Predation risk is a function of alternative prey availability rather than predator abundance in a tropical savanna woodland ecosystem

Typically, factors influencing predation risk are viewed only from the perspective of predators or prey populations but few studies have examined predation risk in the context of a food web. We tested two competing hypotheses regarding predation: (1) predation risk is dependent on predator density; and (2) predation risk is dependent on the availability of alternative prey sources. We use an empirical, multi-level, tropical food web (birds–lizards–invertebrates) and a mensurative experiment (seasonal fluctuations in abundance and artificial lizards to estimate predation risk) to test these hypotheses. Birds were responsible for the majority of attacks on artificial lizards and were more abundant in the wet season. Artificial lizards were attacked more frequently in the dry than the wet season despite a greater abundance of birds in the wet season. Lizard and invertebrate (alternative prey) abundances showed opposing trends; lizards were more abundant in the dry while invertebrates were more abundant in the wet season. Predatory birds attacked fewer lizards when invertebrate prey abundance was highest, and switched to lizard prey when invertebrate abundance reduced, and lizard abundance was greatest. Our study suggests predation risk is not predator density-dependent, but rather dependent on the abundance of invertebrate prey, supporting the alternative prey hypothesis

Afraid of the Dark? The Influence of Natural and Artificial Light at Night on the Behavioral Activity of a Nocturnal Gecko

Both natural and artificial light at night can strongly influence animal behavior. Nocturnal animals often alter activity dependent on lunar light levels, to increase prey capture, minimize detection by predators, or both. Trade-offs among these ecological effects are likely to have a strong influence on behavior and fitness. Here, we examined the influence of light at night on nocturnal geckos that are both predators and prey, and use both natural and anthropogenic habitats. We tested the influence of illumination on the relative abundance and behavioral activity of native geckos in natural woodlands and under laboratory conditions. We hypothesized that Australian native house geckos (Gehyra dubia) would avoid activity on nights with high moon brightness, to minimize exposure to predators, consistent with the predation risk hypothesis. Counter to our prediction, we found a positive relationship between house gecko activity and moon brightness, i.e., house geckos were more active on bright nights. This behavior may allow house geckos to better see their prey while also increasing the visibility of approaching predators. In the laboratory, house geckos had shorter latency times to emerge from a shelter under low light conditions compared to darkness equivalent to a new moon, a trend consistent with higher activity under brighter conditions in the field. Light at night, from both natural and artificial sources, clearly influences the behavior and activity of geckos, but perhaps not in the ways we expect. Reducing the risk of attack from predators in darkness, and increasing prey capture success using vision, may increase the benefits of activity in lit conditions, compared to total darkness

Seasonal, environmental and anthropogenic influences on nocturnal basking in turtles and crocodiles from North-Eastern Australia

Many ectotherms bask in the sun as a behavioural mechanism to increase body temperature and facilitate metabolism, digestion or gamete production, among other functions. Such behaviours are common during the day, but some nocturnal species are also known to thermoregulate at night, in the absence of solar radiation, through shifts in body posture or microhabitat selection. Additionally, recent work has documented nocturnal basking in freshwater turtles in tropical Australia, though the purpose of the behaviour remains unknown. Here, we have built upon that work to test: 1. seasonal differences, 2. the influence of environmental factors and 3. the influence of anthropogenic development (e.g. river-front houses) on nocturnal basking behaviour. We visually surveyed transects repeatedly at night on the Ross River, Townsville, QLD, Australia from March to November 2020 and documented nocturnal basking in both freshwater turtles (Emydura macquarii krefftii) and freshwater crocodiles (Crocodylus johnstoni). For both taxa, we found significantly more nocturnal basking activity during the hotter months. Likewise, water surface temperature significantly influenced nocturnal basking in both taxa, especially when water temperatures were both high and warmer than air temperatures. We propose that nocturnal basking provides a mechanism for thermoregulatory cooling when water temperatures are high (e.g. 30°C) and above-preferred temperatures. After accounting for availability in basking habitat, both turtles and crocodiles basked more frequently on the undeveloped side of the river, suggesting avoidance of human activity or disturbance. This study is the first to document nocturnal basking activity temporally throughout the year as well as the first to identify the influences of environmental factors. Nocturnal thermoregulation has been documented in many reptiles, however, thermoregulatory cooling in tropical systems is less well-known

