Why do Juvenile Chinese Pit‐Vipers (Gloydius shedaoensis) Select Arboreal Ambush Sites?

Ontogenetic shifts in habitat use are widespread, especially in ectothermic taxa in which juveniles may be an order of magnitude smaller than large adult conspecifics. The factors that generate such habitat shifts are generally obscure, but we studied an unusual system that allowed us to compare consequences of habitat selection between adults and juveniles. Pit‐vipers (Gloydius shedaoensis) on a small island in north‐eastern China feed almost entirely on seasonally migrating birds. During the spring bird‐migration period, individual snakes consistently re‐used either arboreal or terrestrial ambush sites. Snakes in trees were smaller (and more philopatric) than snakes on the ground. This ontogenetic shift in habitat use may reflect the difficulty of capturing birds on the ground, especially by small snakes. In laboratory trials, large (adult) pit‐vipers struck faster, further and more accurately than did small (juvenile) snakes. In experiments with free‐ranging snakes, the proportion of strikes hitting the bird was lower for juveniles than for adults, and lower for terrestrial snakes than for arboreal snakes. Additionally, adult snakes generally seized the bird by the head whereas juveniles frequently struck the body or wings (and thus, obtained a less secure grip). Arboreal ambush sites may facilitate prey capture not only because they give access to smaller birds but also because they render the bird's location more predictable and, hence, enable the snake to position itself optimally prior to the prey's arrival. Because juvenile pit‐vipers are less capable strikers, and are small relative to available prey items, they may benefit from the greater ease of prey capture from branches. Thus, the ontogenetic shift in habitat selection within this species may be because of ontogenetic shifts in the vipers’ ability to capture and ingest large, mobile prey.     

Ontogenetic variation in the autecology of the greater earless lizard Cophosaurus texanus

We investigated the potential for ontogenetic resource partitioning within a population of the Texas earless lizard Cophosaurus texanus Results from focal samples, line transects, haphazard observations, and stomach flushes compiled in summer (July 1993) and autumn (September 1995) revealed differences in microhabitat use and diet between adults and juveniles Juveniles use rock perches more frequently than adult males, and consume smaller prey than adults No ontogenetic differences in thermal ecology were observed Diet differences between juveniles and adults may be attributed to the inability of juveniles to handle large prey items Differences between juveniles and adults in prey size and microhabitat use suggest ontogenetic variation in foraging, predator avoidance, and territory defense     

Thermal influences on foraging ability: body size, posture and cooling rate of an ambush predator, the python Morelia spilota

1. The interface between thermal biology and foraging mode has attracted little scientific attention, but may be crucially important to the biology of ectothermic predators. Slip & Shine (1988c) suggested that the ability of large heavy-bodied snakes to ambush nocturnally active mammals relied on the snakes' control of cooling rates through their thermal inertia (via body size and postural adjustments) and microhabitat selection.
2. We tested assumptions underlying this hypothesis, using Diamond Pythons ( Morelia s . spilota ) from southeastern New South Wales. Our laboratory studies confirmed that larger body sizes and coiled postures significantly retarded cooling rates, and that body temperature affected the snakes' ability to detect potential prey items.
3. The magnitude of these effects on cooling rates was great enough to extend the time period substantially over which an adult Diamond Python, lying in ambush in a suitable microhabitat, would be able to detect and capture nocturnally active prey. For example, the times taken for pythons to reach thermal equilibration under our experimental conditions (cooling from 33 to 12°C) were <1h for hatchling pythons regardless of posture, 1 h for outstretched juveniles, 2 h for coiled juveniles and outstretched adults, and almost 8 h for coiled adults.
4. The high rates of cooling of juvenile pythons, even when they are tightly coiled, may force them to rely upon diurnally active prey rather than crepuscular or nocturnal species.     

