Partitioning respiratory and cutaneous evaporative water loss in naked and furred African mole-rats

Abstract

Evaporation is one of the most important pathways of water and heat loss for terrestrial vertebrates and can be partitioned into cutaneous (CEWL) and respiratory (REWL) components of total evaporative water loss (TEWL). Subterranean mammals inhabiting burrows with high humidity and relatively high ambient temperatures (Ta) face significant thermoregulatory challenges. We examined the effects different Tas (27.5, 32 and 37.5 ◦C) on the partitioning of TEWL in two social African mole-rats that differ in pelage. In the naked mole-rat (Heterocephalus glaber), TEWL increases ~33 % with increasing Ta, resulting from the ~63 % increase in CEWL and ~69 % of heat is lost via evaporation. In Micklem´s mole-rat (Fukomys micklemi), TEWL increases ~57 %, but this is related to the ~85 % increase in REWL and only ~35 % of its heat is dissipated via evaporation. In both species, body temperature increased to 39 ◦C at 37.5 ◦C, indicating that heat dissipation was not fully effective. In the naked mole-rat, the increase of CEWL may play a decisive role in complex thermoregulation in hot environments, as cutaneous evaporative cooling does not entail additional metabolic costs. In contrast, Micklem´s mole-rats rely more on non-evaporative mechanisms, as CEWL remains unchanged. Evaporative cooling may therefore be more energetically costly in this species, while fur is likely to enhance thermoregulation in colder environments. Finally, we compared evaporation within thermoneutrality (standard EWL) in both species with other fossorial and surface-living rodents to determine whether lifestyle has an effect on evaporation. We found that standard EWL was higher in fossorial rodents, suggesting that high burrow humidity does not constrain minimal evaporative capacity.