Journal of Herpetology, Vol. 44, No. 2, pp. 236–241, 2010 Copyright 2010 Society for the Study of Amphibians and

Refuge Use in a Patagonian Nocturnal , 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, 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 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 . 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 . 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 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. we used another experiment to test the role of 238 R. AGUILAR AND F. B. CRUZ temperature acclimation as a proxy of refuge RESULTS choice, on locomotor performance. We created We searched for geckos on both slopes of the artificial refuges in glass terraria. Each refuge hill; sampling effort measured in person hours consisted of small flat andesitic stones (approx- was similar for both slopes of the hill (approx- imately 10 3 10 cm). For each trial, we used 100- imately 24 h/person). The distribution of rocks W infrared bulbs suspended at different eleva- in the field was similar on both slopes, too. tions over the refuges to make possible each one Thus, the distribution of potential refuges of the following four temperatures: 18.5u, 22.5u, apparently did not affect our observations. 27.5u,or33uC. Temperature was maintained Homonota darwini were more abundant on the with a thermostat attached to the interior roof of west slope of the hill than on the east slope at the retreat. Lizards were acclimated in these site 1. We collected significantly more geckos on refuges for three consecutive days prior to the west side (87) than on the east side of the hill locomotor trials. Forty-six lizards collected on (4), after the same effort on each side (X2 5 the west slope of the hill were used in these 70.74; df 5 3; P , 0.001, pooled data from four experiments: 11 in the 18.5uC refuge (group A), sampling events). Both sides had andesitic 12 in the 22.5uC refuge (group B), 12 in the igneous rocks. No geckos were found in refuges 27.5uC refuge (group C), and 11 in the 33uC where ant colonies were present (N 5 12, six on refuge (group D). Lizards often refused to feed each slope of the hill). in captivity. Therefore, we did not feed any of We observed a positive relationship between the lizards and conducted the races after three the difference in temperatures of the exposed days to avoid weakness as a consequence of and underneath surfaces of the rocks and rock starvation. On the evening of the third day of thickness (r 5 0.38, P 5 0.006, N 5 54), acclimation, each lizard was sprinted three indicating that thicker rocks are poorer at times in a 2.0 3 0.08-m horizontal racetrack. transferring heat. On the west slope of the hill, The sides of the racetrack were marked every mean temperature of refuges actually used by 0.01 m to facilitate recording sprint rates and geckos was higher than the mean temperature total distances. Lizards were placed on one end of potential refuges (not used) by geckos from of the racetrack and prompted to run with a 0900–1730 h (21.4 6 8.3uC and 18.9 6 9.4uC, light tap on the base of their tails. Races were respectively; Mann-Whitney U 5 7,022; P 5 conducted at night to coincide with the natural 0.047; N 5 160). Mean temperature of potential activity period of these geckos (from 2200– refuges on the west side of the hill, from 0730– 0030 h). Lizard Tb during trials ranged from 2000 h, were significantly higher than those on 22.1–26.1uC(6 2uC of maximal speed perfor- the east side (15.7 6 0 7.5uC and 12.8 6 6.9uC, mance of 24uC; Ibargu¨ engoytı´a et al., 2007). respectively; Mann-Whitney rank test U 5 Lizard sprints were filmed with a digital camera 13,385.5; P 5 0.001, N 5 366). Differences (Panasonic SV200). Each time the gecko stopped between west and east sides were more pro- along the track, we recorded the distance run. nounced during evening hours (Fig. 1). Films were digitally captured with Windows The mean thickness of rocks actually used as Media Player V 11.0 (Microsoft, Redmond, refuges was 10.3 cm (6 5.1, N 5 46), the mean WA). To calculate sprint speed, a chronometer longest diameter was 38.3 cm (6 11.8, N 5 46), window was inserted using the program Pre- and the mean shortest diameter was 21.9 cm mier-pro (Adobe systems, San Jose, CA). (6 8.1, N 5 46). In the case of unused rocks We considered the maximum speed as the (potential refuges), mean thickness was 10.3 cm fastest sprint recorded from the three trials. (6 3.8, N 5 46), mean rock major diameter Maximum distance was the longest sprint 34.2 cm (6 9.3, N 5 46), and finally rock minor distance made by a gecko across the three trials. diameter 20.7 cm (6 8.2, N 5 46). Thus, unused We used the fastest run or longest distance run (potential) refuges were not significantly differ- as an estimate of maximal sprint capacity ent in size or thickness from actually used (Bonine and Garland, 1999; Vanhooydonck refuges (Mann-Whitney rank test P . 0.13 in all and Van Damme, 2001). Four lizards showed cases). evidence of submaximal performance (one at Mean diurnal body temperature (Tb)of the 18.5uC acclimation group and three from the geckos in the field was 21.5uC (range 8.6–40.2 33uC group), and, therefore, were not included S.D. 5 7.84, N 5 91) and increased from in analyses (Losos et al., 2002). morning to evening on the study site (r2 5 We tested for normality and homogeneity of 0.73; P , 0.001; N 5 91). Gecko body temper- variance prior to using data in ANOVA or t- ature also showed a positive relationship with tests. When these assumptions were not met, we refuge temperature (multiple regression F2,90 5 used Kruskal-Wallis or Mann-Whitney non- 423.6, P , 0.001). Both internal roof surface and parametric tests. Means are given 61 SD. substrate temperatures were good predictors of REFUGE USE AND SELECTION IN HOMONOTA DARWINI 239

