Life in the suburbs: artificial heat source selection for nocturnal thermoregulation in a diurnally active tropical lizard

The rapid expansion of urban environments invariably presents a novel series of pressures on wildlife due to changes in external environmental factors. In reptiles, any such changes in temperature are critical since thermoregulation is the key driver in the function of many physiological processes. How reptiles adapt to such changes may vary from those species that are impacted negatively to others that have the behavioural flexibility to exploit new conditions. In this paper we describe retreat site selection, movements and aspects of the thermal ecology of the African lizard Agama agama in urban environments of West Africa. In early evening lizards began movement from late-afternoon core activity areas and ascended the walls of houses for overnight retreats. A high proportion retreated to locations in groups under or on top of warm electrical panels. The thermal potential these panels offered was the attainment of body temperatures equal to or higher than the minimum preferred body temperature (PBT ≈ 36 ∘C in A. agama) and hence increased physiological performance. The lizards that took advantage of the heat sources travelled further each day to and from diurnal activity areas than individuals that spent the night high on walls but not next to heat panels. There were both potential costs (enhanced predation pressures) and benefits (impacts on thermal ecology, retreat site selection) of this behaviour for lizards living in urban environments.

Hydrogen sulfide exposure reduces thermal set point in zebrafish

Behavioural flexibility allows ectotherms to exploit the environment to govern their metabolic physiology, including in response to environmental stress. Hydrogen sulfide (H 2 S) is a widespread environmental toxin that can lethally inhibit metabolism. However, H 2 S can also alter behaviour and physiology, including a hypothesized induction of hibernation-like states characterized by downward shifts of the innate thermal set point (anapyrexia). Support for this hypothesis has proved controversial because it is difficult to isolate active and passive components of thermoregulation, especially in animals with high resting metabolic heat production. Here, we directly test this hypothesis by leveraging the natural behavioural thermoregulatory drive of fish to move between environments of different temperatures in accordance with their current physiological state and thermal preference. We observed a decrease in adult zebrafish ( Danio rerio ) preferred body temperature with exposure to 0.02% H 2 S, which we interpret as a shift in the thermal set point. Individuals exhibited consistent differences in shuttling behaviour and preferred temperatures, which were reduced by a constant temperature magnitude during H 2 S exposure. Seeking lower temperatures alleviated H 2 S-induced metabolic stress, as measured by reduced rates of aquatic surface respiration. Our findings highlight the interactions between individual variation and sublethal impacts of environmental toxins on behaviour.

Body size of ectotherms constrains thermal requirements for reproductive activity in seasonal environments

Body size may influence ectotherm behaviour by influencing heating and cooling rates, thereby constraining the time of day that some individuals can be active. The time of day at which turtles nest, for instance, is hypothesized to vary with body size at both inter- and intra-specific levels because large individuals have greater thermal inertia, retaining preferred body temperatures for a longer period of time. We use decades of data on thousands of individual nests from Algonquin Park, Ontario, Canada, to explore how body size is associated with nesting behaviour in Painted Turtles (Chrysemys picta (Schneider, 1783); small bodied) and Snapping Turtles (Chelydra serpentina (Linnaeus, 1758); large bodied). We found that (i) between species, Painted Turtles nest earlier in the evening and at higher mean temperatures than Snapping Turtles, and (ii) within species, relatively large individuals of both species nest at cooler temperatures and that relatively larger Painted Turtles nest later in the evening compared with smaller Painted Turtles. Our data support the thermal inertia hypothesis and may help explain why turtles in general exhibit geographic clines in body size: northern environments experience more daily variation in temperature, and larger size may evolve, in part, for retention of preferred body temperature during terrestrial forays.

How The Mammalian Sleep Was Born

The Nocturnal Bottleneck explains how mammals evolved from their reptilian ancestors after inverting the chronotype, form diurnal to nocturnal. Pre-mammals traded-off the excellent visual system of their ancestors for improvements in audition and in olfactory telencephalon, needed for efficient orientation in the dark. This was how the mammalian nocturnal telencephalic wakefulness was born. However, the modified visual system of those pre-mammals became sensitive to the dangerous diurnal light and the exposure would involve a high risk of blindness and death. Therefore, pre-mammals had to remain immobile with closed eyes hidden in lightproof burrows during light time. This was the birth of the mammalian sleep. Typical reptiles distribute their wake time cycling between Basking Behavior, to attain the preferred body temperature, and poikilothermic Goal Directed Behavior, to perform life sustaining tasks. These cycles persisted during the new mammalian sleep. However, as the behavioral output had to be blocked during light time, the paralyzed reptilian Basking Behavior and Goal Directed Behavior cycles became the NREM and REM cycles, respectively. This was how NREM and REM cycles remained incorporated within the mammalian sleep. After the Cretaceous-Paleogene extinction, the environmental pressure for nocturnal life was softened, allowing high variability in chronotype and sleeping patterns. This permitted some mammalian groups, e.g., primates, to begin the quest for diurnal wake.Concluding, sleep constituted an additional bottleneck in the mammalian evolution. The reduced population of pre-mammals that was able to develop sleep during light time, including NREM and REM, became full mammals and survived; the remainder perished.

