Attracting unwanted attention: generalization of behavioural adaptation to an invasive predator carries costs

House sparrows use learned information selectively based on whether reward is hidden or visible

Based on past experience, food-related-cues can help foragers to predict the presence and the expected quality of food. However, when the food is already visible there is no need to predict its presence or its other visible attributes, but only those that are still cryptic, such as expected handling time or taste. Optimal foragers should therefore use only knowledge that is relevant to the current setting. Nevertheless, the extent to which they do so is not clear. In a set of experiments, we examined how a change in setting, from hidden to visible reward, affects the reliance of house sparrows (Passer domesticus) on three previously learned attributes of food-related cues (sand colors): the setting of the cue (e.g., whether the food was hidden or exposed), the expected amount of the reward (number of seeds), and the expected handling time. We found that sparrows used all three attributes when the rewards were hidden but reached decisions mainly based on handling time when the rewards were visible. This selective use of cue-related information suggests that animals do not simply associate cues with their average expected value but rather learn different attributes of a cue and use all, or only some of them, in a context-appropriate manner.

The influence of maternal glucocorticoids on offspring phenotype in high- and low-risk environments

Elevated maternal glucocorticoid levels during gestation can lead to phenotypic changes in offspring via maternal effects. Although such effects have traditionally been considered maladaptive, maternally derived glucocorticoids may adaptively prepare offspring for their future environment depending upon the correlation between maternal and offspring environments. Nevertheless, relatively few studies test the effects of prenatal glucocorticoid exposure across multiple environments. We tested the potential for ecologically relevant increases in maternal glucocorticoids in the eastern fence lizard (Sceloporus undulatus) to induce adaptive phenotypic changes in offspring exposed to high or low densities of an invasive fire ant predator. Maternal treatment had limited effects on offspring morphology and behavior at hatching, but by 10 days of age, we found maternal treatment interacted with offspring environment to alter anti-predator behaviors. We did not detect differences in early-life survival based on maternal treatment or offspring environment. Opposing selection on anti-predator behaviors from historic and novel invasive predators may confound the potential of maternal glucocorticoids to adaptively influence offspring behavior. Our test of the phenotypic outcomes of transgenerational glucocorticoid effects across risk environments provides important insight into the context-specific nature of this phenomenon and the importance of understanding both current and historic evolutionary pressures.

Ecoimmune reallocation in a native lizard in response to the presence of invasive, venomous fire ants in their shared environment

Exposure to stressors over prolonged periods can have fitness-relevant consequences, including suppression of immune function. We tested for effects of presence of an invasive species threat on a broad panel of immune functions of a coexisting lizard. Eastern fence lizards (Sceloporus undulatus) have been exposed to invasive fire ants (Solenopsis invicta) for over 80 years. Fire ants sting and envenomate lizards, causing physiological stress, but we do not have a comprehensive understanding of the broad immune consequences of lizard exposure to fire ant presence. We conducted a suite of immune measures on fence lizards caught from areas with long histories of fire ant invasion and lizards from areas not yet invaded by fire ants. The effect of fire ant presence on immunity varied depending on the immune component measured: within fire ant invaded areas, some portions of immunity were suppressed (lymphocytic cell-mediated immunity, complement), some were unaffected (phagocytic respiratory burst, natural antibodies), and some were enhanced (anti-fire ant immunoglobulin M, basophils) compared to within uninvaded areas. Rather than fire ants being broadly immunosuppressing, as generally assumed, the immune response appears to be tailored to this specific stressor: the immune measures that were enhanced are important to the lizards' ability to handle envenomation, whereas those that were unaffected or suppressed are less critical to surviving fire ant encounters. Several immune measures were suppressed in reproductive females when actively producing follicles, which may make them more susceptible to immunosuppressive costs of stressors such as interactions with fire ants.