Habitat use by the inland carpet python (Morelia spilota metcalfei: Pythonidae): Seasonal relationships with habitat structure and prey distribution in a rural landscape

Abstract Squamate reptiles are significant components of woodland vertebrate communities in eastern Australia, but their ecology is poorly understood. We investigated seasonal variation in habitat use by the Inland Carpet Python, Morelia spilota metcalfei Wells and Wellington (Pythonidae), a threatened snake that inhabits the woodland environments of the Murray–Darling Basin. Nine pythons were radiotracked within and near the Mount Meg Flora and Fauna Reserve in north‐eastern Victoria to investigate how habitat structure and prey distribution (namely, that of the European Rabbit, Oryctolagus cuniculus L. (Leporidae)) influenced seasonal movement patterns. Data were analysed over three spatial scales to allow firm interpretations regarding resource selection. Pythons exhibited distinct seasonal trends in habitat use. During the cooler spring months, snakes chose warm, well‐insulated microhabitats, primarily rocky outcrops on north‐ and north‐west‐facing hillsides. Pythons moved widely during the summer months, apparently in search of prey. Snake localities could be readily linked to rabbit distribution at this time. Specifically, snakes moved to more open, disturbed habitats that contained a high density of rabbits, and consistently selected microhabitats in close proximity to rabbit burrows. In autumn, habitat use was transitional, as snakes progressively returned to the rocky hillsides where they overwintered. Thus, trends in habitat use were influenced by the snakes' thermoregulatory and foraging strategies. Careful management of specific habitats and feral prey populations is required to conserve populations of this endangered snake.     

Prey‐driven behavioral habitat use in a low‐energy ambush predator

Food acquisition is an important modulator of animal behavior and habitat selection that can affect fitness. Optimal foraging theory predicts that predators should select habitat patches to maximize their foraging success and net energy gain, likely achieved by targeting areas with high prey availability. However, it is debated whether prey availability drives fine‐scale habitat selection for predators. We assessed whether an ambush predator, the timber rattlesnake (Crotalus horridus), exhibits optimal foraging site selection based on the spatial distribution and availability of prey. We used passive infrared camera trap detections of potential small mammal prey (Peromyscus spp., Tamias striatus, and Sciurus spp.) to generate variables of prey availability across the study area and used whether a snake was observed in a foraging location or not to model optimal foraging in timber rattlesnakes. Our models of small mammal spatial distributions broadly predicted that prey availability was greatest in mature deciduous forests, but T. striatus and Sciurus spp. exhibited greater spatial heterogeneity compared with Peromyscus spp. We found the spatial distribution of cumulative small mammal encounters (i.e., overall prey availability), rather than the distribution of any one species, to be highly predictive of snake foraging. Timber rattlesnakes appear to forage where the probability of encountering prey is greatest. Our study provides evidence for fine‐scale optimal foraging in a low‐energy, ambush predator and offers new insights into drivers of snake foraging and habitat selection.     

Does body size influence thermal biology and diet of a python (Morelia spilota imbricata)?

Current theory predicts that larger‐bodied snakes not only consume larger prey (compared with smaller individuals), but may also have a different range of prey available to them due to their thermal biology. It has been argued that smaller individuals, with lower thermal inertia (i.e. faster cooling rates at nightfall when air temperature falls and basking opportunities are limited), may be thermally restricted to foraging and hunting during the day on diurnally active prey, and have reduced capacity to hunt crepuscular and nocturnal prey species. This predictive theory was investigated by way of dietary analysis, assessment of thermal biology and thermoregulation behaviour in an ambush forager, the south‐west carpet python (Morelia spilota imbricata, Pythonidae). Eighty‐seven scats were collected from 34 individual pythons over a 3‐year radiotelemetry monitoring study. As predicted by gape size limitation, larger pythons took larger prey; however, 65% of prey items of small pythons were represented by nocturnally active, small mammals, a larger proportion than present in larger snakes. Several measures of thermal biology (absolute body temperature, thermal differential of body temperature to air temperature, maximum hourly heating and cooling rates) were not strongly affected by python body mass. Additionally, body temperature was only influenced by the behavioural choice of microhabitat selection and was not affected by python body size or position, suggesting that these behavioural choices do not allow smaller pythons to vastly increase their temporal foraging window. By coupling dietary analysis, measures of body temperature and behavioural observations of free‐ranging animals, we conclude that, contrary to theoretical predictions, a small body size does not thermally restrict the temporal window for ambush foraging in M. s. imbricata. An ontogenetic or size‐determined switch from ambush feeding to actively foraging on slower prey would account for the differences in prey taken by these animals. The concept of altered foraging behaviour warrants further investigation in this species.     

