Functional and Numerical Responses of Predators: Where Do Vipers Fit in the Traditional Paradigms?

Springing into action: Comparing escape responses between bipedal and quadrupedal rodents

Predation is a fundamental selective pressure on animal morphology, as morphology is directly linked with physical performance and evasion. Bipedal heteromyid rodents, which are characterized by unique morphological traits such as enlarged hindlimbs, appear to be more successful than sympatric quadrupedal rodents at escaping predators such as snakes and owls, but no studies have directly compared the escape performance of bipedal and quadrupedal rodents. We used simulated predator attacks to compare the evasive jumping ability of bipedal kangaroo rats (Dipodomys) to that of three quadrupedal rodent groups-pocket mice (Chaetodipus), woodrats (Neotoma), and ground squirrels (Otospermophilus). Jumping performance of pocket mice was remarkably similar to that of kangaroo rats, which may be driven by their shared anatomical features (such as enlarged hindlimb muscles) and facilitated by their relatively small body size. Woodrats and ground squirrels, in contrast, almost never jumped as a startle response, and they took longer to perform evasive escape maneuvers than the heteromyid species (kangaroo rats and pocket mice). Among the heteromyids, take-off velocity was the only jump performance metric that differed significantly between species. These results support the idea that bipedal body plans facilitate vertical leaping in larger-bodied rodents as a means of predator escape and that vertical leaping likely translates to better evasion success.

The tale of the rattle: Using rattle size to understand growth and sexual dimorphism in an insular population of rattlesnakes (Crotalus oreganus caliginis)

Islands have played a key role in our understanding of rapid evolution. A large body of literature has examined morphological changes in response to insularity and isolation, which has yielded useful generalizations about how animals can adapt to live in very small geographic areas. However, understanding the evolution of morphological variation in insular populations often requires detailed data sets on longitudinal patterns of growth and development, and such studies typically necessitate long-term mark-recapture on a large sample of individuals. Rattlesnakes provide a unique opportunity to address some of these difficulties because the addition of rattle segments to the rattle string occurs with regular periodicity and their size directly correlates with the body size of the snake at the time of the ecdysis cycle generating the segment. Here, we used a large database of rattle segment sizes recorded from island (Isla Coronado Sur, Baja California, Mexico) and mainland (Camp Pendleton, California, United States) populations of Western Rattlesnakes (Crotalus oreganus and C. o. caliginis) that separated approximately 10,000 years ago to compare body sizes at different ecdysis cycles, which allowed us to assess differences in growth rates and patterns of sexual size dimorphism. Our results show that rattlesnakes on Isla Coronado Sur appear to be born smaller and grow more slowly than their mainland counterparts, resulting in a "dwarfed" island population. However, despite significant differences in body size, both populations exhibited the same degree of sexual dimorphism. Our study demonstrates the potential to use rattle characteristics to recover detailed estimates of fundamental demographic parameters.

Climate change-related distributional range shifts of venomous snakes: a predictive modelling study of effects on public health and biodiversity

BACKGROUND

Climate change is expected to have profound effects on the distribution of venomous snake species, including reductions in biodiversity and changes in patterns of envenomation of humans and domestic animals. We estimated the effect of future climate change on the distribution of venomous snake species and potential knock-on effects on biodiversity and public health.

METHODS

We built species distribution models based on the geographical distribution of 209 medically relevant venomous snake species (WHO categories 1 and 2) and present climatic variables, and used these models to project the potential distribution of species in 2070. We incorporated different future climatic scenarios into the model, which we used to estimate the loss and gain of areas potentially suitable for each species. We also assessed which countries were likely to gain new species in the future as a result of species crossing national borders. We integrated the species distribution models with different socioeconomic scenarios to estimate which countries would become more vulnerable to snakebites in 2070.