FIG. 1. Hourly variation in refuge mean tempera- FIG. 2. Mean values (6SD) for maximum speed per tures (+SD, N 5 366) from western (white bars) and acclimation group (group 18 N 5 11; group 23 5 12; eastern (black bars) slope of the hill in the study site. group 27 5 12; group 33 5 11). Letters denote groups identified by Tukey post hoc test (alpha 0.05). 2 gecko Tb (multiple regression; r 5 0.91 and 0.84, respectively; N 5 91 and P , 0.001 for to the cooler eastern slope at our study site and both). used the warmer refuges among the available. Preferred mean body temperature obtained Thus, it may be possible that H. darwini uses during the photophase in the thermal gradient these refuges to thermoregulate during photo- was 27.4uC (range 21.6–40.5uC; SD 5 3.7, N 5 phase as has been observed for other geckos 12). At our study site, 54% of Tbs obtained in the (Kearney and Predavec, 2000; Kearney, 2001). field fell below and 16% above the thermal The positive relationship between Tb and interquartile range (25–75%, 23.5 6 1.1uC; 29.5 refuge temperature suggests that H. darwini is 6 3.8uC, respectively) for Tsel obtained in the thigmothermic as are other Neotropical geckos lab. Only 23% of field Tbs fell between upper (Marquet et al., 1990; Aun and Martori, 1994; and lower limits of the set point. Colli et al., 2003). However, body temperature For the trials of lizards acclimated to refuges during diurnal inactivity within refuges of H. under controlled temperatures, we detected darwini is lower than other Homonota species, significant differences among acclimation such as (27.15uC; Aun and groups in maximum speed (ANOVA: F3,39 5 Martori, 1994) and (23.9uC; 3.11, P 5 0.038). Lizards in the acclimated at Werner et al., 1996). These latter studies come 22.5uC running at 0.26 m/sec were the fastest; from warmer localities in northern Argentina, groups A and D were the slowest (Fig. 2). It is but even the same species may show differences interesting that sprint performance peaked with at different periods: H. darwini Tb in our study intermediate refuge temperatures. No signifi- was nearly 2uC lower than those obtained by cant differences were observed in the maximum Ibargu¨ engoytı´a et al. (2007), who included data distance run between acclimation groups (Krus- for a greater number of years (more important, kal-Wallis H 5 3.04, df 5 3, P 5 0.386). Neither they also included Tb data from warmer maximum sprint speed (P 5 0.897, N 5 40) nor months, November and December). This may the distance sprinted (P 5 0.971, N 5 40) were explain the difference between these studies. correlated with lizard body temperature at the In general terms, nocturnal lizards show a moment of running, probably because of the lower Tb than diurnal lizards when active (Vitt, narrow range of Tbs used in the trials (22.4– 1986; Autumn, 1999). Nevertheless, the lower Tb 26.1uC). of geckos must be adequate for capturing prey and avoiding predators, which may be possible because of the low cost of locomotion associated DISCUSSION with low temperatures (Autumn et al., 1994). Refuge temperature is an important aspect for This seems to be true for nocturnal lizards other ectotherms, also (Webb and Shine, 2000; Con- than geckos as well (Hare et al., 2007). Homonota verse and Savidge, 2003; Goldsbrough et al., darwini occurs in the harsh Patagonian climate, 2005). For example, spiders from eastern Aus- which may influence the observed low Tb tralia grow faster in warmer refuges (Golds- related to lowest set point for Tsel (more than brough et al., 2005). Homonota darwini used 50% of the Tb data fell below it). This was diurnal retreats almost exclusively on the observed in other nocturnal geckos (Angilletta warmer west slope of the hill (95.6%) compared and Werner, 1998). Indeed, Patagonian lizards, 240 R. AGUILAR AND F. B. CRUZ

in general, have lower Tbs than species of the LITERATURE CITED same from other localities (Ibargu¨ engoy- ABDALA, V. 1997. Los gecos de la Argentina. Serie tı´a, 2005). Monogra´fica y Dida´ctica, Facultad de Ciencias. The importance of diurnal refuge tempera- Naturales e Instituto Miguel Lillo. Universidad ture may stem from the necessity of reaching Nacional de Tucuma´n 29:1–38. DOLPH high enough Tb to complete digestive and A , S. C. 1990. Influence of behavioral thermo- physiological processes related to the previous regulation on microhabitat use by two Sceloporus night’s activity (Kearney and Predavec, 2000). lizards. Ecology 71:315–327. AGUILAR, R. 2006. Uso diferencial de refugios en Besides, temperature of diurnal refuge may Homonota darwini darwini (Gekkonidae). La impor- affect growth and food intake in Eublepharid tancia de un ambiente te´rmico adecuado determi- nocturnal geckos (Autumn and DeNardo, 1995). nado segu´n el rango de temperatura o´ptimo. Autumn et al. 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Biologı´adeuna highest temperature showed a performance poblacio´ndeHomonota horrida. Cuadernos de significantly lower than for lizards acclimated Herpetologı´a 8:90–96. at 23uC. Refuge temperatures above 32uC were AUTUMN, K. 1999. Secondarily diurnal geckos return to negatively correlated with performance. Refug- cost of locomotion typical of diurnal lizards. es with high temperatures may cause overheat- Physiological and Biochemical Zoology 72:339–351. ing (Arad, 1995; Pough et al., 1998), which may AUTUMN, K., AND D. F. DENARDO. 1995. Behavioral explain the low performance found in trials thermoregulation increases growth rate in a above 32uC in our study. For example, the nocturnal lizard. Journal of Herpetology 29: minimum panting temperature for this species 157–162. was 35.9uC (Aguilar, 2006). Geckos performed AUTUMN, K., R. B. WEINSTEIN, AND R. J. FULL. 1994. 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