Hydrogen sulfide exposure reduces thermal set point in zebrafish

Behavioural flexibility allows ectotherms to exploit the environment to govern their metabolic physiology, including in response to environmental stress. Hydrogen sulfide (H2S) is a widespread environmental toxin that can lethally inhibit metabolism. However, H2S can also alter behaviour and physiology, including a hypothesised induction of hibernation-like states characterised by downward shifts of the innate thermal setpoint (anapyrexia). Support for this hypothesis has proved controversial because it is difficult to isolate active and passive components of thermoregulation, especially in animals with high resting metabolic heat production. Here, we directly test this hypothesis by leveraging the natural behavioural thermoregulatory drive of fish to move between environments of different temperatures in accordance with their current physiological state and thermal preference. We observed a decrease in adult zebrafish (Danio rerio) preferred body temperature with exposure to 0.02% H2S, which we interpret as a shift in thermal setpoint. Individuals exhibited consistent differences in shuttling behaviour and preferred temperatures, which were reduced by a constant temperature magnitude during H2S exposure. Seeking lower temperatures alleviated H2S-induced metabolic stress, as measured by reduced rates of aquatic surface respiration rate. Our findings highlight the interactions between individual variation and sublethal impacts of environmental toxins on behaviour.

Overcoming low environmental temperatures in the primary feeding season: low-level activity and long basking in the tortoise Homopus signatus

Tortoises that live in regions where food plants grow in winter may have to cope with relatively low environmental temperatures to obtain resources. The speckled tortoise, Homopus signatus, inhabits an arid winter rainfall range where it is active in winter and spring at environmental temperatures well below its preferred body temperature. Although H. signatus is a threatened species, we have no information how it deals with low environmental temperatures. Therefore, we made continuous recordings of behaviour in nine female H. signatus on 29 days in the early spring. The group of females as a whole showed activity (i.e., behaviours other than hiding) throughout the day in a unimodal pattern. However, individual tortoises were active only for approximately 4.5 h per day and spent as much as 73% of their active time basking, mostly under the protective cover of shrubs. In addition, a negative relationship between the percentage of active time spent in sun and environmental temperature indicated that H. signatus used active behaviours other than basking to absorb heat, particularly on cold days. Tortoises completed all active behaviours other than basking in 1.2 h per day, including a mere 24 min of feeding, probably facilitated by the abundant availability of food plants in the early spring. We predict that a reduced availability of food plants for H. signatus might lead to increased active time and possibly increased predation pressure, or to a decreased proportion of active time spent basking and reduced body temperatures.

Thermoregulation during the summer season in the Goode’s horned lizard Phrynosoma goodei (Iguania: Phrynosomatidae) in Sonoran Desert

Reptiles in desert environments depend on habitat thermal quality to regulate their body temperature and perform biological activities. Understanding thermoregulation with respect to habitat thermal quality is critical for accurate predictions of species responses to climate change. We evaluated thermoregulation in Goode’s horned lizard, Phrynosoma goodei, and measured habitat thermal quality at the Reserva de la Biosfera El Pinacate y Gran Desierto de Altar, Sonora, Mexico, during the hottest season of the year. We found that field-active body temperature averaged 38.1 ± 0.38°C, preferred body temperature in laboratory averaged 34.9 ± 0.18°C and preferred body temperature range was 32.5-37.3°C. Operative temperature (i.e. environmental temperature available to the lizards) averaged 43.0 ± 0.07°C, with maximum temperature being near 70°C, and 62.9% of operative temperatures were above preferred body temperature range of P. goodei. Microhabitat thermal quality occupied by the lizards was high in the morning (7:00-10:30) and afternoon (5:50-dusk). We found that despite strong thermal constraints P. goodei was highly accurate and efficient in regulating its body temperature and that it presented a bimodal thermoregulatory pattern, being active in the mornings and in the evenings in order to avoid high mid-day environmental temperatures. Despite its thermoregulatory ability, P. goodei may be vulnerable to climate warming.

Field body temperature and thermal preference of the big-headed turtle Platysternon megacephalum

The big-headed turtle Platysternon megacephalum is a stream-dwelling species whose ecology is poorly known. We carried out field and laboratory investigations to determine field body temperatures and thermal preference of this species. In the field, the body temperatures of the turtles conformed to the water temperature, with little diel variation in either summer or autumn. Over the diel cycle, the mean body temperatures ranged from 20.8°C to 22.2°C in summer and from 19.3°C to 21.2°C in autumn; the highest body temperatures ranged from 22.1°C to 25.0°C in summer and from 20.6°C to 23.8°C in autumn. In the laboratory, the preferred body temperature (Tp) was 25.3°C. Food intake was maximized at 24.0°C, whereas locomotor performance peaked at 30.0°C. Consequently, Tp was closer to the thermal optimum for food intake than for locomotion. Therefore, this freshwater turtle has relative low field body temperatures corresponding to its thermal environment. In addition, the turtle prefers low temperatures and has a low optimal temperature for food intake.