FINDINGS

Our results suggest that substantial losses of potentially suitable areas for the survival of most venomous snake species will occur by 2070. However, some species of high risk to public health could gain climatically suitable areas for habitation. Countries such as Niger, Namibia, China, Nepal, and Myanmar could potentially gain several venomous snake species from neighbouring countries. Furthermore, the combination of an increase in climatically suitable areas and socioeconomic factors (including low-income and high rural populations) means that southeast Asia and Africa (and countries including Uganda, Kenya, Bangladesh, India, and Thailand in particular) could have increased vulnerability to snakebites in the future, with potential effects on public human and veterinary health.

INTERPRETATION

Loss of venomous snake biodiversity in low-income countries will affect ecosystem functioning and result in the loss of valuable genetic resources. Additionally, climate change will create new challenges to public health in several low-income countries, particularly in southeast Asia and Africa. The international community needs to increase its efforts to counter the effects of climate change in the coming decades.

FUNDING

German Research Foundation, Conselho Nacional de Desenvolvimento Científico e Tecnológico, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, German Centre for Integrative Biodiversity Research, Ministerio de Ciencia e Innovación de España, European Regional Development Fund.

Hunting behavior and feeding ecology of Mojave rattlesnakes (Crotalus scutulatus), prairie rattlesnakes (Crotalus viridis), and their hybrids in southwestern New Mexico

Predators must contend with numerous challenges to successfully find and subjugate prey. Complex traits related to hunting are partially controlled by a large number of co-evolved genes, which may be disrupted in hybrids. Accordingly, research on the feeding ecology of animals in hybrid zones has shown that hybrids sometimes exhibit transgressive or novel behaviors, yet for many taxa, empirical studies of predation and diet across hybrid zones are lacking. We undertook the first such field study for a hybrid zone between two snake species, the Mojave rattlesnake (Crotalus scutulatus) and the prairie rattlesnake (Crotalus viridis). Specifically, we leveraged established field methods to quantify the hunting behaviors of animals, their prey communities, and the diet of individuals across the hybrid zone in southwestern New Mexico, USA. We found that, even though hybrids had significantly lower body condition indices than snakes from either parental group, hybrids were generally similar to non-hybrids in hunting behavior, prey encounter rates, and predatory attack and success. We also found that, compared to C. scutulatus, C. viridis was significantly more active while hunting at night and abandoned ambush sites earlier in the morning, and hybrids tended to be more viridis-like in this respect. Prey availability was similar across the study sites, including within the hybrid zone, with kangaroo rats (Dipodomys spp.) as the most common small mammal, both in habitat surveys and the frequency of encounters with hunting rattlesnakes. Analysis of prey remains in stomachs and feces also showed broad similarity in diets, with all snakes preying primarily on small mammals and secondarily on lizards. Taken together, our results suggest that the significantly lower body condition of hybrids does not appear to be driven by differences in their hunting behavior or diet and may instead relate to metabolic efficiency or other physiological traits we have not yet identified.

Investigating Welfare Metrics for Snakes at the Saint Louis Zoo

Modern herpetoculture has seen a rise in welfare-related habitat modifications, although ethologically-informed enclosure design and evidence-based husbandry are lacking. The diversity that exists within snakes complicates standardizing snake welfare assessment tools and evaluation techniques. Utilizing behavioral indicators in conjunction with physiological measures, such as fecal glucocorticoid metabolite concentrations, could aid in the validation of evidence-based metrics for evaluating snake welfare. We increased habitat cleaning, to identify behavioral or physiological indicators that might indicate heightened arousal in snakes as a response to the disturbance. While glucocorticoid metabolite concentrations increased significantly during a period of increased disturbance, this increase was not associated with a significant increase in tongue-flicking, a behavior previously associated with arousal in snakes. Locomotion behavior and the proportion of time spent exposed were also not affected by more frequent habitat cleaning. These results demonstrate the need to further investigate the behavioral and physiological responses of snakes to different aspects of animal care at a species and individual level. They also highlight the need to collect baseline behavioral and physiological data for animals, in order to make meaningful comparisons when evaluating changes in animal care.

