Physiological performance declines precipitously at high body temperature ( T b ), but little attention has been paid to adaptive variation in upper T b limits among endotherms. We hypothesized that avian maximum tolerable T b ( T b max ) has evolved in response to climate, with higher T b max in species exposed to high environmental heat loads or humidity-related constraints on evaporative heat dissipation. To test this hypothesis, we compared T b max and related variables among 53 bird species at multiple sites in South Africa with differing maximum air temperature ( T air ) and humidity using a phylogenetically informed comparative framework. Birds in humid, lowland habitats had comparatively high T b max (mean ± SD = 45.60 ± 0.58 °C) and low normothermic T b ( T b norm ), with a significantly greater capacity for hyperthermia ( T b max − T b norm gradient = 5.84 ± 0.77 °C) compared with birds occupying cool montane (4.97 ± 0.99 °C) or hot arid (4.11 ± 0.84 °C) climates. Unexpectedly, T b max was significantly lower among desert birds (44.65 ± 0.60 °C), a surprising result in light of the functional importance of hyperthermia for water conservation. Our data reveal a macrophysiological pattern and support recent arguments that endotherms have evolved thermal generalization versus specialization analogous to the continuum among ectothermic animals. Specifically, a combination of modest hyperthermia tolerance and efficient evaporative cooling in desert birds is indicative of thermal specialization, whereas greater hyperthermia tolerance and less efficient evaporative cooling among species in humid lowland habitats suggest thermal generalization.

Accordingly, we predicted pronounced facultative hyperthermia buffers individual queleas from dehydration risk. To test this prediction, we quantified relationships between body temperature, evaporative heat loss and metabolic heat production in red-billed queleas in South Africa. Methods All experimental procedures were approved by the University of Pretoria's Animal Ethics Committee (NAS181/2019) and the Research Ethics and Scientific Committee of the South African National Biodiversity Institute (SANBI NZG/RES/P19/13) and birds were captured under permit JM 8,057/2019 from the Free State province's Department of Economic, Small Business Development, Tourism and Environmental Affairs. The methods we used for quantifying the upper limits of evaporative cooling capacity and heat tolerance followed those of a recent series of studies of avian heat tolerance 27-30. Study site and species. We trapped 20 red-billed queleas (body mass = 17.94 ± SD 1.19 g) using mist nets in agricultural fields near the town of Harrismith in South Africa (28° 06′ S, 29°10′E, 1754 m asl) during November 2019 (early austral summer). After capture, birds were transported by road (approximately 20-min trip) in cloth bags to a field laboratory, where they were held in cages (600 × 400 × 400 mm) for 1-16 h with ad libitum access to water and wild bird seed. Food was removed at least one hour prior to gas exchange and body temperature measurements, allowing individuals to habituate and ensure they were post-absorptive 31. Air and body temperature measurements. Body temperature was measured using a temperaturesensitive passive integrated transponder (PIT) tag (Biotherm 13, Biomark, Boise, ID, USA) injected intraperitoneally in each bird. Prior to injection, all PIT tags were calibrated in a circulating water bath (model F34, Julabo, Seelbach BW, DE) over temperatures ranging 35 to 50 °C against a thermocouple meter (TC-1000, Sable Systems, Las Vegas, NV, USA), the output of which was verified against a mercury-in-glass thermometer with NIST-traceable accuracy before and after the PIT tag calibration. Temperatures measured by PIT tags deviated by 0.28 ± 0.23 °C (n = 23) from actual values and we corrected all measured values accordingly. Data from the PIT tags were recorded using a reader and transceiver system (HPR + , Biomark, Boise ID, USA). To measure air temperature during the gas exchange measurements, we inserted a thermistor probe (TC-100, Sable Systems, Las Vegas, NV, USA) through a hole sealed with a rubber grommet in the side of each metabolic chamber.

This study aimed to assess the seasonal reproductive strategy occurring in the male Lesser Egyptian jerboa (Jaculus jaculus) and determine which environmental cues are responsible for initiating reproductive recrudescence. Body mass, morphometry of the reproductive tract, the histology of the testes and the circulating testosterone concentrations in wild male J. jaculus, from central Saudi Arabia, were studied over 12 consecutive months. Furthermore, sixteen additional males were collected and subsequently placed on either a short‐day (SD) or a long‐day (LD) light schedule under controlled laboratory conditions. Using these male reproductive parameters, we investigated the potential proximate environmental cues that may trigger the onset of reproduction. Unexpectedly, males were reproductively active during three out of the four seasons, namely autumn, winter and spring possibly due to the rainfall that fell at an unprecedented frequency and quantity during this time. However, we revealed the importance of other environmental triggers, namely photoperiod and ambient temperature, on the control of male reproduction in J. jaculus. Decreasing photoperiod (SD) and ambient temperature were observed to activate the reproductive system by increasing plasma testosterone concentration. The male Lesser Egyptian jerboa was found to exhibit a marked seasonal reproduction controlled by the environmental cues of ambient temperature and photoperiod.