Abstract
Ectotherms, especially terrestrial squamates, rely on suitable microclimatic conditions to maintain critical physiological processes. Selection of appropriate microhabitat determines how well squamates will function in fluctuating thermal environments. Understanding the use of microhabitat by cool-climate lizards, including the long-lived gecko, Woodworthia “Otago/Southland”, will give insight into the future implications of global climate change for New Zealand lizards. These viviparous, rock-dwelling geckos from southern New Zealand (N 45° S) live in an environment with an air temperature frequently lower than their preferred body temperature. They are nocturnally foraging, and females in a mid-elevation population at Macraes have the most extended gestation length found in reptiles. Factors determining microhabitat use, and how changes in current weather conditions might influence the use of microhabitat by Otago/Southland geckos remain unclear. I investigated microhabitat use by Otago/Southland geckos of different life-history groups at Macraes, including the influence of current weather conditions in different seasons, using a combination of laboratory- and field-based approaches.
Body temperatures (Tb) can be used to predict microhabitat use and activity time of squamates and require accurate measurements using a reliable device, especially in field situations. I tested the accuracy of an inexpensive, hand-held mini-infrared thermometer (mini-IRT) in measuring skin surface temperature of small lizards or their models. I assessed the effect of model width on the accuracy of the measurement and compared the measurement of skin surface temperatures using the mini-IRT with a thermal infrared camera and thermocouple thermometer. Mean discrepancy for skin surface temperatures collected with a mini-IRT was low for Otago/Southland geckos with a relatively wider abdominal width, but higher for the sympatric McCann’s skink (Oligosoma maccanni) with a narrower abdominal width. I concluded that the mini-IRT is acceptable for field studies on small lizards with an abdominal width ≥ 7 mm.
For rock-dwelling squamates, microhabitats may be abandoned on hot days if the Tb reaches a threshold that cannot be tolerated voluntarily. In my second study, I identified the upper voluntary thermal limit (VTmax) of Otago/Southland geckos, namely the body temperature (Tb) at which lizards are forced to abandon their daytime retreats when heated. Using a new laboratory protocol, I developed for determining VTmax in a retreat-dwelling species, I compared Tb at VTmax, and duration of heating, between two source groups with different thermal histories and among three reproductive groups. I also assessed total evaporative water loss (EWL) during heating for the wild geckos. I found VTmax and duration of heating varied between source groups (and thus potentials with prior thermal experience), but not among reproductive groups. Non-pregnant females lost more water than other reproductive groups. In the wild, pregnant female geckos with attached biologgers reached a Tb matching VTmax, and the temperatures of some separately monitored microhabitats exceeded VTmax in hot weather, implying that some retreats must be abandoned to avoid overheating. The results suggest that Otago/Southland geckos at Macraes may abandon retreats more frequently if climate warming persists, implying a trade-off between retention of the originally occupied shelter and ongoing water loss due to overheating.
The selection of appropriate retreats affects the microhabitat use of Otago/Southland geckos in their field environment. Using a thermal infrared camera and field survey, I found that geckos inhabit daytime retreats under rock slabs that are warmer, wider, thicker, closer to vegetation and with less depression on the lower substrate than slabs that are unoccupied. Also, 2-y thermal profiles of initially occupied retreats indicated that for thin slabs, the temperatures of the underside of retreat slab or Tretreat-top, and the surface of the rocky substrate beneath the slabs, Tretreat-bottom, exceeded VTmax on hot days in summer. In contrast, thick rock slabs and deep crevice temperatures were cooler and lower than the geckos’ preferred body temperature over time. Path analysis indicated that field Tb was positively correlated with maximum Trock-surface, maximum Tretreat-bottom, and air temperature. Also, daytime field Tb varied across seasons, life-history groups and with the time of capture. This study demonstrates the importance of microclimate conditions in influencing retreat-site choice.
As in other parts of the world, air temperature at Macraes is predicted to increase with global climate change. This increase may be beneficial to cool-climate lizards in creating a thermal environment that better supports physiological processes and activity. Using a combination of trail camera and nighttime field survey, I assessed the seasonal diurno-nocturnal activity (daytime basking and nighttime emergence) of the geckos, concurrently with field operative temperature (Te) exposed to the open and nighttime field Tb using a thermal infrared camera. The activity patterns showed that Otago/Southland geckos at Macraes are thermal generalists, active over a wide range of body temperatures, from a low of 1.4 °C at night, to 31.9°C by day. Air temperature positively influenced the Te available to emerged geckos, the geckos’ emergence activity by day and night, and their field Tb. However, higher wind speed reduced their diurnal and nocturnal activity. I identified that geckos that basked during the day in spring are exclusively pregnant females. My study also, for the first time, recorded evidence of basking and nocturnal emergence (at reduced levels) in winter. The results suggest that present weather conditions, the activity of Woodworthia “Otago/Southland” geckos was high. Also, Otago/Southland geckos are capable of activity at current low temperatures in winter, and sparsely use loose rock slabs in winter. These observations suggest that greater opportunities to bask in winter are likely to increase under future climate scenarios.
Collectively, my thesis results provide evidence of weather-dependent effects and life-history traits on microhabitat use by Otago/Southland geckos at Macraes. The findings suggest that future climate heating may provide some initial benefits to these geckos, but longer-term outcomes likely hinge on the magnitude of heating and the availability of suitable refugia below the voluntary thermal maximum.