Valentina S. A. Mella

A practical guide to avoid giving up on giving-up densities

The giving-up density (GUD) framework provides a powerful experimental approach with a strong theoretical underpinning to quantify foraging outcomes in heterogeneous landscapes. Since its inception, the GUD approach has been applied successfully to a vast range of foraging species and foraging scenarios. However, its application is not simple, as anyone who has tried to use it for the first time might attest. Limitations of the technique were noted at its conception, yet only the artificiality of the patches, the appropriateness of the food resource, and the possibility of multiple visiting foragers were identified. Here we show the current uses of GUD and outline the practical benefits as well as the often overlooked limitations of the technique. We define seven major points that need to be addressed when applying this methodology: (1) the curvilinearity between harvest rate and energy, (2) the energetic state of the forager, (3) the effect of group foraging, (4) food quality and substrate properties, (5) the predictability of the patch, (6) behavioral traits of the forager, and (7) nontarget species. We also suggest how GUD experiments can be enhanced by incorporating complementary methods (such as cameras) to better understand the foraging processes involved in the GUD itself. We conclude that the benefits of using GUD outweigh the costs, but that its limitations should not be ignored. Incorporating new methods when using GUD can potentially offer novel and important insights into the study of foraging behavior.

Personality affects the foraging response of a mammalian herbivore to the dual costs of food and fear

Predators attack and plants defend, so herbivores face the dilemma of how to eat enough without being eaten. But do differences in the personality of herbivores affect the foraging choices of individuals? We explored the ecological impact of personality in a generalist herbivore, the brushtail possum (Trichosurus vulpecula). After quantifying personality traits in wild individuals brought temporarily into captivity, we tested how these traits altered foraging by individuals when free-ranging in their natural habitat. To measure their responses to the dual costs of predation risk and plant toxin, we varied the toxin concentration of food in safe foraging patches against paired, non-toxic risky patches, and used a novel synthesis of a manipulative Giving-Up-Density (GUD) experiment and video behavioural analysis. At the population level, the cost of safe patches pivoted around that of risky patches depending on food toxin concentration. At the individual level, boldness affected foraging at risky high-quality food patches (as behavioural differences between bold and shy), and at safe patches only when food toxin concentration was low (as differences in foraging outcome). Our results ecologically validate the personality trait of boldness, in brushtail possums. They also reveal, for the first time, a nuanced link between personality and the way in which individuals balance the costs of food and fear. Importantly, they suggest that high plant defence effectively attenuates differences in foraging behaviour arising from variation in personality, but poorly defended plants in safe areas should be differentially subject to herbivory depending on the personality of the herbivore.

Ventilatory frequency as a measure of the response of tammar wallabies (Macropus eugenii) to the odour of potential predators.

This study uses changes in ventilatory frequency to quantify the physiological response of an Australian terrestrial herbivore, the tammar wallaby (Macropus eugenii), to olfactory cues suggesting the presence of potential predators. Ventilatory frequency proved to be a quantifiable measure to assess the response of this macropod marsupial to olfactory cues. Ventilatory frequency increased from mean resting levels of 45 ± 5.1 breaths min–1 to 137 ± 11.2 breaths min–1 during the first minute of exposure to all odours. These physiological responses diminished over time, with ventilatory frequency in the first minute after introduction of the scents greater than that during the subsequent four, suggesting that the initial reaction was due to disturbance and was investigative in nature. However, the ratio of ventilatory frequency in the remaining 4 min after introduction of the odours compared with before was greater for fox (3.58 ± 0.918) and cat (2.44 ± 0.272) odours than for snake (2.27 ± 0.370), distilled water (1.81 ± 0.463) and quoll (1.71 ± 0.245) odours, suggesting that fox and cat odour provoked a greater response. However, the wallabies’ response to the odour of these introduced predators and to horse odour (2.40 ± 0.492) did not differ. Our study indicates that a long period of co-history with particular predators is not a prerequisite for detection of potentially threatening species. We do not find any support for the hypothesis that an inability to interpret olfactory cues to detect and respond to potential predation by introduced predators is responsible for the decline of these macropod marsupials.

