Snow bunting respirometry data

1. Arctic animals inhabit some of the coldest environments on the planet
and have evolved physiological mechanisms for minimizing heat loss under
extreme cold. However, the Arctic is warming faster than the global
average and how well Arctic animals tolerate even moderately high air
temperatures (Ta) is unknown. 2. Using flow-through respirometry we
investigated the heat tolerance and evaporative cooling capacity of snow
buntings (Plectrophenax nivalis; ≈ 31g, N = 42), a cold specialist, Arctic
songbird. We exposed buntings to increasing Ta and measured body
temperature (Tb), resting metabolic rate (RMR), rates of evaporative water
loss (EWL) and evaporative cooling efficiency (the ratio of evaporative
heat loss to metabolic heat production). 3. Buntings had an average (±SD)
Tb of 41.3 ± 0.2 °C at thermoneutral Ta, and increased Tb to a maximum of
43.5 ± 0.3 °C. Buntings started panting at Ta of 33.2 ± 1.7 °C, with rapid
increases in EWL starting at Ta = 34.6 °C, meaning they experienced heat
stress when air temperatures were well below their body temperature.
Maximum rates of EWL were only 2.9x baseline rates at thermoneutral Ta, a
markedly lower increase than seen in more heat tolerant arid-zone species
(e.g., ≥ 4.7x baseline rates). Heat stressed buntings also had low
evaporative cooling efficiencies, with 95% of individuals unable to
evaporatively dissipate an amount of heat equivalent to their own
metabolic heat production. 4. Our results suggest that buntings’
well-developed cold tolerance may come at the cost of reduced heat
tolerance. As the Arctic warms, and this and other species experience
increased periods of heat stress, a limited capacity for evaporative
cooling may force birds to increasingly rely on behavioural
thermoregulation, such as minimizing activity, at the expense of
diminished performance or reproductive investment.