The structure of communities may be largely a result of evolutionary changes that occurred many millions of years ago. We explore the historical ecology of squamates (lizards and snakes), identify ...historically derived differences among clades, and examine how this history has affected present‐day squamate assemblages globally. A dietary shift occurred in the evolutionary history of squamates. Iguanian diets contain large proportions of ants, other hymenopterans, and beetles, whereas these are minor prey in scleroglossan lizards. A preponderance of termites, grasshoppers, spiders, and insect larvae in their diets suggests that scleroglossan lizards harvest higher energy prey or avoid prey containing noxious chemicals. The success of this dietary shift is suggested by dominance of scleroglossans in lizard assemblages throughout the world. One scleroglossan clade, Autarchoglossa, combined an advanced vomeronasal chemosensory system with jaw prehension and increased activity levels. We suggest these traits provided them a competitive advantage during the day in terrestrial habitats. Iguanians and gekkotans shifted to elevated microhabitats historically, and gekkotans shifted activity to nighttime. These historically derived niche differences are apparent in extant lizard assemblages and account for some observed structure. These patterns occur in a variety of habitats at both regional and local levels throughout the world.
Animal diets crucially affect fitness, yet many aspects of their ultimate
determinants are unknown. The distribution and extent of herbivory in lizards, its evolutionary
history, and ecological ...factors that may favour it are discussed. Most lizards are exclusively or
primarily carnivorous, yet many species eat some plants and a few are almost exclusively herbivorous.
Based on a literature survey of diets of over 450 lizard species, the distribution and degree of
omnivory and herbivory are described. Some plants occur in the diets of slightly over half of lizard
species, and plants formed 10% or more of the dietary volume of 12.1% of species, and 90% or more of
the diet of 0.8% of species. The greatest percentage of omnivorous species (> 10% plant diet),
over 30% in each, and highest mean percentage plant matter in the diet are in Iguanidae,
Corytophanidae, Gerrhosauridae, Agamidae, Xantusiidae, and Tropiduridae. Numerous other omnivores
occur in Lacertidae and Scincidae and fewer in several additional families. Herbivorous lizards
(> 90% plant volume) tend to be folivorous and to possess adaptations for processing leaves,
including specialized dentition for cutting or reducing leaves, elongated intestines, colic valves
that slow passage of food, and intestinal flora that digest cellulose. Omnivorous lizards lacking
such specializations may eat some leaves, but consume much more fruit, flowers, and seeds, plant
parts that are easy to digest, likely to be very abundant seasonally, and may be highly nutritious.
Some lizards eat nectar and pollen; even sap is eaten by at least one gecko. Ontogenetic increase in
plant consumption and decrease in prey consumption is known, but its generality has been
controversial. Such ontogeny has been demonstrated in three iguanid species, a skink, a lacertid,
two tropidurids, a phrynosomatid, and two corytophanids, but it does not occur in some other species. The
importance of specific foods may vary with age. Omnivory and/or herbivory have originated in many lizard
families, with at least nine origins in Iguania and 23 in Scleroglossa. Origins have been rare in Gekkonoidea
and Anguimorpha and common in Scincomorpha, especially in Lacertidae and Scincidae. Losses of omnivory have
been much less frequent than gains. Only a few origins can account for all the herbivory in lizards.