Predator odour does not influence trappability of southern brown bandicoots (Isoodon obesulus) and common brushtail possums (Trichosurus vulpecula)

Predators cause changes in the behaviour of many prey species. This study investigated whether trappability of wild southern brown bandicoots (Isoodon obesulus) and common brushtail possums (Trichosurus vulpecula) was influenced by odour cues suggesting the presence of potential predators. Trapping success was compared between traps with predator scents and controls in two different experiments. The first measured trapping success of single clean traps, traps with unfamiliar herbivore (horse) odour and traps scented with predator (fox, cat, quoll and dingo) odours, while the second offered three choices (fox, dingo and clean traps) simultaneously. Frequency of capture of bandicoots and possums was not influenced by odour, mass or sex in either experiment. The lack of avoidance response observed in this study suggests that neither southern brown bandicoots nor common brushtail possums alter foraging behaviour in response to olfactory cues that suggest the presence of exotic or Australian predators. This is consistent with previous studies, which indicate that native Australian marsupials do not avoid predator odours.

Effective field-based methods to quantify personality in brushtail possums (Trichosurus vulpecula)

Abstract Context. There is growing recognition of the importance of animal personality in wildlife ecology and management. Individuals that differ consistently in their behaviours from others of the same population are considered to exhibit different personalities. Personality can be easily quantified with repeat tests on animals held in captivity. However, captive-based tests may not always be possible for logistical reasons and tests conducted in captivity can alter naturally occurring behaviours. Development of methods to assess personality in the field over short-term capture is an important alternative to long-term captive tests. Aims. To develop and refine field-based tests that can be used to easily define personality traits of wild common brushtail possum (Trichosurus vulpecula), eliminating the need to bring individuals into captivity for an extended period of time. Methods. We developed a series of short-term capture protocols to quantify personality traits of the common brushtail possum in the field, using handling-bag tests, modified open-field tests, trap-behaviour tests and release tests. Key results. Personality traits of brushtail possums could be measured in several different ways, but refining methods was crucial to reveal traits efficiently. Behaviours related to several personality traits (boldness, docility and activity/exploration) could be measured rapidly in the field with our methods, and were repeatable over time. Conclusions. Rapid, reliable and easy-to-perform quantification of personality of brushtail possums in the field is possible. This may sometimes be the only available option, and has advantages over long captive trials in terms of animal welfare and cost. Implications. The present study provides a baseline for future research on the personality of common brushtail possums and its application to management and control strategies for the species.

Effects of historically familiar and novel predator odors on the physiology of an introduced prey

Abstract Predator odors can elicit fear responses in prey and predator odor recognition is generally associated with physiological responses. Prey species are often more likely to respond to the odor of familiar rather than alien predators. However, predator naïvety in an introduced prey species has rarely been investigated. We examined the physiological response, as shown by changes in ventilatory variables, of an introduced terrestrial herbivore, the European rabbit Oryctolagus cuniculus, in Australia, to the odor of potential predators and to control odors (distilled water and horse), to explore if responses were limited to historical (cat and fox) predators, or extended to historically novel predators (snake and quoll). All odors except distilled water elicited a response, with rabbits showing long-term higher respiratory frequencies and lower tidal volumes after introduction of the odors, indicating an increase in alertness. However, the intensity of the rabbits’ reaction could not be directly linked to any pattern of response with respect to the history of predator–prey relationships. Rabbits exhibited significantly stronger reactions in response to both cat and quoll odors than they did to distilled water, but responses to horse, fox, and snake odor were similar to that of water. Our results show that the introduced rabbit can respond to both historical and novel predators in Australia, and suggest that shared evolutionary history is not necessarily a prerequisite to predator odor recognition.