Invasive snake causes massive reduction of all endemic herpetofauna on Gran Canaria

Invasive snakes represent a serious threat to island biodiversity, being responsible for far-reaching impacts that are noticeably understudied, particularly regarding native reptiles. We analysed the impact of the invasive California kingsnake, Lampropeltis californiae-recently introduced in the Canary Islands-on the abundance of all endemic herpetofauna of the island of Gran Canaria. We quantified the density in invaded and uninvaded sites for the Gran Canaria giant lizard, Gallotia stehlini, the Gran Canaria skink, Chalcides sexlineatus, and Boettger's wall gecko, Tarentola boettgeri. We used spatially explicit capture-recapture and distance-sampling methods for G. stehlini and active searches under rocks for the abundance of the other two reptiles. The abundance of all species was lower in invaded sites, with a reduction in the number of individuals greater than 90% for G. stehlini, greater than 80% for C. sexlineatus and greater than 50% for T. boettgeri in invaded sites. Our results illustrate the severe impact of L. californiae on the endemic herpetofauna of Gran Canaria and highlight the need for strengthened measures to manage this invasion. We also provide further evidence of the negative consequences of invasive snakes on island reptiles and emphasize the need for further research on this matter on islands worldwide.

Kelp Forests: Catastrophes, Resilience, and Management

Resilient kelp forests provide foundation habitat for marine ecosystems and are indicators of the ecosystems’ sustainable natural capital. Loss of resilience and imperfectly reversible catastrophic shifts from kelp forests to urchin barrens, due to pollution or loss of a top predator, are part of an ecological tipping point phenomenon, and involve a loss in sustainable natural capital. Management controls to prevent or reverse these shifts and losses are classified in a number of ways. Systemic controls eliminate the cause of the problem. Symptomatic controls use leverage points for more direct control of the populations affected, urchin harvesting or culling, or kelp enhancement. There is a distinction between ongoing structural (press) controls versus temporary or intermittent perturbation (pulse) controls, and one between shift preventing versus shift reversing or restorative controls. Adaptive management and the options it creates both focus on reductions in uncertainty and control policies with the flexibility to take advantage of those reductions. The various management distinctions are most easily understood by modeling the predator-urchin-kelp marine ecosystem. This paper develops a mathematical model of the ecosystem that has the potential for two different catastrophic shifts between equilibria. Pulse disturbances, originating from exogenous abiotic factors or population dynamics elsewhere in the metacommunity, can activate shifts. A measure of probabilistic resilience is developed and used as part of an assessment of the ecosystem’s sustainable stock of natural capital. With perturbation outcomes clustered around the originating equilibrium, hysteresis is activated, resulting imperfect reversibility of catastrophic shifts, and a loss in natural capital. The difficulty of reversing a shift from kelp forest to urchin barren, with an associated loss in sustainable natural capital, is an example. Management controls are modeled. I find that systemic and symptomatic, and press and pulse, controls can be complementary. Restorative controls tend to be more difficult or costly than preventative ones. Adaptive management, favoring flexible, often preventative, controls, creates option value, lowering control costs and/or losses in sustainable natural capital. Two cases are used to illustrate, Tasmania, Australia and Haida Gwaii, Canada.

Big City Living: A Global Meta-Analysis Reveals Positive Impact of Urbanization on Body Size in Lizards

