Low Cost of Locomotion in Lizards That Are Active at Low Temperatures

Ecological responses of squamate reptiles to nocturnal warming

Nocturnal temperatures are increasing at a pace exceeding diurnal temperatures in most parts of the world. The role of warmer nocturnal temperatures in animal ecology has received scant attention and most studies focus on diurnal or daily descriptors of thermal environments' temporal trends. Yet, available evidence from plant and insect studies suggests that organisms can exhibit contrasting physiological responses to diurnal and nocturnal warming. Limiting studies to diurnal trends can thus result in incomplete and misleading interpretations of the ability of species to cope with global warming. Although they are expected to be impacted by warmer nocturnal temperatures, insufficient data are available regarding the night-time ecology of vertebrate ectotherms. Here, we illustrate the complex effects of nocturnal warming on squamate reptiles, a keystone group of vertebrate ectotherms. Our review includes discussion of diurnal and nocturnal ectotherms, but we mainly focus on diurnal species for which nocturnal warming affects a period dedicated to physiological recovery, and thus may perturb activity patterns and energy balance. We first summarise the physical consequences of nocturnal warming on habitats used by squamate reptiles. Second, we describe how such changes can alter the energy balance of diurnal species. We illustrate this with empirical data from the asp viper (Vipera aspis) and common wall lizard (Podarcis muralis), two diurnal species found throughout western Europe. Third, we make use of a mechanistic approach based on an energy-balance model to draw general conclusions about the effects of nocturnal temperatures. Fourth, we examine how warmer nights may affect squamates over their lifetime, with potential consequences on individual fitness and population dynamics. We review quantitative evidence for such lifetime effects using recent data derived from a range of studies on the European common lizard (Zootoca vivipara). Finally, we consider the broader eco-evolutionary ramifications of nocturnal warming and highlight several research questions that require future attention. Our work emphasises the importance of considering the joint influence of diurnal and nocturnal warming on the responses of vertebrate ectotherms to climate warming.

Extreme tolerance for nocturnal emergence at low body temperatures in a high-latitude lizard: implications for future climate warming

High-latitude lizards live in environments where ambient air temperature at night is frequently below retreat temperatures, which likely has implications for nocturnal emergence and activity. However, patterns of lizard activity at night under current temperate climates are poorly understood, a situation that limits our understanding of potential effects of climate change. We investigated patterns of nocturnal emergence and activity in the cold-adapted, viviparous gecko (Woodworthia 'Otago/Southland'). We measured operative environmental temperature (T _e) available to geckos that emerged at night and simultaneously assessed nighttime emergence activity using time-lapse trail cameras. Also, we assessed field body temperature (T _b) of emerged geckos of various life history groups at night using thermography to understand how current weather conditions affect field T _b of emerged geckos. Our results show that T_e , nocturnal emergence activity and field-active T _b increased with nighttime air temperature. Nocturnal emergence was highest in spring and summer but also occurred in autumn and (unexpectedly) in winter. Geckos were active over a broad range of T _b down to 1.4°C (a new record low for lizards) and on rock surfaces typically warmer than air temperature or T _b. We conclude that this nocturnal, high-latitude lizard from the temperate zone is capable of activity at low winter temperatures, but that current climate limits emergence and activity at least in autumn and winter. Activity levels for cool-temperate reptiles will probably increase initially as climates warm, but the consequences of increased nocturnal activity under climate change will probably depend on how climate change affects predator populations as well as the focal species' biology.

A nocturnally foraging gecko of the high-latitude alpine zone: Extreme tolerance of cold nights, with cryptic basking by day

