 100% of active metabolic heat production was stored in core body tissues. We estimate that during nocturnal activity, in the absence of solar radiation, between 5 and 64% of metabolic heat production would be stored in core tissues. Potentially lethal rates of heat storage in active elephants and Edmontosaurus could be behaviorally regulated by nocturnal activity.

FOOTNOTES

  • AUTHOR CONTRIBUTIONS

    M.F.R., G.S.B. and V.A.L. concieved this study and designed the experiments. Execution of the exercise trials in elephants was conducted by M.F.R. and J.J.R. Data analysis and interpretation was performed by M.F.R.

  • COMPETING INTERESTS

    No competing interests declared.

  • FUNDING

    This research received funding from the Indiana State University College of Graduate and Professional Studies, the Lilly Endowment Graduate Fellowship and the Pittsburgh Zoo and Aquarium Conservation Fund (POT 523/R85523).

  • LIST OF SYMBOLS AND ABBREVIATIONS

    A
    total skin surface area (m2) calculated from the relationship A=0.1mb0.67
    A1
    cross-sectional surface area of a spherical elephant (m2)
    A2
    50% of the total skin surface area (m2)
    C
    convective heat loss (W)
    Cex
    convective heat loss during exercise (W)
    cp
    heat capacity of air (1003.5 J °C−1 kg−1)
    dt
    change in time (s)
    dTb
    change in core body temperature (°C)
    E
    evaporative heat loss (W)
    Eb
    respiratory evaporative heat loss (W)
    Er
    evaporative heat loss from skin (W)
    hc
    convection coefficient (W m−2 °C−1)
    I
    whole-animal effective tissue insulation (°C W−1)
    K
    conductive heat loss (W)
    k
    thermal conductivity of air (2.47 to 2.65×10−2 W m−1 °C−1)
    M
    resting metabolic heat production (W)
    mb
    body mass (kg)
    Mex
    wet exercise metabolic heat production (W)
    MexEb
    dry exercise (active) metabolic heat production (W)
    Qa
    environmental radiation absorbed by a spherical elephant (W)
    Qn
    net radiant heat transfer (W)
    Qn,night
    net radiant heat transfer at night (W)
    Qn,sun
    net radiant heat transfer in full sun (W)
    r
    reflectance of the asphalt track surface (15%)
    Ra
    long-wave thermal radiation from the atmosphere (W m−2)
    Rg
    long-wave thermal radiation from the track surface (W m−2)
    RH
    relative humidity (%)
    Rs
    radiant heat loss from a spherical elephant (W)
    s
    diffuse short-wave solar radiation scattered in the atmosphere
    Sh
    direct short-wave solar radiation falling on a horizontal plane (W m−2)
    Sn
    direct short-wave solar radiation perpendicular to the body (W m−2)
    Ta
    ambient air temperature (°C)
    Tb
    core body (rectal) temperature (°C)
    TE
    temperature of exhaled air (°C)
    Tg
    radiant ground temperature (°C)
    TI
    temperature of inspired air (°C)
    Tr
    mean radiant skin temperature (°C)
    u
    sustained environmental wind speed (m s−1)
    V
    respiratory minute volume (l s−1)
    vf
    walking speed (m s−1)
    WE
    water content of exhaled air (kg H2O l−1 air)
    WI
    water content of inhaled air (kg H2O l−1 air)
    x
    distance from body core to skin surface (m)
    X
    heat storage in core tissues (W)
    α1
    percent absorptance of elephant skin for short-wave solar radiation
    α2
    percent absorptance of elephant skin for long-wave thermal radiation
    ε
    emissivity of elephant skin (0.96) and asphalt (0.93)
    λ
    latent heat of water vaporization (≈2.43×106 J kg−1 H2O)
    ρE
    density of air (0.0011–0.0012 kg l−1).
    σ
    Stefan–Boltzmann constant (5.67×10−8 W m−2 K−4).
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