DOI: 10.5061/dryad.crjdfn39p
title: Keeping cool with poop: Urohidrosis lowers leg surface temperature by up to 6ºC in breeding White storks

description: Storks (Ciconiidae) are renowned for defecating on their legs when exposed
 to high temperatures, a phenomenon known as ‘urohidrosis’. Previous work
 suggested that this behaviour can cool down the blood supply to the legs
 and thus prevent hyperthermia in captive storks when overheated. However,
 no study has quantified the magnitude or duration of its evaporative
 cooling effect in free-ranging birds. Here, we combine urohidrosis data
 with thermal imaging and microclimate data to investigate the
 thermoregulatory significance of urohidrosis in White storks Ciconia
 ciconia during the breeding season. We show that urohidrosis can reduce
 leg surface temperature by up to 6.7 ºC (4.40 ± 1.04 ºC). Yet its cooling
 effect was of short duration (lasting no more than 2.5 min) and decreased
 with time since defecation. Thus, for urohidrosis to significantly
 contribute to heat dissipation, storks must perform it repeatedly when
 overheated. Indeed, individuals can perform up to 11 urohidrosis events
 per hour; our estimates indicate that repeated urohidrosis could represent
 a significant amount of heat loss during short-time spans — equivalent to
 4% of daily field metabolic rate for an adult stork. Our results points to
 an absence of differences in the cooling efficiency of urohidrosis between
 adults and nestlings, probably because all nestlings were recorded during
 the last phase of the ontogeny of thermoregulation. Besides, during the
 hottest days adult storks delivered water to their nestlings, which might
 allow them to replenish body water reserves to sustain urohidrosis and
 other heat dissipation behaviours such as panting or gular fluttering. Our
 results indicate that urohidrosis might buffer the impact of high
 temperatures in breeding storks, preventing overheating and lethal
 hyperthermia. Gaining knowledge about behavioural thermoregulation in the
 heat is therefore crucial to better predict the future persistence and
 vulnerability of species under different climate warming scenarios.
description: During the spring of 2021
 (from late March to mid-June), we conducted a study on the
 thermoregulation and heat exchange of White storks
 <em>Ciconia</em> <em>ciconia</em> at a nesting
 colony in Extremadura, southwestern Spain (38º52’ N; 6º19’W). This colony
 hosted 12 active nests that were visited twice a week to obtain thermal
 images to study the potential role of different body parts (particularly
 beaks and legs) as thermal windows along a range of different
 environmental temperatures. We collected thermal images of each nest
 throughout the study period, from early incubation to the end of the
 breeding season. Thermal imaging sessions were performed from 1 h
 after sunrise until maximum ambient daily temperature was reached (which
 was confirmed by tracking temperature with a portable weather station; see
 below), measuring no more than 2 nests per day (switching the camera
 between focal nests during thermal imaging sessions). We employed a
 handheld thermal infrared camera (FLIR E95, resolution = 464 x 348 pixels,
 FLIR Systems, USA), equipped with a 14º lens. We automatically took a
 thermal image every 20 seconds and simultaneously recorded local weather
 data – environmental temperature, Ta (Ta_Kestrel, ºC); wind speed, (m
 s-1); and relative humidity (%) – every 30 s using a portable weather
 station (Kestrel 5400, Kestrel Instruments, USA) mounted on a vane and
 placed on the colony (at 1.5 m height). 
 Although thisurohidrosis
 behaviour appeared to be frequent in White storks during late spring, we
 only could obtain thermal images of 22 urohidrosis events (four in adults
 and 18 in nestlings), from five different nests and seven individuals
 (three adults and four nestlings) during the 31th of May, and the 7th, 9th
 and 10th of June. A total of 173 thermal images of individuals showing
 urohidrosis were taken, but we discarded those images in which legs were
 not completely visible or those in which storks’ position could compromise
 accuracy of thermal measures, as incidence angle influences apparent
 emissivity – with increased angle resulting on declines in apparent
 emissivity (Playà-Montmany & Tattersall 2021). Then, we selected
 for the analyses those pictures in which a lateral (or almost lateral)
 view of the legs could be taken. Finally, 81 images from these 22
 urohidrosis events met this criterion and were analyzed using the program
 FLIR ResearchIR (FLIR Systems, USA). For each image, we assumed an
 emissivity of 0.95 and set ambient temperature and relative humidity from
 values obtained from the weather station. Average surface temperature of
 the leg area covered by excreta was measured using the ‘freehand roi’
 function, while average surface temperature from the adjacent region free
 of excreta was measured by using the ‘line roi’ function. We employed the
 ‘freehand roi’ function to measure urohidrosis due to the irregular shape
 of excreta marks along the leg. Overall, temperature measures were
 calculated on areas that had, on average, 20.73 ± 7.97 pixels. We
 calculated <em>urohidrosis</em> gradient as the difference
 between the surface temperature of the bare skin leg region and that of
 the region covered by excreta. When possible, we repeatedly calculated
 urohidrosis gradient for the same event to estimate its duration and
 magnitude over time.