The stoichiometric approach in determining total evaporative water loss and the relationship between evaporative and non-evaporative heat loss in two resting bird species: passerine and non-passerine

It is a central paradigm of comparative physiology that the effect of humidity on evaporative water loss (EWL) is determined for most mammals and birds, in and below thermoneutrality, essentially by physics and is not under physiological regulation. Fick's law predicts that EWL should be inversely proportional to ambient relative humidity (RH) and linearly proportional to the water vapour pressure deficit (Δwvp) between animal and air. However, we show here for a small dasyurid marsupial, the little kaluta ( Dasykaluta rosamondae ), that EWL is essentially independent of RH (and Δwvp) at low RH (as are metabolic rate and thermal conductance). These results suggest regulation of a constant EWL independent of RH, a hitherto unappreciated capacity of endothermic vertebrates. Independence of EWL from RH conserves water and heat at low RH, and avoids physiological adjustments to changes in evaporative heat loss such as thermoregulation. Re-evaluation of previously published data for mammals and birds suggests that a lesser dependence of EWL on RH is observed more commonly than previously thought, suggesting that physiological independence of EWL of RH is not just an unusual capacity of a few species, such as the little kaluta, but a more general capability of many mammals and birds.

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Can birds do it too? Evidence for convergence in evaporative water loss regulation for birds and mammals

Proceedings of The Royal Society B Biological Sciences ◽

10.1098/rspb.2017.1478 ◽

2017 ◽

Vol 284 (1867) ◽

pp. 20171478 ◽

Cited By ~ 5

Author(s):

E. C. Eto ◽

P. C. Withers ◽

C. E. Cooper

Keyword(s):

Water Vapour ◽

Water Loss ◽

Vapour Pressure ◽

Vapour Pressure Deficit ◽

Ambient Air ◽

Evaporative Water Loss ◽

Water Vapour Pressure ◽

Evaporative Heat Loss ◽

Evaporative Water ◽

Water Vapour Pressure Deficit

Birds have many physiological characteristics that are convergent with mammals. In the light of recent evidence that mammals can maintain a constant insensible evaporative water loss (EWL) over a range of perturbing environmental conditions, we hypothesized that birds might also regulate insensible EWL, reflecting this convergence. We found that budgerigars ( Melopsittacus undulatus ) maintain EWL constant over a range of relative humidities at three ambient temperatures. EWL, expressed as a function of water vapour pressure deficit, differed from a physical model where the water vapour pressure deficit between the animal and the ambient air is the driver of evaporation, indicating physiological control of EWL. Regulating EWL avoids thermoregulatory impacts of varied evaporative heat loss; changes in relative humidity had no effect on body temperature, metabolic rate or thermal conductance. Our findings that a small bird can regulate EWL are evidence that this is a common feature of convergently endothermic birds and mammals, and may therefore be a fundamental characteristic of endothermy.

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Respiratory and cutaneous evaporative water loss at high environmental temperatures in a small bird

Journal of Experimental Biology ◽

10.1242/jeb.199.2.451 ◽

1996 ◽

Vol 199 (2) ◽

pp. 451-457 ◽

Cited By ~ 18

Author(s):

B Wolf ◽

G Walsberg

Keyword(s):

Metabolic Rate ◽

Air Temperature ◽

Water Loss ◽

Heat Dissipation ◽

Evaporative Water Loss ◽

Co2 Production ◽

Evaporative Heat Loss ◽

Evaporative Water ◽

Metabolic Chamber ◽

Air Temperatures

We measured rates of respiratory and cutaneous evaporative water loss as a function of air temperature in a small desert bird, the verdin Auriparus flaviceps. Birds were placed in a two-compartment metabolic chamber that separately collected water evaporated from the bird's head and body. Cutaneous and respiratory evaporative water loss, as well as CO2 production, were measured in resting birds at 2 °C intervals between 30 and 50 °C. Metabolic rate was lowest at 38 °C (19 mW g-1) and increased to 28 mW g-1 at 50 °C. At the lowest air temperature, 30 °C, resting metabolic rate was 34 mW g-1. As air temperature increased from 30 to 50 °C, cutaneous water loss increased from 3.3 to 10.3 mg g-1 h-1 and respiratory water loss increased from 2.1-64.1 mg g-1 h-1. At moderate air temperatures (30-36 °C), water loss was divided almost evenly between respiratory and cutaneous components. As air temperature increased, however, verdins became heavily dependent on respiratory evaporation for heat dissipation. Evaporative water loss data for other species at high air temperatures suggest that partitioning of water loss may follow two different patterns. Evaporative heat dissipation may depend primarily on either cutaneous or respiratory modes of evaporative heat transfer. The physiological mechanisms and functional significance of these contrasting patterns of evaporative heat loss remain unknown.

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