Lizards that inhabit high-latitude alpine zones are exposed to extreme temperatures and long winters and most are diurnal heliotherms. Yet some poorly known nocturnal species exist in such locations, including several viviparous geckos from New Zealand. We studied the orange-spotted gecko (Mokopirirakau 'Roy's Peak'), a cryptic, nocturnal and viviparous lizard known only from the alpine zone (1150-1800 m a.s.l.) in the South Island (~44°S). Our field study investigated (1) the influence of female reproductive condition and sex on daytime body temperatures, including relationships with microhabitat rock temperatures, (2) the influence of temperature and other weather conditions on gecko emergence by night and day, and (3) the thermal microclimates available year-round to orange-spotted geckos. Building a better understanding of these lizards aids in species conservation efforts, for example in developing monitoring programmes, and provides insights into the evolution of thermal mechanisms in cold environments. Reproductive females maintained higher daytime body temperatures than non-reproductive females and males, suggesting pregnancy-related thermophily. On summer days, all reproductive groups reached similar body temperatures to New Zealand geckos from lower elevations, suggesting similar thermal preferences. Using trail cameras, we obtained evidence of geckos openly basking during the day (previously undocumented for this species) when temperatures of exposed lizard models (=T_exp) were 3.2-39.3 °C. We also observed emergence at night at low T_exp (-0.8-14.6 °C), when some T_bs were probably 0-6 °C. Diurnal activity increased as T_exp rose to peak at ~30 °C before dropping again at higher temperatures, whereas nocturnal activity unexpectedly decreased with increasing T_exp. Our study provides evidence of diurnal activity in a 'nocturnal' gecko that may be essential to squamate viviparity at high-latitude, high-elevation sites. It also suggests remarkable capacity for locomotor activity at extremely low T_b.

Shifts in space and time: ecological transitions affect the evolution of resting metabolic rates in microteiid lizards

Ecological diversification often encompasses exposure to new thermal regimes given by the use of specific spatial (microhabitat) and temporal (activity periods) niches. Empirical evidence provides links between temperature and physiology (e.g. rates of oxygen consumption), fostering predictions of evolutionary changes in metabolic rates coupled with ecological shifts. One example of such correspondence is the evolution of fossoriality and nocturnality in vertebrate ectotherms, where changes in metabolic rates coupled with niche transitions are expected. Because most studies address single transitions (fossoriality or nocturnality), metabolic changes associated with concomitant shifts in spatial and temporal components of habitat usage are underestimated, and it remains unclear which transition plays a major role for metabolic evolution. Integrating multiple ecological aspects that affect the evolution of thermosensitive traits is essential for a proper understanding of physiological correlates in niche transitions. Here, we provide the first phylogenetic multidimensional description of effects from ecological niche transitions both in space (origin of fossorial lineages) and in time (origin of nocturnal lineages) on the evolution of microteiid lizard (Gymnophthalmidae) metabolic rates. We found that evolution of resting metabolic rates was affected by both niche transitions, but with opposite trends. Evolution of fossoriality in endemic diurnal microteiids is coupled with a less thermally sensitive metabolism and higher metabolic rates. In contrast, a reduction in metabolic rates was detected in the endemic fossorial-nocturnal lineage, although metabolic thermal sensitivity remained as high as that observed in epigeal species, a pattern that likely reduces locomotion costs at lower temperatures and also favors thermoregulation in subsuperficial sand layers.

Locomotion, Energetics, Performance, and Behavior: A Mammalian Perspective on Lizards, and Vice Versa

SYNOPSIS

Animals are constrained by their abilities and by interactions with environmental factors, such as low ambient temperatures. These constraints range from physical impossibilities to energetic inefficiencies, and may entail trade-offs. Some of the constraints related to locomotion and activity metabolism can be illustrated through allometric comparisons of mammals and lizards, as representative terrestrial vertebrate endotherms and ectotherms, respectively, because these lineages differ greatly in aerobic metabolic capacities, resting energetic costs, and thermoregulatory patterns. Allometric comparisons are both useful and unavoidable, but "outlier" species (unusual for their clade) can also inform evolutionary scenarios, as they help indicate extremes of possible adaptation within mammalian and saurian levels of organization. We compared mammals and lizards for standard metabolic rate (SMR), maximal oxygen consumption during forced exercise (VO2max), net (incremental) cost of transport (NCT), maximal aerobic speed (MAS), daily movement distance (DMD), daily energy expenditure (DEE) during the active season, and the ecological cost of transport (ECT = percentage of DEE attributable to locomotion). (Snakes were excluded because their limbless locomotion has no counterpart in terrestrial mammals.) We only considered lizard SMR, VO2max, NCT, MAS, and sprint speed data if measured at 35-40 °C. On average, MAS is ∼7.4-fold higher in mammals, whereas SMR and VO2max are ∼6-fold greater, but values for all three of these traits overlap (or almost overlap) between mammals and lizards, a fact that has not previously been appreciated. Previous studies show that sprint speeds are similar for smaller mammals and lizards, but at larger sizes lizards are not as fast as some mammals. Mammals move ∼6-fold further each day than lizards, and DMD is by far the most variable trait considered here, but their NCT is similar. Mammals exceed lizards by ∼11.4-fold for DEE. On average for both lineages, the ECT is surprisingly low, somewhat higher for lizards, and positively allometric. If a lizard and mammal of 100 g body mass were both to move their entire DMD at their MAS, they could do so in ∼21 and 17 min, respectively, thus de-emphasizing the possible importance of time constraints. We conclude that ecological-energetic constraints related to locomotion are relatively more likely to occur in large, carnivorous lizards. Overall, our comparisons support the idea that the (gradual) evolution of mammalian endothermy did not necessarily require major changes in locomotor energetics, performance, or associated behaviors. Instead, we speculate that the evolution of thermoregulatory responses to low temperatures (e.g., shivering) may have been a key and "difficult" step in this transition.