Urban environments pose different selective pressures than natural ones, leading to changes in animal behavior, physiology, and morphology. Understanding how animals respond to urbanization could inform the management of urban habitats. Non-avian reptiles have important roles in ecosystems worldwide, yet their responses to urbanization have not been as comprehensively studied as those of mammals and birds. However, unlike mammals and birds, most reptiles cannot easily move away from disturbances, making the selective pressure to adapt to urban environments especially strong. In recent years, there has been a surge in research on the responses of lizards to urbanization, yet no formal synthesis has determined what makes an urban lizard, in other words, which phenotypic traits are most likely to change with urbanization and in which direction? Here, we present a qualitative synthesis of the literature and a quantitative phylogenetic meta-analysis comparing phenotypic traits between urban and non-urban lizard populations. The most robust finding from our analysis is that urban lizards are larger than their non-urban counterparts. This result remained consistent between sexes and taxonomic groups. Hence, lizards that pass through the urban filter have access to better resources, more time for foraging, and/or there is selection on attaining a larger body size. Other results included an increase in the diameters of perches used and longer limb and digit lengths, although this may be a result of increased body size. Urban lizards were not bolder, more active or exploratory, and did not differ in immune responses than non-urban populations. Overall, studies are biased to a few geographic regions and taxa. More than 70% of all data came from three species of anoles in the family Dactyloidae, making it difficult to generalize patterns to other clades. Thus, more studies are needed across multiple taxa and habitats to produce meaningful predictions that could help inform conservation and management of urban ecological communities.

Predatory functional responses under increasing temperatures of two life stages of an invasive gecko

The direct effects of temperature increases and differences among life-history might affect the impacts of native and invasive predators on recipient communities. Comparisons of functional responses can improve our understanding of underlying processes involved in altering species interaction strengths and may predict the effect of species invading new communities. Therefore, we investigated the functional responses of the mourning gecko Lepidodactylus lugubris (Duméril & Bibron, 1836) to explore how temperature, body-size and prey density alter gecko predatory impacts in ecosystems. We quantified the functional responses of juvenile and adult geckos in single-predator experiments at 20, 23 and 26 °C. Both displayed saturating Type-II functional responses, but juvenile functional responses and the novel Functional Response Ratio were positively affected by temperature as juvenile attack rates (a) increased as a function of increased temperature. Handling times (h) tended to shorten at higher temperature for both predator stages. We demonstrate that the effects of temperature on functional responses of geckos differ across ontogeny, perhaps reflecting life-history stages prioritising growth and maturation or body maintenance. This indicates that temperature-dependent gecko predatory impacts will be mediated by population demographics. We advocate further comparisons of functional responses to understand the invasiveness and future predatory impacts of geckos, and other invasive species globally, as temperatures change.

Relationship between the Paradox of Enrichment and the Dynamics of Persistence and Extinction in Prey-Predator Systems

The paradox of the enrichment phenomenon, considered one of the main counterintuitive observations in ecology, likely destabilizes predator–prey dynamics by increasing the nutrition of the prey. We use two systems to study the occurrence of the paradox of enrichment: The prey–predator system and the one prey, two predators system, with Holling type I and type II functional and numerical responses. We introduce a new approach that involves the connection between the occurrence of the enrichment paradox and persistence and extinction dynamics. We apply two main analytical techniques to study the persistence and extinction dynamics of two and three trophics, respectively. The linearity and nonlinearity of functional and numerical responses plays important roles in the occurrence of the paradox of enrichment. We derive the persistence and extinction conditions through the carrying capacity parameter, and perform some numerical simulations to demonstrate the effects of the paradox of enrichment when increasing carrying capacity.

Seed ingestion and germination in rattlesnakes: overlooked agents of rescue and secondary dispersal

Seed dispersal is a key evolutionary process and a central theme in the population ecology of terrestrial plants. The primary producers of most land-based ecosystems are propagated by and maintained through various mechanisms of seed dispersal that involve both abiotic and biotic modes of transportation. By far the most common biotic seed transport mechanism is zoochory, whereby seeds, or fruits containing them, are dispersed through the activities of animals. Rodents are one group of mammals that commonly prey on seeds (granivores) and play a critical, often destructive, role in primary dispersal and the dynamics of plant communities. In North America, geomyid, heteromyid and some sciurid rodents have specialized cheek pouches for transporting seeds from plant source to larder, where they are often eliminated from the pool of plant propagules by consumption. These seed-laden rodents are commonly consumed by snakes as they forage, but unlike raptors, coyotes, bobcats, and other endothermic predators which eat rodents and are known or implicated to be secondary seed dispersers, the role of snakes in seed dispersal remains unexplored. Here, using museum-preserved specimens, we show that in nature three desert-dwelling rattlesnake species consumed heteromyids with seeds in their cheek pouches. By examining the entire gut we discovered, furthermore, that secondarily ingested seeds can germinate in rattlesnake colons. In terms of secondary dispersal, rattlesnakes are best described as diplochorous. Because seed rescue and secondary dispersal in snakes has yet to be investigated, and because numerous other snake species consume granivorous and frugivorous birds and mammals, our observations offer direction for further empirical studies of this unusual but potentially important channel for seed dispersal.