Concentrated changes tests show that there is a significant association in Lacertidae, Lacertiformes,
Lacertoidea, Scincidae, and Scleroglossa between insularity and omnivory. Insular lizards may broaden
their diets to compensate for limited availability of prey. Addition of other factors that reduce availability
of prey, i.e. extreme aridity and cave-dwelling, to insularity, strengthened the relationship to omnivory
in Lacertidae and Lacertoidea. We were unable to demonstrate a role of aridity independent of insularity, but
present anecdotal evidence suggests that it may promote evolution of plant consumption. Large body size in
lizards has long been associated with herbivory, and more recently, with omnivory in lacertid lizards. Using
a conventional regression approach in which each species is considered to supply an independent data point,
this relationship was confirmed for all lizards. Although larger species have diets with more plants, plant
consumption accounts for only 9% of the variation in body length, which is not surprising given that other
factors such as predation, competition, and sexual selection affect body size. The frequency of transitions
body size associated with transitions to omnivory or carnivory was also examined. In Iguania, Scleroglossa,
and all lizards, transitions supporting the hypothesis that omnivory favours increase in body size were significantly
more frequent than non-supporting transitions. This suggests that substantial plant consumption favours
evolution of larger size, probably because of the energetic considerations first presented by Pough (1973).
Because actively foraging lizards move widely through the habitat to locate prey and tongue-flick to locate
prey by chemical cues, we hypothesized that they may be more likely to evolve omnivory than ambush foragers,
which wait motionless for prey and do not tongue-flick to locate or identify prey. The basis of this prediction is that
the wider seaching of active foragers predisposes them to contact with a greater variety and quantity of plants
and that chemosensory tongue-flicking used by omnivores to identify plant food might be easier to evolve in active
foragers that already use pre-chemical discrimination. The prediction is supported by a significantly greater
per species frequency of origins of omnivory by active foragers than by ambushers. A scenario for the progressive
evolution of omnivory and herbivory from ancestrally carnivorous lizards is discussed.
Approach distance (flight initiation distance) and escape methods depend on predation risk. I studied escape methods and effects of risk factors (temperature, perch height and orientation, ...conspicuousness) on approach distance in seven Puerto Rican anoles. Approach distance increased as temperature decreased in
Anolis gundlachi
Peters, 1876, presumably because of decreasing running speed, but not in other species (probably because of narrow temperature range). Perch height and approach distance varied inversely in four arboreal species that escape upward, positively in two grass-bush species that are more conspicuous when higher and flee downward, and were unrelated in cryptic
Anolis stratulus
Cope, 1861. Approach distance was shortest in cryptic A. stratulus and shorter intraspecifically in three species for partially concealed lizards and at sites providing more cover in two species. Approach distance was shorter for A. gundlachi on vertical than nonvertical perches, suggesting that ease of escape upward affects assessed risk. Escape behaviours have been proposed to vary among anole ecomorphs. Grass-bush species fled downward as proposed, or horizontally near ground level. Contrary to predictions of escape downward by trunk-ground and upward by trunk-crown anoles, all arboreal species escaped upward. Only trunk anoles were proposed to use squirreling, but species from four ecomorphs did as well.
Costly anti-predatory defences are used in ecological time and maintained in evolutionary time by natural selection favouring individuals that survive through their use. Autotomy of expendable body ...parts is a striking example of a defence having multiple substantial costs, including loss of ability to use the same defence, loss of energy, and decreased growth, reproductive success and survival following autotomy, plus the energetic cost of replacing the lost body part in species capable of regenerating them. Our study shows that autotomy in the lacertid lizard Podarcis lilfordi reduces sprint speed, indicating decreased capacity to escape as well as the loss of energy. Autotomy carries substantial cost, and thus should be avoided except as a last resort. Ease of autotomy and post-autotomic movements were studied in three populations of lacertid lizards. Two were islet populations of P. lilfordi from Aire (lowest predation pressure) and Colom (intermediate predation pressure) off Minorca. The third was a mainland population of Podarcis hispanica, a closely related species from the mainland of the Iberian Peninsula where predation pressure is higher than on the islets. As predicted, a suite of autotomic traits increases the effectiveness of autotomy as a defence as predation pressure increases. With increasing predation pressure, the frequency of voluntary autotomy increases, latency to autotomy decreases, pressure on the tail needed to induce autotomy decreases, vigour of post-autotomic tail movements increases, and distance moved by the shed tail increases. Additional changes that might be related to predation pressure, but could have other causes, are the presence of tail coloration contrasting with body coloration except under the lowest predation pressure (Aire) and longer tails in the mainland species P. hispanica. Correspondence between predation pressure and the suite of autotomic traits suggests that autotomy is an important defence that responds to natural selection. Comparative data are needed to establish the generality of relationships suggested in our study of only three populations.