The Relationship Between Foraging Ecology and Lizard Chemoreception: Can a Snake Analogue (Burton’s Legless Lizard,Lialis burtonis) Detect Prey Scent?

In lizards and snakes, foraging mode (active vs. ambush) is highly correlated with the ability to detect prey chemical cues, and the way in which such cues are utilized. Ambush-foraging lizards tend not to recognize prey scent, whereas active foragers do. Prey scent often elicits strikes in actively-foraging snakes, while ambushers use it to select profitable foraging sites. We tested the influence of foraging ecology on the evolution of squamate chemoreception by gauging the response of Burton's legless lizard ( Lialis burtonis Gray, Pygopodidae) to prey chemical cues. Lialis burtonis is the ecological equivalent of an ambush-foraging snake, feeding at infrequent intervals on relatively large prey, which are swallowed whole. Captive L. burtonis did not respond to prey odour in any manner: prey chemical cues did not elicit elevated tongue-flick rates or feeding strikes, nor were they utilized in the selection of ambush sites. Like other ambushing lizards, L. burtonis appears to be a visually oriented predator. In contrast, an active forager in the same family, the common scaly-foot ( Pygopus lepidopodus ), did tongue-flick in response to odours of its preferred prey. These results extend the correlation between lizard foraging mode and chemosensory abilities to a heretofore-unstudied family, the Pygopodidae.     

Seasonal variation in terrestrial resource subsidies influences trophic niche width and overlap in two aquatic snake species: a stable isotope approach

Quantifying diet is essential for understanding the functional role of species with regard to energy processing, transfer, and storage within ecosystems. Recently, variance structure in the stable isotope composition of consumer tissues has been touted as a robust tool for quantifying trophic niche width, a task that has previously proven difficult due to bias in direct dietary analyses and difficulties in integrating diet composition over time. We used carbon and nitrogen stable isotope analyses to examine trophic niche width of two sympatric aquatic snakes, banded watersnakes Nerodia fasciata and black swamp snakes Seminatrix pygaea inhabiting an isolated wetland where seasonal migrations of amphibian prey cause dramatic shifts in resource availability. Specifically, we characterized snake and prey isotope compositions through time, space, and ontogeny and examined isotope values in relation to prey availability and snake diets assessed by gut content analysis. We determined that prey cluster into functional groups based on similarity of isotopic composition and seasonal availability. Isotope variance structure indicated that the trophic niche width of the banded watersnake was broader (more generalist) than that of the black swamp snake. Banded watersnakes also exhibited seasonal variation in isotope composition, suggesting seasonal diet shifts that track amphibian prey availability. Conversely, black swamp snakes exhibited little seasonal variation but displayed strong ontogenetic shifts in carbon and nitrogen isotope composition that closely paralleled ontogenetic shifts in their primary prey, paedomorphic mole salamanders Ambystoma talpoideum. Although niche dimensions are often treated as static, our results demonstrate that seasonal shifts in niche dimensions can lead to changes in niche overlap between sympatric species. Such short‐term fluctuations in niche overlap can influence competitive interactions and consequently the composition and dynamics of communities and ecosystems.     