The Influence of Sex and Season on Conspecific Spatial Overlap in a Large, Actively-Foraging Colubrid Snake

Understanding the factors influencing the degree of spatial overlap among conspecifics is important for understanding multiple ecological processes. Compared to terrestrial carnivores, relatively little is known about the factors influencing conspecific spatial overlap in snakes, although across snake taxa there appears to be substantial variation in conspecific spatial overlap. In this study, we described conspecific spatial overlap of eastern indigo snakes (Drymarchon couperi) in peninsular Florida and examined how conspecific spatial overlap varied by sex and season (breeding season vs. non-breeding season). We calculated multiple indices of spatial overlap using 6- and 3-month utilization distributions (UD) of dyads of simultaneously adjacent telemetered snakes. We also measured conspecific UD density values at each telemetry fix and modeled the distribution of those values as a function of overlap type, sex, and season using generalized Pareto distributions. Home range overlap between males and females was significantly greater than overlap between individuals of the same sex and male home ranges often completely contained female home ranges. Male home ranges overlapped little during both seasons, whereas females had higher levels of overlap during the non-breeding season. The spatial patterns observed in our study are consistent with those seen in many mammalian carnivores, in which low male-male overlap and high inter-sexual overlap provides males with greater access to females. We encourage additional research on the influence of prey availability on conspecific spatial overlap in snakes as well as the behavioral mechanisms responsible for maintaining the low levels of overlap we observed.

Ecological aspects of the horned viper, Cerastes cerastes gasperettii in the central region of Saudi Arabia

Feeding ecology of any species can help us to understand its natural history, ecological requirements and approaches involved in searching for food. Feeding ecology and sexual dimorphism in the horned viper, Cerastes cerastes gasperetti collected from the Al-Thumama area, central region of Saudi Arabia was described. The gut content of Cerastes c. gasperetti mainly consisted of rodents (70%) in addition to arthropods (15%) and lizards (10%). Least sexual size dimorphism was noticed in the species in terms of total length. Significant difference was noticed between males and females in terms of two correlation points vent tail length (VT) and total length (TL) with the males attaining a larger size (P < 0.05). The mean number of the dorsal body scales, ventrals and subcaudals for the females was 102, 156 and 33 scales respectively which were significantly different from respective ones in males 95, 160 and 38 scales. There are many aspects of the feeding of this snake that remain unknown and further studies are clearly needed.

Problems with mitigation translocation of herpetofauna

Mitigation translocation of nuisance animals is a commonly used management practice aimed at resolution of human-animal conflict by removal and release of an individual animal. Long considered a reasonable undertaking, especially by the general public, it is now known that translocated subjects are negatively affected by the practice. Mitigation translocation is typically undertaken with individual adult organisms and has a much lower success rate than the more widely practiced conservation translocation of threatened and endangered species. Nonetheless, the public and many conservation practitioners believe that because population-level conservation translocations have been successful that mitigation translocation can be satisfactorily applied to a wide variety of human-wildlife conflict situations. We reviewed mitigation translocations of reptiles, including our own work with 3 long-lived species (Gila monsters [Heloderma suspectum], Sonoran desert tortoises [Gopherus morafkai], and western diamond-backed rattlesnakes [Crotalus atrox]). Overall, mitigation translocation had a low success rate when judged either by effects on individuals (in all studies reviewed they exhibited increased movement or increased mortality) or by the success of the resolution of the human-animal conflict (translocated individuals often returned to the capture site). Careful planning and identification of knowledge gaps are critical to increasing success rates in mitigation translocations in the face of increasing pressure to find solutions for species threatened by diverse anthropogenic factors, including climate change and exurban and energy development.