Ecological niche and microhabitat use of Australian geckos

Modern biological research often uses global datasets to answer broad-scale questions using various modelling techniques. But detailed information on species–habitat interactions are often only available for a few species. Australian geckos, a species-rich group of small nocturnal predators, are particularly data-deficient. For most species, information is available only as scattered, anecdotal, or descriptive entries in the taxonomic literature or in field guides. We surveyed gecko communities from 10 sites, and 15 locations across central and northern Queensland, Australia, to quantify ecological niche and habitat use of these communities. Our surveys included deserts, woodlands, and rainforests, examining 34 gecko species. We assigned species to habitat niche categories: arboreal (9 species), saxicoline (4), or terrestrial (13), if at least 75% of our observations fell in one microhabitat; otherwise we classified geckos as generalists (8). For arboreal species, we described perch height and perch diameter and assigned them to ecomorph categories, originally developed for Anolis lizards. There was lower species richness in rainforests than in habitats with lower relative humidity; the highest species richness occurred in woodlands. Most arboreal and generalist species fit the trunk-ground ecomorph, except those in the genus Strophurus, whose members preferred shrubs, twigs of small trees, or, in two cases, spinifex grass hummocks, thus occupying a perch space similar to that of grass-bush anoles. Habitat use by Pseudothecadactylus australis, Saltuarius cornutus, and Gehyra dubia fit the trunk-crown ecomorph. We provide quantified basic ecological data and habitat use for a large group of previously poorly documented species

Nonlinear variation in clinging performance with surface roughness in geckos

Understanding the challenges faced by organisms moving within their environment is essential to comprehending the evolution of locomotor morphology and habitat use. Geckos have developed adhesive toe pads that enable exploitation of a wide range of microhabitats. These toe pads, and their adhesive mechanisms, have typically been studied using a range of artificial substrates, usually significantly smoother than those available in nature. Although these studies have been fundamental in understanding the mechanisms of attachment in geckos, it is unclear whether gecko attachment simply gradually declines with increased roughness as some researchers have suggested, or whether the interaction between the gekkotan adhesive system and surface roughness produces nonlinear relationships. To understand ecological challenges faced in their natural habitats, it is essential to use test surfaces that are more like surfaces used by geckos in nature. We tested gecko shear force (i.e., frictional force) generation as a measure of clinging performance on three artificial substrates. We selected substrates that exhibit microtopographies with peak-to-valley heights similar to those of substrates used in nature, to investigate performance on a range of smooth surfaces (glass), and fine-grained (fine sandpaper) to rough (coarse sandpaper). We found that shear force did not decline monotonically with roughness, but varied nonlinearly among substrates. Clinging performance was greater on glass and coarse sandpaper than on fine sandpaper, and clinging performance was not significantly different between glass and coarse sandpaper. Our results demonstrate that performance on different substrates varies, probably depending on the underlying mechanisms of the adhesive apparatus in geckos

Geckos cling best to, and prefer to use, rough surfaces

Background: Fitness is strongly related to locomotor performance, which can determine success in foraging, mating, and other critical activities. Locomotor performance on different substrates is likely to require different abilities, so we expect alignment between species' locomotor performance and the habitats they use in nature. In addition, we expect behaviour to enhance performance, such that animals will use substrates on which they perform well. Methods: We examined the associations between habitat selection and performance in three species of Oedura geckos, including two specialists, (one arboreal, and one saxicolous), and one generalist species, which used both rocks and trees. First, we described their microhabitat use in nature (tree and rock type) for these species, examined the surface roughnesses they encountered, and selected materials with comparable surface microtopographies (roughness measured as peak-to-valley heights) to use as substrates in lab experiments quantifying behavioural substrate preferences and clinging performance. Results The three Oedura species occupied different ecological niches and used different microhabitats in nature, and the two specialist species used a narrower range of surface roughnesses compared to the generalist. In the lab, Oedurageckos preferred substrates (coarse sandpaper) with roughness characteristics similar to substrates they use in nature. Further, all three species exhibited greater clinging performance on preferred (coarse sandpaper) substrates, although the generalist used fine substrates in nature and had good performance capabilities on fine substrates as well. Conclusion: We found a relationship between habitat use and performance, such that geckos selected microhabitats on which their performance was high. In addition, our findings highlight the extensive variation in surface roughnesses that occur in nature, both among and within microhabitats

Too cold is better than too hot: Preferred temperatures and basking behaviour in a tropical freshwater turtle

Thermoregulation is critical to the survival of animals. Tropical environments can be particularly thermally challenging as they reach very high, even lethal, temperatures. The thermoregulatory responses of tropical freshwater turtles to these challenges are poorly known. One common thermoregulatory behaviour is diurnal basking, which, for many species, facilitates heat gain. Recently, however, a north-eastern Australian population of Krefft's river turtles (Emydura macquarii krefftii) has been observed basking nocturnally, possibly to allow cooling. To test this, we determined the thermal preference (central 50% of temperatures selected) of E. m. krefftii in an aquatic thermal gradient in the laboratory. We then conducted a manipulative experiment to test the effects of water temperatures, both lower and higher than preferred temperature, on diurnal and nocturnal basking. The preferred temperature range fell between 25.3°C (±SD: 1.5) and 27.6°C (±1.4) during the day, and 25.3°C (±2.4) and 26.8°C (±2.5) at night. Based on this, we exposed turtles to three 24 h water temperature treatments (‘cool’ [23°C], ‘preferred’ [26°C] and ‘warm’ [29°C]) while air temperature remained constant at 26°C. Turtles basked more frequently and for longer periods during both the day and night when water temperatures were above their preferred range (the ‘warm’ treatment). This population frequently encounters aquatic temperatures above the preferred thermal range, and our results support the hypothesis that nocturnal basking is a mechanism for escaping unfavourably warm water. Targeted field studies would be a valuable next step in understanding the seasonal scope of this behaviour in a natural environment