Variation in the diet of the viperine snake Natvix mama in relation to prey availability

The diet of the viperine snake was compared with food availability in the Ebro Delta, a wetland largely occupied by rice fields, in 1990 and 1991. Snake selection of prey type and size was studied seasonally and by snake group: males, females and immature snakes. Overall, feeding activity (percentage of individuals with prey and number of prey per stomach) increased with food availability. Diet analysis showed that viperine snakes mainly foraged on the green frog Rana perezi (adults and tadpoles) and the carp Cyprinus earpio. Conversely, viperine snakes rejected the mosquito fish Gambusia holbroki which is the most abundant species in autumn, when Natrix maura has a low feeding activity. Statistical comparisons between viperine snake diet and prey availability showed that males selected small carp, immature snakes selected tadpoles and, in spring, females selected frogs. The selection of small carp by males may reflect a sexual divergence of trophic niche related to sexual size dimorphism, as females are larger than males. As tadpoles are presumably easier to catch than fish, tadpole selection by immature individuals may reflect variance in capture abilities. In spring, the selection of frogs by females overlapped with vitellogenesis, suggesting that females compensate for the cost of reproduction by selecting green frogs, which have a greater biomass and higher energy content than fish. Carps eaten in spring were smaller than in summer. Moreover, in summer viperine snakes selected smaller carp than the available mean size. This divergent tendency between carp size selection and carp size availability reveals how seasonal diet shifts in prey size selection may be a response to an increase in prey size.     

Ontogenetic change in blood oxygen capacity and maximum activity in garter snakes (Thamnophis sirtalis)

Summary There is an ontogenetic increase in the time that garter snakes (Thamnophis s. sirtalis) can maintain maximum activity at 25°C. Newborn snakes are exhausted by 3–5 min of activity while adults can be active for 20–25 min. The increased endurance of adult snakes results from ontogenetic increases in both aerobic and anaerobic energy generation. At rest juvenile and adult snakes have the same whole-body lactic acid concentrations, but at exhaustion adult lactic acid concentrations are 1.5 times those of juveniles. This increase in anaerobic energy production accounts for part of the endurance of adult snakes, but increased aerobic metabolism appears to be more important. Among the mechanisms increasing aerobic metabolism are more effective pulmonary ventilation and a 3-fold ontogenetic increase in blood oxygen capacity.The rapid exhaustion of small garter snakes probably limits the microhabitats they can occupy and the sorts of hunting methods they can employ. Small garter snakes feed only on small prey that are easily subdued. There is an ontogenetic increase in the relative size of prey eaten by garter snakes that parallels the ontogenetic increase in endurance. Adult feeding habits are adopted at the same body size at which adult blood oxygen capacity and endurance are attained.     

Migration patterns in a population of cottonmouths (Agkistrodon piscivorus) inhabiting an isolated wetland

Few studies have examined the spatial and temporal migration patterns of snakes to and from active-season habitats. We conducted a year-long population-level analysis of cottonmouth Agkistrodon piscivorus migration patterns by monitoring snakes entering and leaving a Carolina bay wetland that was encircled by a continuous terrestrial drift fence. Cottonmouths used the wetland during the active season and left the bay in the fall to overwinter in other habitats. Adults and juveniles did not differ in time of arrival at the bay but juveniles left the bay earlier than adults. Spatially, captures of adult cottonmouths entering and leaving the bay were distributed non-randomly, with capture peaks corresponding to the directions to the nearest permanent aquatic habitats. Juveniles' immigration patterns in the spring were biased in the same directions as those of the adults, but they left non-directionally in the fall. This suggests that neonates do not rely on adult scent trailing to locate hibernacula, and that in a region with moderate winter temperatures, suitable overwintering sites may not be a limited resource. Additionally, our study demonstrates that cottonmouths make extensive use of upland habitats and underscores the importance of both critical upland habitat and forested corridors between wetlands and hibernacula for the conservation of wetlands herpetofauna.     