Anthropogenic Change Alters Ecological Relationships via Interactive Changes in Stress Physiology and Behavior within and among Organisms

Anthropogenic change has well-documented impacts on stress physiology and behavior across diverse taxonomic groups. Within individual organisms, physiological and behavioral traits often covary at proximate and ultimate timescales. In the context of global change, this means that impacts on physiology can have downstream impacts on behavior, and vice versa. Because all organisms interact with members of their own species and other species within their communities, the effects of humans on one organism can impose indirect effects on one or more other organisms, resulting in cascading effects across interaction networks. Human-induced changes in the stress physiology of one species and the downstream impacts on behavior can therefore interact with the physiological and behavioral responses of other organisms to alter emergent ecological phenomena. Here, we highlight three scenarios in which the stress physiology and behavior of individuals on different sides of an ecological relationship are interactively impacted by anthropogenic change. We discuss host-parasite/pathogen dynamics, predator-prey relationships, and beneficial partnerships (mutualisms and cooperation) in this framework, considering cases in which the effect of stressors on each type of network may be attenuated or enhanced by interactive changes in behavior and physiology. These examples shed light on the ways that stressors imposed at the level of one individual can impact ecological relationships to trigger downstream consequences for behavioral and ecological dynamics. Ultimately, changes in stress physiology on one or both sides of an ecological interaction can mediate higher-level population and community changes due in part to their cascading impacts on behavior. This framework may prove useful for anticipating and potentially mitigating previously underappreciated ecological responses to anthropogenic perturbations in a rapidly changing world.

Sheep in wolves' clothing: prey rely on proactive defences when predator and non-predator cues are similar

Predation produces intense selection and a diversity of defences. Reactive defences are triggered by predator cues, whereas proactive defences are always in effect. We assess whether prey rely on proactive defences when predator cues do not correlate well with predation risk. Many bats use echolocation to hunt insects, and many insects have evolved to hear bats. However, in species-rich environments like Neotropical forests, bats have extremely diverse foraging strategies, and the presence of echolocation corresponds only weakly to the presence of predators. We assess whether katydids that live in habitats with many non-dangerous bat species stop calling when exposed to echolocation. For 11 species of katydids, we quantified behavioural and neural responses to predator cues, and katydid signalling activity over 24 h periods. Despite having the sensory capacity to detect predators, many Neotropical forest katydids continued calling in the presence of predator cues, displaying proactive defences instead (short, infrequent calls totalling less than 2 cumulative seconds of sound per 24 h). Neotropical katydid signalling illustrates a fascinating case where trophic interactions are probably mediated by a third group: bats with alternative foraging strategies (e.g. frugivory). Although these co-occurring bats are not trophically connected, their mere presence disrupts the correlation between cue and predation risk.

Rapid adaptation to invasive predators overwhelms natural gradients of intraspecific variation

Invasive predators can exert strong selection on native populations. If selection is strong enough, populations could lose the phenotypic variation caused by adaptation to heterogeneous environments. We compare frog tadpoles prior to and 14 years following invasion by crayfish. Prior to the invasion, populations differed in their intrinsic developmental rate, with tadpoles from cold areas reaching metamorphosis sooner than those from warm areas. Following the invasion, tadpoles from invaded populations develop faster than those from non-invaded populations. This ontogenetic shift overwhelmed the intraspecific variation between populations in a few generations, to the point where invaded populations develop at a similar rate regardless of climate. Rapid development can have costs, as fast-developing froglets have a smaller body size and poorer jumping performance, but compensatory growth counteracts some costs of development acceleration. Strong selection by invasive species can disrupt local adaptations by dampening intraspecific phenotypic variation, with complex consequences on lifetime fitness.

