Refuge Use in a Patagonian Nocturnal Lizard, Homonota Darwini: the Role of Temperature
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Journal of Herpetology, Vol. 44, No. 2, pp. 236–241, 2010 Copyright 2010 Society for the Study of Amphibians and Reptiles Refuge Use in a Patagonian Nocturnal Lizard, Homonota darwini: The Role of Temperature 1,2 1,3,4 ROCI´O AGUILAR AND FE´ LIX B. CRUZ 1Centro Regional Universitario Bariloche, Universidad Nacional del Comahue, Quintral 1250, 8400 Bariloche, Rı´o Negro, Argentina 3INIBIOMA (CONICET–UNComahue), Instituto Nacional de Investigaciones en Biodiversidad y Medioambiente, CRUB–Universidad Nacional del Comahue, 8400 Bariloche, Rı´o Negro, Argentina; E-mail: [email protected] ABSTRACT.—The thermal quality of diurnal refuges is important to the performance and survival of nocturnal reptiles. We studied refuge use on both slopes of an east–west-oriented hill by the thigmothermic gecko Homonota darwini, the southernmost-distributed nocturnal lizard in the world, in the vicinity of Bariloche, Rio Negro, in the Patagonia of Argentina. Because of the harsh climatic conditions in Patagonia, suitable refuges are limited, and retreat-site use is important for these geckos. Homonota darwini used refuges significantly more frequently on the warmer western slope in our study site. Geckos on the western slope used those refuges with higher temperatures regardless of size and thickness of rocks that acted as retreats. We tested whether refuge temperature affected locomotor performance of these lizards. Performance experiments showed that maximum sprint speed was affected by the temperature of the refuges. Refuges at 22.5uC allowed lizards to achieve their fastest sprint performance. Unexpectedly, sprint performance of lizards that used refuges with temperatures .32uC was the lowest among all tested refuge temperatures (18u, 22.5u, 27.5u, and 33uC). Our data illustrate the importance of the thermal quality of refuges for reptiles living in extreme environments. Nocturnal ectotherms depend on diurnal constitute the only microhabitats where noctur- refuges for shelter and as sites for thermoreg- nal lizards can achieve adequate body temper- ulation (Kerney and Predavec, 2000; Webb and atures for such processes. Shine, 2000). Such retreats may be key to We studied the nocturnal lizard Homonota survival. For example, in Australia, removal of darwini from Patagonian steppe habitat of diurnal shelters caused a decline of nocturnal southern Argentina (Cei, 1986; Abdala, 1997; reptiles (Webb and Shine, 2000). Although Piantoni et al., 2006), where the availability of physiological mechanisms can be important adequate thermal microhabitats is restricted. for some thermoregulating reptiles (Bartholo- Homonota darwini are small Patagonian geckos mew, 1982; Autumn and DeNardo, 1995; Dzia- (approximate mean adult snout–vent length lowski and O’Connor, 2001), behavior seems to 43 mm; mass 2–4 g) that use rocks as diurnal be the most common way that small reptiles refuges (Cruz et al., 2004; Ibargu¨ engoytı´a et al., cope with variable thermal environments (Hill, 2007). If these geckos use daytime refugia for 1980; Hertz et al., 1993; Huey et al., 2003). To thermoregulation, as other gecko species do, achieve body temperature, a direct or indirect then we predicted that they would use warmer source of heat must be available. Interestingly, over cooler available potential refuge locations. many nocturnal lizards thermoregulate during If so, this may be a benefit for digestion during daytime (Bustard, 1967; Autumn, 1999; Kear- daytime and may contribute to a better perfor- ney, 2001). Consequently, nocturnal lizards mance to acquire food during nighttime. Final- often have higher body temperatures during ly, we tested nighttime locomotor ability capa- the day than during the night (Gil et al., 1994; bilities after establishing refuges with different Autumn and DeNardo, 1995; Autumn, 1999; thermal characteristics during daytime, as a Cruz et al., 2004). The availability of thermally proxy of the effect of temperature of refuges suitable microhabitats allows nocturnal reptiles used during daytime. to elevate performance and metabolic processes and, thus, is critical for lizards to maintain their MATERIALS AND METHODS physiology within their acceptable range (Adolph, 1990). Therefore, diurnal refuges may Fieldwork was conducted at El Co´ndor ranch in the vicinity of San Carlos de Bariloche, Rı´o Negro province, Argentina (41u069S, 71u089W; 2 E-mail: [email protected] 900 m). The habitat has rocky promontories 4 Corresponding Author. surrounded by rubble of different sizes on REFUGE USE AND SELECTION IN HOMONOTA DARWINI 237 sandy soils. The vegetation is typical Patagonian At site 2, we collected eighty-four lizards, shrub steppe (Correa, 1998). Field measure- later transferred to the lab for experimental ments were recorded February through March trials in cloth bags on the same day of capture. 