Food resources influence spatial ecology, habitat selection, and foraging behavior in an ambush-hunting snake (Viperidae: Bothrops asper): an experimental study

Prey availability affects many aspects of predators' life history and is considered a primary factor influencing individuals' decisions regarding spatial ecology and behavior, but few experimental data are currently available. Snakes may represent ideal model organisms relative to other animal groups for addressing such resource dependency, due to a presumably more direct link between food resources and many aspects of behavior and natural history. We experimentally investigated the relationship between food intake and spatial behavior in a population of the snake Bothrops asper in a Costa Rican lowland rainforest. Six adult snakes were allowed to forage naturally while six were offered supplemental food in the field, with both groups monitored using radiotelemetry. Mean home range size did not differ between groups presumably due to small sample size, but supplementally fed snakes demonstrated altered patterns of macro- and microhabitat selection, shorter and less frequent movements, and increased mass acquisition. Fed snakes also devoted less time to foraging efforts, instead more frequently remaining inactive and utilizing shelter. Because snakes were always fed in situ and not at designated feeding stations, observed shifts in habitat selection are not explained by animals simply moving to areas of higher food availability. Rather, B. asper may have moved to swamps in order to feed on amphibians when necessary, but remained in preferred forest habitat when food was otherwise abundant. The strong behavioral and spatiotemporal responses of snakes in this population may have been influenced by an overall scarcity of mammalian prey during the study period.     

Foraging Behavior in the Arboreal Boid Corallus grenadensis

Corallus grenadensis is an arboreal boa endemic to the Grenada Bank. Thirty-five encounters with boas resulted in 17.65 hours of observations, including 6.3 hours of video-tape (which included two acts of predation). Boas under 100 cm are largely active foragers that move slowly through bushes and trees and tongue-flick leaf and branch surfaces apparently seeking chemosensory evidence of nocturnally quiescent lizard (Anolis) prey. Significantly more search time was directed to branches below the snake rather than to either the branches supporting the snake or to those above the snake, and tongue-flick rates were significantly higher for moving snakes than for those that were stationary. Smaller snakes prey on nocturnally quiescent lizards and they spent more time moving than did large snakes that feed on nocturnally active rodents and often employ an ambush foraging strategy. Once visual and, presumably, thermal information was received from a sleeping anole, C. grenadensis adopted a lengthy stalking process devoid of tongue-flicks. Snakes approached inactive lizards from adjacent branches with great stealth, moving at a rate of about 1 cm/min. The strike was made from close range (within 3 cm), and the prey was never released once contact was made. We conclude that, if chemosensory cues successfully lead a treeboa to a visual encounter with a sleeping lizard, subsequent behavior ensures a high rate of predation success.     

Arboreal ambush site selection by pit-vipers Gloydius shedaoensis

For a sit-and-wait predator, the choice of ambush site may be a crucial determinant of foraging success. During fieldwork on a small island in northeastern China, we explored the availability and use of arboreal ambush sites (tree branches) selected by Shedao pit-vipers, Gloydius shedaoensis. The snakes were highly selective at a variety of spatial scales. For example, they displayed strong biases in terms of which tree species were used, which individual trees within each species were used and which branches were used within a tree. Snakes disproportionately used trees that were on the edge rather than the interior of thickets, and branches that faced out towards the clearing rather than back towards the thicket. Branches at an angle slightly above horizontal were preferred. The snakes used branches visited at high rates by potential prey, that provided effective camouflage, and with thermal and visual backgrounds (cool, bright) that contrasted strongly with avian prey items (hot, dark). The snakes used perches close to the ground (the area of greatest bird activity) despite suboptimal visual and thermal backgrounds. Use of thicker branches by larger snakes, and by snakes containing recently ingested prey items, may contribute to effective camouflage. Thermoregulation did not appear to influence foraging site selection.     

