The role of insensible evaporative water loss control for endotherms

• Applicant: Dr. Stephanie Reher
• Subject Area: Animal Physiology and Biochemistry ; Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 512659895

Factors influencing the water budget and thermoregulation of animals will be important determinants for the survival of many species in the face of increasing temperature and aridity that accompany anthropogenic climate change. For endotherms, evaporative water loss (EWL) is critical for thermoregulation at high ambient temperatures. However, also at moderate temperatures below and within the thermoneutral zone, endotherms inevitably lose water through respiratory and cutaneous pathways. Since endotherms experience moderate conditions for much of their time, this “insensible” EWL can account for up to 70% of the daily water budget and therefore has profound effects on maintaining homeostasis and thermoregulation. Despite its major physiological importance, insensible EWL is surprisingly poorly understood: it has long been considered an entirely passive process driven by the water vapour pressure deficit between an animal and its environment. Only recently, it was shown, though, that insensible EWL is not simply a passive consequence of the evaporative environment, but is under physiological control. Nonetheless, neither the phylogenetic spread, avenue or role for this control are clear; few studies on insensible EWL in only a handful of species exist. I will therefore examine insensible EWL control in two large endotherm groups: passerine birds and bats. I will use respirometry to obtain standard physiological variables (e.g., EWL, metabolic rate) at different evaporative environments by altering humidity and temperature, and by exposing the animals to both ambient air and helox (21% O2 in helium), changing the diffusion coefficient of the ambient air. Thus, I will provide two independent lines of evidence to build my conclusions. During these measurements, I will also measure skin resistance and expired air temperature to disentangle cutaneous and respiratory insensible EWL, allowing to assess the potential avenue of insensible EWL control. Lastly, insensible EWL may be related to water balance, or it may support thermoregulation by controlling evaporative heat loss. I will investigate the role of insensible EWL control by examining EWL during development for birds and mammals, some of which are ectothermic at early stages and attain endothermy during later development stages. If EWL control was related to thermoregulation, I would expect physiological control of EWL only during the later endothermic phases. The results of my study will help to determine the spread, avenue and role of insensible EWL control, to better understand this novel and important physiological trait. Considering the contribution of insensible EWL to the daily water budget and the role of water for heat balance and ultimately survival, studying how a regulatory process of EWL relates to different environmental conditions will give us a clearer picture of how endotherms currently function and, particularly, how they may function in a warmer and drier world.

• DFG Programme: WBP Fellowship
• International Connection: Australia
• Host: Dr. Christine Cooper