Optimal escape theory seeks to explain variation in the distance to an approaching predator at which the prey initiates escape (flight initiation distance). Flight initiation distance increases when ...predators pose a greater threat and decreases when escape costs increase. Although optimal escape theory has been highly successful, its predictions have been tested primarily for species that escape to discrete refuges, and most studies have focused on single risk or cost factors. We present data from two experiments in which two risks or a risk and a cost varied in Bonaire whiptail lizards (Cnemidophorus murinus) that escaped without entering refuges. Our data verify several predictions about optimal escape for nonrefuging lizard prey. Two risk factors, speed and directness of approach by the predator, interacted. Directly approached lizards had greater flight initiation distances than did indirectly approached lizards when approached rapidly, but shorter flight initiation distances when approached slowly. Flight initiation distance was shorter in the presence of food and during slow versus rapid approaches, but contrary to expectation, food presence and approach speed did not interact. This would be explained if cost curves are nonlinear or if they are parallel rather than intersecting when the predator reaches the prey. More empirical work is needed to determine which risk and cost factors act additively and which act synergistically. The absence of interaction between the risk and cost factors suggests that cost curves were nonlinear.
AbstractForaging, feeding, and escape decisions may all be modified by threat of predation. Field experiments using a human simulated predator show that a lizard, the broad - headed skink, Eumeces ...laticeps, alters several aspects of feeding behavior in ways suggesting tradeoffs between predation risk and feeding. When food (cricket) was closer to the predator, the lizards more frequently did not attack it, and often retreated to safety before consuming it, reducing the duration of exposure to predation. The probability of attacking a cricket decreased with distance of the lizard from refuge, reflecting greater risk due to increased time required to reach refuge. Latency to attack increased with distance of the lizard from refuge, suggesting that lizards assessed the risk as acceptable after observing the predator's continued immobility. Large crickets were attacked with higher probability and shorter latency than small crickets, indicating that greater risk was acceptable for greater energetic benefit. The lizards more frequently carried large than small crickets to refuges before eating them, reducing duration of exposure because handling time was greater for larger crickets. Smaller crickets were consumed where captured, again indicating modification of feeding behavior in response to predation risk. The skinks also reduced risk by reducing handling time when closer to the predator. Escape was delayed until a predator approached closer when lizards were eating than when not eating. This delay may reflect a tradeoff between predation risk and acquisition of food.
Escape from predators by desert iguanas (Dipsosaurus dorsalis) conforms to predictions of optimal escape theory based on risk. I simulated an approaching predator to study risk factors. The primary ...response variable was approach distance (= flight-initiation distance), i.e., the distance between predator and prey when the prey initiates escape. In additional studies, I recorded whether lizards permitted me to approach close enough to noose them (an indicator of wariness) and the method of escape. Approach distance was greater when the predator approached rapidly than slowly and directly than indirectly, and when the predator turned toward the lizard rather than away. It was greater in open than in more densely covered habitats, which may reflect greater risk due to conspicuousness and (or) a greater distance to refuge. Early in the day at lower air temperatures, desert iguanas permitted a closer approach before initiating escape. While basking after emergence from burrows, lizards escaped into burrows; later in the day they fled. Lizards that fled had high body temperatures; a single individual captured immediately after entering a burrow had a lower body temperature. Lizards presumably enter burrows when low body temperature limits the running speed, but burrow use is costly because attainment of the activity temperature is delayed because of time elapsed and the temperature decrease in burrows.