Microhabitat use and prey selection of the coral-feeding snail <Emphasis Type="Italic">Drupella cornus</Emphasis> in the northern Red Sea

Corallivorous gastropods of the genus Drupella are known for population outbreaks throughout the Indo-Pacific region. Despite their potential to destroy wide areas of coral reef, prey preferences have never been analyzed with respect to prey availability, and juvenile ecology and food selectivity remain largely unknown. Here, the influence of water depth, coral abundance, colony shape, prey species, and intraspecific attraction among snails on distribution patterns, prey selection, and microhabitat use of D. cornus was studied in the northern Red Sea. Special emphasis was put on ontogenetic differences. The snails were most abundant in the shallowest reef zone (1 m depth). Adults were associated with several substrates and coral growth forms, whereas juveniles were highly cryptic and restricted to live branching corals. The genus Acropora was significantly preferred over other acroporid and pocilloporid corals. As revealed by resource selection ratios, Acropora acuminata was preferred by juveniles, A. selago by adults. In aquarium experiments, intraspecific attraction was high among both life stages. Overall, significant differences in juvenile and adult microhabitat and prey use suggest that juveniles have more specific habitat requirements, and indicate ecological impacts on coral communities different from that of adults. Prey preferences seem to depend on both coral genus and colony shape. Acropora corals provide the best combination of food and shelter and therefore determine distribution patterns of D. cornus.     

Thermal correlates of foraging-site selection by Chinese pit-vipers (Gloydius shedaoensis,Viperidae)

• 1.Do thermal factors influence foraging-site selection by ectothermic predators? Snake species that obtain their prey from ambush must remain immobile for long periods, precluding overt behavioural thermoregulation; and some “ambush” snakes use thermal cues to detect endothermic prey. Plausibly, alternative ambush sites might differ either in equilibrial body temperatures available to snakes, or in the thermal “background” against which prey items must be detected.
• 2.We examined this topic with field data on pit-vipers (Gloydius shedaoensis) on a small island in northeastern China. Adult snakes feed only on migrating passerine birds. The snakes ambush birds both from arboreal perches (branches of small trees) and from the ground.
• 3.Arboreal versus terrestrial ambush sites differed both in operative temperatures and thermal “backgrounds” available to the snakes. Operative temperatures inside copper models were lower in trees than on the ground (because of wind), and snakes in arboreal ambush sites were cooler than those in terrestrial sites. Thermal backgrounds from arboreal perches were cooler (and thus, provided more contrast against prey items) than did backgrounds available from terrestrial ambush-sites.
• 4.Thermal factors thus modify the suitability of alternative ambush locations for these pit-vipers, but with a trade-off: a snake in a tree can “see” its prey more clearly, but may not be warm enough (and hence, able to strike fast enough) to capture it. Further work is required to determine whether or not snakes actually use such thermal differences as criteria for the selection of ambush sites.

     

Microhabitat use by foraging white-clawed crayfish (<Emphasis Type="Italic">Austropotamobius pallipes</Emphasis>) in stream pools in the NE Iberian Peninsula

The white-clawed crayfish (Austropotamobius pallipes) is an endangered species across most of its distribution range, and information on its ecological requirements is needed to implement effective conservation measures. Its habitat use has been studied in different areas and at various spatial scales. However, being a nocturnal species, there is scarce information on its habitat selection during foraging periods. In this work we analyse nocturnal habitat use of white-clawed crayfish in pools of a small stream in the northeastern Iberian Peninsula at two different scales: (1) microhabitat selection and (2) pool characteristics. Large crayfish showed a clear positive selection for deeper microhabitats, a selection pattern that was weaker for medium-sized crayfish and absent for small ones. On the other hand, crayfish of all sizes avoided cobble and boulder microhabitats and positively selected fine substrate and more exposed microhabitats. Crayfish abundance in pools was positively influenced by pool area, pool depth and the availability of fine substrates, especially silt. While studies on white-clawed crayfish habitat use have often stressed the importance of rough substrates as crayfish refuge, our results show that fine substrates are positively selected by foraging crayfish of all size classes and promoted active crayfish abundance in pools. These apparently contradictory results may be due to the differences in microhabitat preferences exhibited by active and inactive crayfish. Thus, our results help to better complete the picture of white-clawed crayfish habitat requirements.     