2004 and 2005, during which period geckos are Only adults with intact tails were used for these not reproductively active (Ibargu¨ engoytı´a and purposes; sex was not determined because no Casalins, 2007). evident sexual dimorphism is present in these Our study is composed of two parts: first, lizards. To avoid using the same individuals in determining refuge use and body temperatures different experiments (Tsel trials, nonacclima- in the field; and second, sprint performance tion performance trials and acclimation trials), experiments and thermal preferenda in the lab. geckos were claw clipped when released at the Lizards used for the field and lab components original point of capture. were captured at two different rock promonto- We measured preferred body temperature ries 150 m apart from each other. (Tsel) of 12 adult individuals in two thermal At site 1, we collected refuge measurements, gradients that consisted of six lanes each (lanes presence or absence of geckos under rocks, and were 1.20 3 0.12 m). Each track was separated field temperatures of lizards and refuges. We from the next by a 0.4-m high opaque wall. For sampled both sides of a hill with slopes oriented the hot side, we used a 1,000-W infrared stove east–west along this study area. The sampling with a potentiometer that was suspended 0.50 m effort was the same for both slopes, allowing us above one end of the gradient. The cool side to compare the presence of geckos on the was maintained at approximately 18uC. The sunnier and, thus, warmer west face of the hill thermal gradient provided a temperature range to the colder east face. We obtained field body from 18–44uC. Small flat rocks (,0.07 3 0.07 3 temperatures (Tb) of geckos encountered in 0.02 m) were placed every 0.15 m and separated their diurnal refuges (1,000–1,700), by introduc- from one another by 0.05 m along each track of ing a type K (60.1uC) thermocouple (Extech the gradient (Cruz et al., 2009). We recorded 421502, Waltham, MA) into the cloacae within geckos Tsel by inserting an ultra-fine thermo- 20 sec of capture (Schwarzkopf and Shine, couple into the cloacae and grabbing the 1991). At the moment of capture, soil (inserting specimens from their shoulders to avoid heat thermocouple tip 0.01 m into the substratum) transfer when handling; then lizards were and internal roof (contacting thermocouple tip released in the middle section of the track (> to the rock surface) temperatures were recorded 0.60 m from each extreme) to allow them to as well. We compared temperatures and mea- choose their retreats. Data were acquired every surements (length, width, and thickness) of hour during photophase (0900–1900) for two actual used rocks by geckos and potential rocks consecutive days (18 Tb data from each one of (not used or random rock) during the first two the 12 geckos). Thermal gradient temperatures, of the four samplings (because the area studied taken every 15 cm along the lanes, fitted was the same across the entire study, and we significantly on a linear function (r2 5 0.61, P marked the used rocks). Immediately after 5 0.02). Because these trials took place over a collecting a lizard in a refuge, we recorded the short period (two days), lizards were not fed data, and then we collected the same data from during these experiments. Water was sprayed a nearby randomly selected rock that was two times a day across the tracks. Set-point chosen among five similar rocks at >1–2 m temperatures were estimated from Tsel. For each distance. The random choice was conducted by lizard the bounding interquartile range (middle numbering the five nearby rocks, chosen from 50% of observations) was used to represent the among those with similar characteristics of the upper and lower limits of set point temperature used rock (we excluded those rocks that were range (Hertz et al., 1993). too small or to deeply imbedded in ground) and Because one of our interests was to test then rolling a die to choose the random rock whether locomotor performance was better for (Goldsbrough et al., 2005). specimens collected on the west slope of the hill, We also compared temperatures between we measured speed performance for four potential refuges on each side of the hill (west geckos captured on the eastern slope of the hill and east). Outdoor/industrial HOBO (Pocasset, and for 22 individuals collected on the western MA) data logger probes (60.1uC) were attached slope. This was set after transporting the lizards to the internal surface of six potential refuges immediately to the lab; trials were carried out (three on the west and three on the east slope of on the same night of capture (i.e., no acclima- the hill). Thus, we acquired temperatures every tion). hour for 10 days from 6 to 15 March 2005. From Because of few captures of lizards on the these data, we analyzed the daytime hours eastern slope, we were not able to compare how (0800–2000) when geckos were confirmed to be lizards perform in the field between sites; thus, in their refuges.