The ability of squamates to detect chemical cues from adaptively important sources including prey, predators, and conspecifics has been tested frequently by presenting stimuli on cotton-tipped swabs ...or ceramic tiles. In many such studies the primary response variable is tongue-flicking, which is widely interpreted to indicate sampling for vomerolfaction. I review the basic experimental method and consider limitations regarding its application and interpretation and ways to overcome them. Effects of experimenter proximity and the assumed invisibility of chemical stimuli are considered, as are use of cologne as a pungency control, senses used in making chemical discriminations, and interpretation of results when there are no significant response differences among stimulus classes. Although the assumption that tongue-flicking reveals vomerolfactory sampling and the necessity of an intact vomeronasal system for normal responses to pheromones have been demonstrated where tested, very few species have been examined. In some squamates for which these assumptions have not been examined experimentally, especially eublepharid geckos, attacks on swabs bearing prey chemicals and performance of antipredatory displays in response to predator chemicals occur with no prior tongue-flicking. Not only are assays based on tongue-flicking useless in such cases, but the discriminations are likely based on olfaction. Issues specific to the study of responses to prey chemicals, predator chemicals, and pheromones are discussed. For many purposes, swab tests provide rapid, conclusive assays of ability to respond differentially to biologically relevant stimuli. However, other methods may be superior for studying some responses, and swab tests are not always applicable.PUBLICATION ABSTRACT
Two-species studies should not be used to infer adaptive relationships between ecological variables, but may be useful to generate hypotheses for comparative testing, to obtain data for comparative ...analysis, or to obtain information specific to a particular paired comparison. I present a case in which a two-species study was essential to obtaining data for comparative analysis. Measurements of multiple individuals per taxon are needed to provide estimates of variables and their differences, but comparative methods using dichotomous variables cannot themselves demonstrate the existence of true differences among taxa. Previous studies using the concentration of correlated changes test and the method of independent contrasts showed that correlated evolution has occurred in lizards between foraging mode and detection of prey chemicals by tongue-flicking. Active foragers exhibit prey chemical discrimination, but ambush foragers do not. Effects of changes in foraging mode among very close relatives are unknown and more data on changes in foraging mode are needed to confirm and extend the relationship. In a two-species experiment using congeneric lacertid lizards differing in foraging mode, both species discriminated prey chemicals from control substances, but the discrimination was significantly stronger in the active forager. Change in foraging mode was accompanied by rapid change in prey chemical discrimination that could not be detected by correlative methods. Concentration of correlated changes tests including data from the experiment strengthened evidence for correlated evolution between foraging mode and prey chemical discrimination in lizards as a whole and the major subtaxa Scleroglossa and Autarchoglossa, and permitted its first detection in Scincomorpha.
To efficiently locate and assess foods, animal sensory capacities and behavioral discriminations based on them must be appropriate for the diet and method of hunting. In lizards, actively foraging ...insectivores identify animal prey using lingually sampled chemical cues, but ambush foragers do not. Among plant eaters derived from active foragers, plant chemical discrimination is added to prey chemical discrimination, resulting in correlated evolution of plant diet and plant chemical discrimination. Here I present comparative evidence on the relationships between plant diet and food chemical discrimination in Iguania, which consists primarily of ambush foragers and is one of two major lizard clades, and for ambushing lizards in general. Comparative analyses conducted using phylogenetic methods show that (1) all but one species of omnivore studied exhibited both prey and plant chemical discrimination, whereas ambush foragers exhibited neither; (2) significant correlated evolution occurred between plant diet and plant chemical discrimination in Iguania and in omnivores and herbivores derived from ambush foragers; and (3) correlated evolution has occurred between prey and plant chemical discrimination in Iguania and, more generally, in taxa derived from ambush foragers. These results are explained by selection on plant eaters to assess the nutritional value and possible toxicity of plants and by continued consumption of some animal prey even in herbivores combined with freedom from factors that select against prey chemical discrimination in ambush foragers.
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