Presence of an invasive species reverses latitudinal clines of multiple traits in a native species

Understanding the processes driving formation and maintenance of latitudinal clines has become increasingly important in light of accelerating global change. Many studies have focused on the role of abiotic factors, especially temperature, in generating clines, but biotic factors, including the introduction of non-native species, may also drive clinal variation. We assessed the impact of invasion by predatory fire ants on latitudinal clines in multiple fitness-relevant traits-morphology, physiological stress responsiveness, and antipredator behavior-in a native fence lizard. In areas invaded by fire ants, a latitudinal cline in morphology is opposite both the cline found in museum specimens from historical populations across the species' full latitudinal range and that found in current populations uninvaded by fire ants. Similarly, clines in stress-relevant hormone response to a stressor and in antipredator behavior differ significantly between the portions of the fence lizard range invaded and uninvaded by fire ants. Changes in these traits within fire ant-invaded areas are adaptive and together support increased and more effective antipredator behavior that allows escape from attacks by this invasive predator. However, these changes may mismatch lizards to the environments under which they historically evolved. This research shows that novel biotic pressures can alter latitudinal clines in multiple traits within a single species on ecological timescales. As global change intensifies, a greater understanding of novel abiotic and biotic pressures and how affected organisms adapt to them across space and time will be central to predicting and managing our changing environment.

Population history with invasive predators predicts innate immune function response to early life glucocorticoid exposure

Early life stress can suppress immune function, but it is unclear if transgenerational stress exposure modulates the immune consequences of early stress. In populations where, historically, the immune system is frequently activated, e.g. persistent stressors that cause injury, it may be maladaptive to suppress immune function after early life stress. Thus, the relationship between early life stress and immune function may vary with population-level historical stressor exposure. We collected gravid fence lizards (Sceloporus undulatus) from populations that naturally differ in long-term exposure to invasive fire ants (Solenopsis invicta). We manipulated early life stress in resulting offspring via weekly exposure to fire ants, application of the stress-relevant hormone corticosterone, or control treatment from 2 to 43 weeks of age. We quantified adult immune function in these offspring with baseline and antigen-induced hemagglutination and plasma bacterial killing ability. Early life corticosterone exposure suppressed baseline hemagglutination in offspring of lizards from populations without fire ants but enhanced hemagglutination in those from populations with fire ants. This enhancement may prepare lizards for high rates of wounding, toxin exposure, and infection associated with fire ant attack. Adult bacterial killing ability and hemagglutination were not affected by early life exposure to fire ants, but the latter was higher in offspring of lizards from invaded sites. A population's history of persistent wounding may thus alter individual long-term immunological responses to early life stressors. Further consideration of historical stressor exposure (type and duration) may be important to better understand how early life stressors affect adult physiology.

Maternal stress alters the phenotype of the mother, her eggs and her offspring in a wild-caught lizard

While biomedical researchers have long appreciated the influence of maternally derived glucocorticoids (GCs) on offspring phenotype, ecologists have only recently begun exploring its impact in wild animals. Interpreting biomedical findings within an ecological context has posited that maternal stress, mediated by elevations of maternal GCs, may play an adaptive role preparing offspring for a stressful or rigorous environment. Yet, the influence of maternal stress on offspring phenotype has been little studied in wild animals. We experimentally elevated GCs to ecologically relevant levels (mimicking increases in maternal stress hormones following a nonlethal predator encounter, a heat challenge, or a chasing or confinement stressor) in female eastern fence lizards Sceloporus undulatus during gestation. We tested the hypothesis that maternally derived stress hormones themselves are sufficient to alter offspring phenotype. Specifically, we examined the effects of experimentally elevated maternal GCs on fitness-relevant traits of the mother, her eggs and her subsequent offspring. We found that daily maternal GC elevation: (a) increased maternal antipredator behaviours and postlaying glucose levels; (b) had no effect on egg morphology or caloric value, but altered yolk hormone (elevated GC) and nutrient content; and (c) altered offspring phenotype including stress-relevant physiology, morphology and behaviour. These findings reveal that maternally derived GCs alone can alter offspring phenotype in a wild animal, changes that may be mediated via maternal behaviour, and egg hormone and nutrient content. Understanding the ecological consequences of these effects under different environmental conditions will be critical for determining the adaptive significance of elevated maternal GCs for offspring.