Do Individual Differences in Behavior Influence wild Rodents more than Predation Risk

Prey can enhance their survival by eliciting an appropriate response to predators. Theoretically, prey should distinguish odors of predators and nonpredators. The manifestation of defensive antipredator behaviors has been extensively researched in domestic species (i.e., the relationship between laboratory-bred rats and domestic cats). However, little is known about the expression of these behaviors in wild rodents. Studies have so far focused on quantitative assessments of cost—benefit trade-offs or giving-up densities. We examined the expression of finescale defensive behaviors in Arnhem rock rats (Zyzomys maini) in response to fecal cues from 2 predators (the northern quoll [Dasyurus hallucatus] and the dingo [Canis dingo]), a nonpredator (the short-eared rock-wallaby [Petrogale brachyotis]), and a control (water). We adapted a predator-odor avoidance apparatus that has been widely used for domestic rodent studies to film the behavior of wild rock rats in a captive environment. Rock rats did not alter their behavior in the presence of odors of nonpredators, predators, or controls. In the current study, individual rock rats behaved in a consistent manner across time, and we identified 3 individually consistent behaviors which may suggest the existence of personality traits in this species. We suggest that these individual differences may influence wild rock rat behavior more than predation risk. These differences should therefore be taken into consideration when investigating behavioral responses to predators in wild populations.

Visit, consume and quit: Patch quality affects the three stages of foraging

Foraging is a three-stage process during which animals visit patches, consume food and quit. Foraging theory exploring relative patch quality has mostly focused on patch use and quitting decisions, ignoring the first crucial step for any forager: finding food. Yet, the decision to visit a patch is just as important as the decision to quit, as quitting theories can only be used if animals visit patches in the first place. Therefore, to better understand the foraging process and predict its outcomes, it is necessary to explore its three stages together. We used the common brushtail possum (Trichosurus vulpecula) as a model to investigate foraging decisions in response to food varying in quality. In particular, we tested whether patch nutritional quality affected the following: (1) patch visits; (2) behaviours at the patch during a foraging visit; and (3) patch quitting decisions (quantified using giving up density-GUD). Free-ranging possums were presented with diets varying in nitrogen content and concomitantly volatile organic compound (VOC) composition at feeding stations in the wild. We found that possums were able to distinguish between different quality foods from afar, despite the location of the diets changed daily. Possums used VOC (i.e. odour cues) emitted by the diets to find and select patches from a distance. High-quality diets with higher protein and lower fibre were visited more often and for longer. Possums spent more time foraging on diets high in nutritional content, resulting in lower GUDs. Our study provides important quantitative evidence that foraging efficiency plays out during all the three stages of the foraging process (i.e. visit, consume and quit), and demonstrates the significance of considering all these stages together in future studies and foraging models. Sensory cues such as food odours play a critical role in helping foragers, including mammalian herbivores, find high-quality food. This allows foragers to make quick, accurate and important decisions about food patches well before patch quitting decisions come into play.

Foxes in trees: a threat for Australian arboreal fauna?

We document the first evidence of tree climbing by red foxes (Vulpes vulpes) in Australia. Camera traps recorded foxes in trees on the Liverpool Plains, New South Wales. This finding prompts a reassessment of the impact that this invasive predator has on Australian fauna: from purely terrestrial to also potentially arboreal.

You Are What You Eat: The Interplay Between Animal Personality and Foraging Ecology

Foraging animals face a constant dilemma—balancing the need to acquire food without putting themselves at risk. Individuals within a population vary in personality, and as a consequence they differ in how they perceive, interpret, and respond to foraging choices. Despite the inherent necessity for animals to acquire food, and thus direct link to fitness, the interplay between foraging and animal personality has received little attention. In this chapter, we first summarize studies that have integrated animal personality and foraging decisions in wild and captive populations, then highlight research deficiencies and suggest future research opportunities to unite the study of personality and foraging ecology. We also introduce and discuss patch-use theory as a specific research methodology that may help to facilitate future assessment of individual foraging differences. We believe that further investigation into the influence of personality on individual foraging decisions is key to understanding a valuable and largely understudied aspect of overall animal success and the ecological consequences of personality.