Age-Related Microhabitat Segregation in Willow Tit Parus montanus Winter Flocks

It is expected that through flexibility in behaviour, flock living birds respond to the asymmetries in resource access derived from dominance relationships. We analysed the microhabitat use of willow tits in winter flocks and assessed possible factors which shape habitat segregation between adults and juveniles in different temperature regimes. When foraging in mild conditions (ambient temperature > 0°C), flocks split up into subgroups with adults foraging in inner parts of trees more often than juveniles. However, no differences were recorded in the vertical position occupied in trees. In harsh conditions (< ? 4°C), flocks re‐united and juveniles further moved to outer parts of trees, increasing horizontal segregation between age classes. In mild conditions, vigilance behaviour was not related to the position of birds in trees, but in harsh conditions, scanning frequency was higher in outer parts of trees only for adults. In mild weather, juvenile position in trees was associated with body size and mass. The foraging microhabitat segregation detected in harsh conditions fits the age‐related hoarding distribution previously described in the same population. This supports the hypothesis that hoarded food is important in determining future foraging habitat use. Adult preference and intraspecific competition for safer or richer inner parts of trees as foraging sites during harsh conditions seems to determine the habitat segregation between adults and juveniles. Furthermore, we suggest that in mild weather, when foraging in the absence of adults, juveniles balance the costs of using a potentially dangerous microhabitat with the benefits of building energetically cheap and large food reserves through hoarding. The expected patterns of microhabitat segregation may differ in parids, depending on whether predation risk or other factors such as food availability are the main factors controlling habitat quality.     

Ambush or active life: foraging mode influences haematocrit levels in snakes

The rate at which organisms acquire resources is a critical trait and foraging mode can vary from sit‐and‐wait tactics to being highly mobile and active. Snakes provide a robust opportunity to examine the physiological correlates of contrasted foraging strategies. In this context, haematocrit (Hct), a proxy of blood oxygen carrying capacity, should be a reliable indicator of aerobic activity levels. We used phylogenetically informed models to examine the relationship between foraging mode and Hct in 80 snake species. After accounting for clade and habitat effects, we found a significant relationship of Hct with foraging mode; Hct is lower in snakes that ambush prey compared to active foragers across habitats. Species using both foraging tactics had marginally lower Hct than active foragers. Ambush foraging tactics are widespread in snakes, notably among low‐energy specialists that usually display low feeding frequency, as well as limited activity and daily movements. Because Hct influences blood viscosity, low levels may thus be advantageous by reducing maintenance and locomotory costs. Further studies are required to better understand the implication of foraging mode on blood characteristics and other aspects of snake physiology. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 636–645.     

Adult Male Density Influences Juvenile Microhabitat Use in a Territorial Lizard

Habitat choice often has strong effects on performance and fitness. For many animals, optimal habitats differ across age or size classes, and individuals shift habitat use through ontogeny. Although many studies document ontogenetic habitat shifts for various taxa, most are observational and do not identify the causal factor of size‐specific habitat variation. Field observations of the brown anole lizard (Anolis sagrei) show that juveniles perch on shorter and thinner vegetation than adults. We hypothesized that this variation is due to adult males forcing smaller juveniles to less preferred habitat. To test this assertion, we manipulated adult male densities in mesh enclosures with artificial trees to examine the response of juvenile microhabitat choice. We found that adult male density had strong effects on juvenile perch height, perch width, and substrate use, suggesting that age‐class competition contributes to the observed ontogenetic differences in habitat choice. We also found that time of day significantly affected juvenile perch height and substrate use. In many cases, our results suggest that juveniles distance themselves from adults using different microhabitats from those used in our control ‘no‐adult’ treatment. However, these findings were often body size dependent and varied depending upon time of day. This study highlights the complexity of juvenile perching behavior and demonstrates the role of intraspecific interactions in shaping habitat use by juvenile animals.