Phylogenetic comparisons of pedestrian locomotion costs: confirmations and new insights

The energetic costs for animals to locomote on land influence many aspects of their ecology. Size accounts for much of the among-species variation in terrestrial transport costs, but species of similar body size can still exhibit severalfold differences in energy expenditure. We compiled measurements of the (mass-specific) minimum cost of pedestrian transport (COT_min, mL/kg/m) for 201 species - by far the largest sample to date - and used phylogenetically informed comparative analyses to investigate possible eco-evolutionary differences in COT_min between various groupings of those species. We investigated number of legs, ectothermy and endothermy, waddling, and nocturnality specifically in lizards. Thus, our study primarily revisited previous theories about variations in COT_min between species, testing them with much more robust analyses. Having accounted for mass, while residual COT_min did not differ between bipedal and other species, specifically waddling bipeds were found to have relatively high COT_min. Furthermore, nocturnal lizards have relatively low COT_min although temperature does not appear to affect COT_min in ectotherms. Previous studies examining across-species variation in COT_min from a biomechanical perspective show that the differences between waddling birds and nonwaddling species, and between nocturnal lizards and other ecotherms, are likely to be attributable to differences in ground reaction forces, posture, and effective limb length.

Unusual change in activity pattern at cool temperature in a reptile (Sphenodon punctatus)

Animals that can be active both during day and night offer unique opportunities to identify factors that influence activity pattern. By experimental manipulations of temperatures under constant photoperiod, we aimed to determine if emergence, activity and thermoregulatory behaviour of juvenile tuatara (Sphenodon punctatus) varied at different temperatures (20°C, 12°C and 5°C). To help clarify its activity pattern, we compared tuatara with two lizard species endemic of the South Island of New Zealand for which activity pattern is known and clearly defined: the nocturnal common gecko Woodworthia "Otago/Southland" and the diurnal McCann׳s skink Oligosoma maccanni. Tuatara showed similar responses to both species of lizards. Similar to the diurnal skinks, tuatara emerged quickly at 20°C and 12°C while nocturnal geckos took more time to emerge. Like nocturnal geckos, tuatara continued to be active at 5°C, but only during the day. Interestingly, tuatara shifted from diurno-nocturnal activity at 20°C and 12°C to being strictly diurnal at 5°C. We suggest that this temperature-dependent strategy maximises their survival during cold periods.

Low metabolic cost of locomotion in ornate box turtles, Terrapene ornata

Evolution has produced a wide range of body plans, but for a given body mass, the energetic cost of transport (COT) of terrestrial animals falls in a relatively narrow range. Previous research indicates that the COT depends on the proficiency of minimizing mechanical work performed, efficiency of performing that work, and cost of generating force to support weight. Turtles are unique in that their protective shell and shoulder-girdle articulation may eliminate the need for the ;muscular sling'. In addition, turtles have slower, more efficient muscles than other vertebrates. However, slow locomotion may raise the COT by confounding mechanical-energy conservation via the inverted-pendulum mechanism. Our goal was to determine the metabolic COT and efficiency of a terrestrial turtle species during locomotion. We studied 18 ornate box turtles, Terrapene ornata. Walking speed was extremely slow (0.07+/-0.005 m s(-1)). The average minimum COT was 8.0+/-0.70 J kg(-1) m(-1) attained at approximately 0.1 m s(-1). Ornate box turtles consume only half the energy predicted by the allometric relationship for all terrestrial animals (15.9+/-0.35 J kg(-1) m(-1)), and, thus, appear to be very economical walkers. When walking up a 24 deg. incline turtles moved significantly slower (0.04+/-0.004 m s(-1)), but performed the extra work required to walk uphill with very high efficiencies (>49%). It appears that the co-evolution of a protective shell, the associated shoulder morphology, and very slow, efficient muscles produce both economical level walking and efficient uphill walking.