Abstract:
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, namely, the African cuckoo
(Cuculus gularis, Cuculiformes), lilac-breasted roller (Coracias
caudatus, Coraciiformes) and Burchell’s starling (Lamprotornis
australis, Passeriformes). All three species initiated respiratory
mechanisms to increase evaporative heat dissipation when body
temperature (Tb) approached 41.5°C in response to increasing Ta, with
gular flutter observed in cuckoos and panting in rollers and starlings.
Resting metabolic rate and evaporative water loss increased by
quantitatively similar magnitudes in all three species, although
maximum rates of evaporative water loss 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.