Simple fence modification increases land movement prospects for freshwater turtles on floodplains

Installing conservation fences to prohibit feral animal access to wetlands can become a barrier for non-target species of interest. We collected 161 turtles (Chelodina rugosa, Emydura subglobosa worrelli, Myuchelys latisternum) from twenty floodplain and riverine wetlands during post-wet (June–August) and late-dry season (November–December) surveys (2015–2018) in northern Australia. Wetlands were fenced (150 × 150 mm square, 1.05 m high wire mesh) or unfenced around the wet perimeter. Ninety-seven percent of individuals caught in either fenced or unfenced wetlands had a shell carapace width greater than mesh width, of these 44 (46%) were captured inside fenced wetlands, while 50 were caught in unfenced wetlands. The remaining 35 turtles were smaller than 150 mm and would likely pass easily through fence mesh. Sixty-five turtles partook in a fencing manipulative experiment. Turtles with carapace widths wider than mesh often successfully escaped through fences by lifting one side of their shell and passing diagonally through the mesh. In a second experiment where a piece of vertical wire (1500 × 300 mm) was removed, turtles located ‘gates' after prospecting and fitting through meshing areas that were too small to pass. Ninety-two percent of turtles were able to locate and pass through gates, while 8% failed to locate a gate after 2 h. Gates applied every 4 m showed an 83% passage rate, every 2 m was 91%, and every 1 m was 100%. Combing field and manipulative experiments revealed that large turtles will prospect and move along a fence until they find suitable passage, which has important consequences when considering that gates could be easily retrofitted to existing sites, as well in new fencing programs, which has enormous positive conservation benefits for turtles in an already challenging and changing floodplain environment

Predation risk is a function of alternative prey availability rather than predator abundance in a tropical savanna woodland ecosystem

Typically, factors influencing predation risk are viewed only from the perspective of predators or prey populations but few studies have examined predation risk in the context of a food web. We tested two competing hypotheses regarding predation: (1) predation risk is dependent on predator density; and (2) predation risk is dependent on the availability of alternative prey sources. We use an empirical, multi-level, tropical food web (birds–lizards–invertebrates) and a mensurative experiment (seasonal fluctuations in abundance and artificial lizards to estimate predation risk) to test these hypotheses. Birds were responsible for the majority of attacks on artificial lizards and were more abundant in the wet season. Artificial lizards were attacked more frequently in the dry than the wet season despite a greater abundance of birds in the wet season. Lizard and invertebrate (alternative prey) abundances showed opposing trends; lizards were more abundant in the dry while invertebrates were more abundant in the wet season. Predatory birds attacked fewer lizards when invertebrate prey abundance was highest, and switched to lizard prey when invertebrate abundance reduced, and lizard abundance was greatest. Our study suggests predation risk is not predator density-dependent, but rather dependent on the abundance of invertebrate prey, supporting the alternative prey hypothesis

Body temperatures and winter activity in overwintering Timber Rattlesnakes (Crotalus horridus) in Tennessee, USA

At high latitudes and elevations, snakes spend considerable time in overwintering refugia. Although brumation is generally associated with periods of inactivity, some evidence supports the occurrence of limited above and below ground activity during winter. Observations of such events are rare due to the inaccessibility of the typical subterranean refugia of snakes. Our study examined occurrences of both surface and subterranean activity during winter in the Timber Rattlesnake, Crotalus horridus. We monitored hourly body temperatures (Tbs) and small-scale above and below ground movement bouts throughout the overwintering period in Tennessee, USA. High frequency monitoring of body temperatures and movement patterns allowed us to identify mid-winter activity as well as shuttling behavior during ingress and egress. We recorded environmental temperatures and snake operative temperatures to estimate periods when snakes were surface active. Snake ingress into brumation occurred on 10 October ± 12 d, and egress occurred on 7 April ± 17 d. We recorded 53,041 Tbs (mean snake Tb = 11.0 ± 3.6°C; range 1.1–33.7°C) collected over two overwintering periods (2011–2012 and 2012–2013). Snakes made on average 6.1 ± 1.2 movement bouts throughout winter, accumulating a total distance of 146.4 ± 35.5 m. All individuals made small