Repeated exposure and handling effects on the escape response of fence lizards to encounters with invasive fire ants

Eastern Fence Lizards (Sceloporus undulatus) display an ontogenetic shift in relative consumption of native and invasive prey

Interactions between invasive prey and native predators can provide an opportunity to better understand predator–prey dynamics and how these may change through ontogeny. Eastern Fence Lizards (Sceloporus undulatus (Bosc and Daudin in Sonnini and Latreille, 1801)) are ant specialist, particularly as juveniles. Invasive red imported fire ants (Solenopsis invicta Buren, 1972) pose a lethal risk to S. undulatus that eat them, especially smaller-bodied juveniles. We examine ontogenetic shifts in S. undulatus consumption of toxic invasive fire ants versus palatable native pyramid ants (Dorymyrmex bureni (Trager, 1988)). We predicted that hatchlings should avoid eating fire ants in favor of native ants, whereas less-vulnerable adults should take advantage of both prey sources. However, when given the choice between fire ants and native ants, hatchlings consumed similar numbers of these species, whereas adults consumed nearly three times as many native ants as invasive fire ants. Increased consumption of fire ants in adulthood could be the result of lifetime experience, strategies to safely consume fire ants, ontogenetic shifts in the ability to distinguish between ants, or reduced costs to adults of eating venomous ants. Future research should aim to distinguish these alternative mechanisms and examine the long-term consequences of native species incorporating toxic invasive prey into their diets.

Endocrine stress response of Eastern Fence Lizards in fire-disturbed landscapes

Landscape disturbances can alter habitat structure and resource availability, often inducing physiological responses by organisms to cope with the changing conditions. Quantifying the endocrine stress response through measurement of glucocorticoids has become an increasingly common method for determining how organisms physiologically respond to challenges imposed by their environment. We tested the hypothesis that Eastern Fence Lizards cope with fire disturbance effects by modulating their secretion of corticosterone (CORT). We measured the baseline and stress-induced plasma CORT of male Eastern Fence Lizards in a chronosequence of fire-altered habitats (recently burned, recovering from burn, and unburned). Although habitat use by lizards differed among burn treatments, including differences in use of canopy cover, leaf litter, and vegetation composition, we did not detect a significant effect of fire-induced habitat alteration on plasma CORT concentration or on body condition. In addition, we found no effect of blood draw treatment (baseline or stress-induced), body temperature, body condition, or time taken to collect blood samples on concentration of plasma CORT. Low intensity burns, which are typical of prescribed fire, may not be a sufficient stressor to alter CORT secretion in Eastern Fence Lizards (at least during the breeding season). Instead, lizards may avoid allostatic overload using behavioral responses and by selecting microsites within their environment that permit thermoregulatory opportunities necessary for optimal performance and energy assimilation.

Does repeated human handling of study animals during the mating season affect their offspring?

Minimizing disturbance of study animals is a major consideration in ethological and ecological research design. One nearly universal type of disturbance is the handling of study animals as a component of trial setup. Even low to moderate levels of handling can be a substantial stressor to study animals, which may negatively affect their offspring via maternal effects. Understanding how routine human handling and manipulation may affect the outcome of research studies is therefore critical for interpreting study outcomes. We tested whether repeatedly handling and manipulating (i.e., manually disengaging) amplexed pairs of wood frogs (Rana sylvatica [Lithobates sylvaticus]), which have an explosive breeding season, would affect their reproductive output and offspring fitness. Handling and manipulation did not alter any parameter that we measured: reproductive timing, hatching success, and offspring larval duration, survival, and size at metamorphosis. These results suggest that handling and manipulation by researchers may have a negligible effect on wood frog reproduction and offspring fitness. It is possible that many species that are commonly used in reproductive studies because they suppress behavioral and physiological responses during the mating season are likewise unaffected by human handling. Nevertheless, researchers should examine possible consequences of methodological interventions on their study species in order to determine any potential influence on their results. Having a broad understanding of these effects on species that have robust or dampened stress responsiveness during the breeding season would be useful for making generalizations about potential effects.

Is naïveté forever? Alien predator and aggressor recognition by two endemic island reptiles

The disproportionate impacts of invasive predators are often attributed to the naïveté (i.e., inefficient or non-existing anti-predator behavior) of island native species having evolved without such predators. Naïveté has long been regarded as a fixed characteristic, but a few recent studies indicate a capacity for behavioral adaptation in native species in contact with alien predators. Here, we tested whether two reptiles endemic to New Caledonia, a skink, Caledoniscincus austrocaledonicus, and a gecko, Bavayia septuiclavis, recognized and responded to the odor of six introduced species (two rodents, the feral cat, and three species of ants). We used an experimental design in which reptiles had a choice of retreat sites with or without the odor of predators or aggressors. Skinks avoided two or three of the predators, whereas geckos avoided at most one. These results suggest that diurnal skinks are more responsive than nocturnal geckos to the odor of introduced predators. Neither skinks nor geckos avoided the three species of ants. Thus, the odors of alien predators are shown to influence retreat site selection by two native island reptiles. Moreover, the study suggests that this loss of naïveté varies among native species, probably as a consequence of the intensity of the threat and of time since introduction. These findings argue for re-thinking the behavioral flexibility of ectothermic reptiles in terms of their responses to biological invasion.

Evading invaders: the effectiveness of a behavioral response acquired through lifetime exposure

Understanding the mechanisms driving adaptations to survive agonistic interactions, and their function, provides insight into how native species adapt to aggressive invaders, a growing global threat. We staged encounters between native fence lizards (Sceloporus undulatus) and red imported fire ants (Solenopsis invicta) on and off the ant mound (nest) to examine the effectiveness of lizard antipredator behavior through ontogeny while focusing on the impact of lifetime and evolutionary exposure to this invasive threat. We used field-caught and lab-reared lizards from a fire ant–invaded and an uninvaded site. In ∼90% of cases, fire ants found lizards within 12 min in natural lizard habitat. Lizards that performed rapid twitches of their body and/or fled after initial encounter with a fire ant scout reduced their risk of having additional fire ants recruit to the attack. The majority of lizards that had been exposed to fire ants within their lifetime (field-caught lizards from the invaded site) behaviorally responded to attack, whereas relatively few lizards that were naïve to fire ants (all lab-reared lizards and field-caught lizards from the uninvaded site) responded. Because fewer adult lizards responded to fire ants than juveniles, they were recruited to by additional attacking ants significantly more than were juveniles. Our data suggest that the higher percentage of responsive adults within invaded populations is the result of within-lifetime selection acting against unresponsive adults, and/or lifetime exposure to fire ants triggering the retention of this juvenile behavior into adulthood, rather than selection acting on a heritable trait across generations.