Acclimatization of Thermoregulation in Desert‐Inhabiting Jackrabbits(Lepus Alleni and Lepus Californicus

Abstract

Seasonal physiological responses to ambient temperature were determined for the black‐tailed jackrabbit, Lepus californicus and the antelope jackrabbit, Lepus alieni at Tucson, Arizona. Both species exhibited similar responses to changes in season although differences were noted. From winter to summer the thermoneutral zone shifted upwards, with the shift occurring at both critical temperatures in L. californicus and at only the lower critical temperature in L. alieni. Standard metabolism above thermoneutrality was higher in winter than in summer, while below thermoneutrality it was lower in winter than in summer. Basal metabolism in winter was 15% lower in L. alieni and only slightly lower in L. californicus. Water losses during the summer were higher than in winter at ambient temperatures below 20°C and lower at the higher air temperatures. During both seasons all metabolic heat production was dissipated by evaporative heat Joss at an ambient temperature of ≈ 40°C. Seasonal differences in body temperature were not apparent except that in summer L. alieni exhibited hypothermia at ambient temperatures < 20°C. The dry‐heat‐transfer coefficients were higher during summer below 30°C; they also increased at higher ambient temperatures than in winter and at a faster rate. The dry‐heat‐transfer coefficient in L. alieni decreased during summer when ambient exceeded body temperature. This was not the case in winter for L. alieni or in either season for L. californicus.

Avian Adjustments to Cold

“…A precedent for a seasonal change in the effects of wind is provided by data on pelage insulation in the black-tailed jackrabbit Lepus californicus. The difference in insulation found in this mammal between summer and winter (Hinds 1977) is substantially magnified by the effects of wind (Harris et al 1985). A seasonal shift in heat loss due to the combined effects of cold and wind could be of particular significance during flight.…”

Evolutionary History of the Antelope Jackrabbit,Lepus alleni

“…Lepus californicus is also more cold tolerant, forms of the species occurring in the Great Basin and Great Plains, while L. alleni is confined to habitats having annual temperatures >15.4°C with <60 mean days a year below freezing. Hinds (1977) noted that L. alleni went into hypothermia when body temperatures dropped to 28ºC.…”

Is the large size of the pinna of the ear of the European hare (Lepus europaeus) due to its role in thermoregulation or in anterior capital shock absorption?

“…Although the pinna of leporids at rest is a thermoregulatory organ (Schmidt-Nielsen et al, 1965;Dawson and Schmidt-Nielsen, 1966;Hinds, 1977;Marai et al, 2005), we have demonstrated that the size of the pinna in this species is not determined by the requirements of sedentary diurnal thermoregulation. The pinna of the Australian population of the hare, unlike that of the European rabbit (Parer and Libke, 1985), had not increased in length relative to its English antecedents (Fig.…”

“…The pinna reaches its most extreme sizes in the genus Lepus. Schmidt-Nielsen et al (1965), Dawson and Schmidt-Nielsen (1966), and Hinds (1977) concluded that the large sizes of the pinnae of two North American species, Lepus alleni and Lepus californicus, were determined by the need for radiant heat loss under the thermal loads prevalent in summers in the south-west of the United States of America (e.g. mean July maximum for Las Vegas, NM: 28.68C).…”