McKechnie, A. E. | Search bioRxiv

bioRxiv2018

Avian thermoregulation in the heat: is evaporative cooling more economical in nocturnal birds?

Evaporative cooling is a prerequisite for avian occupancy of hot, arid environments, and is the only avenue of heat dissipation when air temperatures (Ta) exceed body temperature (Tb). Whereas diurnal birds can potentially rehydrate throughout the day, nocturnal species typically forgo drinking between sunrise and sunset. We hypothesized that nocturnal birds have evolved reduced rates of evaporative water loss (EWL) and more economical evaporative cooling mechanisms than those of diurnal species that permit them to tolerate extended periods of intense heat without becoming lethally dehydrated. We used phylogenetically-informed regressions to compare EWL and evaporative cooling efficiency (ratio of evaporative heat loss [EHL] and metabolic heat production [MHP]; EHL/MHP) among nocturnal and diurnal birds at high Ta. We analyzed variation in three response variables: 1) slope of EWL at Ta between 40 and 46{degrees}C, 2) EWL at Ta = 46{degrees}C, and 3) EHL/MHP at Ta = 46{degrees}C. Nocturnality emerged as a weak, negative predictor, with nocturnal species having slightly shallower slopes and reduced EWL compared to diurnal species of similar mass. In contrast, nocturnal activity was positively correlated with EHL/MHP, indicating a greater capacity for evaporative cooling in nocturnal birds. However, our analysis also revealed conspicuous differences among nocturnal taxa. Caprimulgids and Australian-owlet nightjars had shallower slopes and reduced EWL compared to similarly-sized diurnal species, whereas owls had EWL rates comparable to diurnal species. Consequently, our results did not unequivocally demonstrate more economical cooling among nocturnal birds. Owls predominately select refugia with cooler microclimates, but the more frequent and intense heat waves forecast for the 21st century may increase microclimate temperatures and the necessity for active heat dissipation, potentially increasing owls vulnerability to dehydration and hyperthermia.

physiology↗

bioRxiv2017

Avian thermoregulation in the heat: phylogenetic variation among avian orders in evaporative cooling capacity and heat tolerance

Little is known about the phylogenetic variation of avian evaporative cooling efficiency and heat tolerance in hot environments. We quantified thermoregulatory responses to high air temperature (Ta) in ~100-g representatives of three orders: African cuckoo (Cuculus gularis, Cuculiformes), lilac-breasted roller (Coracias caudatus, Coraciiformes), and Burchells starling (Lamprotornis australis, Passeriformes). All three species initiated respiratory mechanisms to increase evaporative heat dissipation when body temperature (Tb) approached 41.5{degrees}C in response to increasing Ta, with gular flutter observed in cuckoos and panting in rollers and starlings. Resting metabolic rate (RMR) and evaporative water loss (EWL) increased by quantitatively similar magnitudes in all three species, although maximum rates of EWL were proportionately lower in starlings. Evaporative cooling efficiency [defined as the ratio of evaporative heat loss (EHL) to metabolic heat production (MHP)] generally remained below 2.0 in cuckoos and starlings, but reached a maximum of ~3.5 in rollers. The high value for rollers reveals a very efficient evaporative cooling mechanism, and is similar to EHL/MHP maxima for similarly sized columbids which very effectively dissipate heat via cutaneous evaporation. This unexpected phylogenetic variation among the orders tested in the physiological mechanisms of heat dissipation is an important step toward determining the evolution of heat tolerance traits in desert birds.\n\nSummary statementWe show that avian evaporative cooling efficiency and heat tolerance display substantial taxonomic variation that are, unexpectedly, not systematically related to the use of panting versus gular flutter processes.

zoology↗