Mating systems, reproductive success, and sexual selection in secretive species: a case study of the western diamond-backed rattlesnake, Crotalus atrox

Long-term studies of individual animals in nature contribute disproportionately to our understanding of the principles of ecology and evolution. Such field studies can benefit greatly from integrating the methods of molecular genetics with traditional approaches. Even though molecular genetic tools are particularly valuable for species that are difficult to observe directly, they have not been widely adopted. Here, we used molecular genetic techniques in a 10-year radio-telemetric investigation of the western diamond-backed rattlesnake (Crotalus atrox) for an analysis of its mating system and to measure sexual selection. Specifically, we used microsatellite markers to genotype 299 individuals, including neonates from litters of focal females to ascertain parentage using full-pedigree likelihood methods. We detected high levels of multiple paternity within litters, yet found little concordance between paternity and observations of courtship and mating behavior. Larger males did not father significantly more offspring, but we found evidence for size-specific male-mating strategies, with larger males guarding females for longer periods in the mating seasons. Moreover, the spatial proximity of males to mothers was significantly associated with reproductive success. Overall, our field observations alone would have been insufficient to quantitatively measure the mating system of this population of C. atrox, and we thus urge more widespread adoption of molecular tools by field researchers studying the mating systems and sexual selection of snakes and other secretive taxa.

Could controlling mammalian carnivores lead to mesopredator release of carnivorous reptiles?

Emerging evidence increasingly illustrates the importance of a holistic, rather than taxon-specific, approach to the study of ecological communities. Considerable resources are expended to manage both introduced and native mammalian carnivores to improve conservation outcomes; however, management can result in unforeseen and sometimes catastrophic outcomes. Varanid lizards are likely to be apex- or mesopredators, but being reptiles are rarely considered by managers and researchers when investigating the impacts of mammalian carnivore management. Instances of mesopredator release have been described for Varanus gouldii as a result of fox and cat management in Australia, with cascading effects on faunal community structure. A meta-analysis showing extensive dietary niche overlap between varanids, foxes and cats plus a review of experimental and circumstantial evidence suggests mesopredator release of V. gouldii and about five other medium to large species of varanid lizard is likely in other regions. This highlights the need for managers to adopt a whole-of-community approach when attempting to manage predators for sustained fauna conservation, and that additional research is required to elucidate whether mesopredator release of varanids is a widespread consequence of carnivore management, altering the intended faunal responses.

Tail shedding in island lizards [Lacertidae, Reptilia]: decline of antipredator defenses in relaxed predation environments

The ability of an animal to shed its tail is a widespread antipredator strategy among lizards. The degree of expression of this defense is expected to be shaped by prevailing environmental conditions including local predation pressure. We test these hypotheses by comparing several aspects of caudal autotomy in 15 Mediterranean lizard taxa existing across a swath of mainland and island localities that differ in the number and identity of predator species present. Autotomic ease varied substantially among the study populations, in a pattern that is best explained by the presence of vipers. Neither insularity nor the presence of other types of predators explain the observed autotomy rates. Final concentration of accumulated tail muscle lactate and duration of movement of a shed tail, two traits that were previously thought to relate to predation pressure, are in general not shaped by either predator diversity or insularity. Under conditions of relaxed predation selection, an uncoupling of different aspects of caudal autotomy exists, with some elements (ease of autotomy) declining faster than others (duration of movement, lactate concentration). We compared rates of shed tails in the field against rates of laboratory autotomies conducted under standardized conditions and found very high correlation values (r > 0.96). This suggests that field autotomy rates, rather than being a metric of predatory attacks, merely reflect the innate predisposition of a taxon to shed its tail.