A battle of wits? Problem-solving abilities in invasive eastern grey squirrels and native Eurasian red squirrels

'Ripple effects' of urban environmental characteristics on cognitive performances in Eurasian red squirrels

Urban areas are expanding exponentially, leading more species of wildlife living in urban environments. Urban environmental characteristics, such as human disturbance, induce stress for many wildlife and have been shown to affect some cognitive traits, such as innovative problem-solving performance. However, because different cognitive traits have common cognitive processes, it is possible that urban environmental characteristics may directly and indirectly affect related cognitive traits (the ripple effect hypothesis). We tested the ripple effect hypothesis in urban Eurasian red squirrels residing in 11 urban areas that had different urban environmental characteristics (direct human disturbance, indirect human disturbance, areas of green coverage and squirrel population size). These squirrels were innovators who had previously repeatedly solved a food extraction task (the original task). Here, we examined whether and how urban environmental characteristics would directly and indirectly influence performance in two related cognitive traits, generalisation and (long-term) memory. The generalisation task required the innovators to apply the learned successful solutions when solving a similar but novel problem. The memory task required them to recall the learned solution of the original task after an extended period of time. Some of the selected urban environmental characteristics directly influenced the task performance, both at the population level (site) and at individual levels. Urban environmental characteristics, such as increased direct and indirect human disturbance, decreased the proportion of success in solving the generalisation task or the memory task at the population (site) level. Increased direct human disturbance and less green coverage increased the solving efficiency at individual levels. We also found an indirect effect in one of the urban environmental characteristics, indirect human disturbance, in the generalisation task, but not the memory task. Such an effect was only seen at the individual level but not at the population level; indirect human disturbance decreased the first original latency, which then decreased the generalisation latency across successes. Our results partially support the ripple effect hypothesis, suggesting that urban environmental characteristics are stressors for squirrels and have a greater impact on shaping cognitive performance than previously shown. Together, these results provide a better understanding of cognitive traits that support wildlife in adapting to urban environments.

Trained quantity discrimination in invasive red-eared slider and a comparison with the native stripe-necked turtle

Little is known about the behavioral and cognitive traits that best predict invasion success. Evidence is mounting that cognitive performance correlates with survival and fecundity, two pivotal factors for the successful establishment of invasive populations. We assessed the quantity discrimination ability of the globally invasive red-eared slider (Trachemys scripta elegans). We further compared it to that of the native stripe-necked turtle (Mauremys sinensis), which has been previously evaluated for its superior quantity discrimination ability. Specifically, our experimental designs aimed to quantify the learning ability as numerosity pairs increased in difficulty (termed fixed numerosity tests), and the immediate response when turtles were presented with varied challenges concurrently in the same tests (termed mixed numerosity tests). Our findings reaffirm the remarkable ability of freshwater turtles to discern numerical differences as close as 9 vs 10 (ratio = 0.9), which was comparable to the stripe-necked turtle's performance. However, the red-eared slider exhibited a moderate decrease in performance in high ratio tests, indicating a potentially enhanced cognitive capacity to adapt to novel challenges. Our experimental design is repeatable and is adaptable to a range of freshwater turtles. These findings emphasize the potential importance of cognitive research to the underlying mechanisms of successful species invasions.

Grow up, be persistent, and stay focused: keys for solving foraging problems by free-ranging possums

Individuals within a species often vary in both their problem-solving approach and ability, affecting their capacity to access novel food resources. Testing problem-solving in free-ranging individuals is crucial for understanding the fundamental ecological implications of problem-solving capacity. To examine the factors affecting problem-solving in free-ranging animals, we presented three food-extraction tasks of increasing difficulty to urban common brushtail possums (Trichosurus vulpecula). We quantified two measures of problem-solving performance: trial outcome (success/failure) and time to solve and tested the influence of a range of potential drivers, including individual traits (personality, body weight, sex, and age), mechanistic behaviors that quantify problem-solving approach (work time, functional behavior time, behavioral diversity, and flexibility), and prior experience with the puzzles. We found that mechanistic behaviors were key drivers of performance. Individuals displaying greater persistence (higher work and functional behavior time) were more likely to solve a food-extraction task on their first attempt. Individuals also solved problems faster if they were more persistent and had lower behavioral flexibility. Personality indirectly affected time to solve one of the three problems by influencing time allocated to functional behaviors. Finally, adults solved the most difficult problem faster than juveniles. Overall, our study provides rare insight into the drivers underlying the problem-solving performance of wild animals. Such insight could be used to improve management strategies and conservation efforts, such as food or bait deployment, tailored to suit the innovative foraging abilities of target individuals in new and changing environments.

Making meaning from fragmentary fossils: Early Homo in the Early to early Middle Pleistocene

In celebration of the 50th anniversary of the Journal of Human Evolution, we re-evaluate the fossil record for early Homo (principally Homo erectus, Homo habilis, and Homo rudolfensis) from early diversification and dispersal in the Early Pleistocene to the ultimate demise of H. erectus in the early Middle Pleistocene. The mid-1990s marked an important historical turning point in our understanding of early Homo with the redating of key H. erectus localities, the discovery of small H. erectus in Asia, and the recovery of an even earlier presence of early Homo in Africa. As such, we compare our understanding of early Homo before and after this time and discuss how the order of fossil discovery and a focus on anchor specimens has shaped, and in many ways biased, our interpretations of early Homo species and the fossils allocated to them. Fragmentary specimens may counter conventional wisdom but are often overlooked in broad narratives. We recognize at least three different cranial and two or three pelvic morphotypes of early Homo. Just one postcranial morph aligns with any certainty to a cranial species, highlighting the importance of explicitly identifying how we link specimens together and to species; we offer two ways of visualizing these connections. Chronologically and morphologically H. erectus is a member of early Homo, not a temporally more recent species necessarily evolved from either H. habilis or H. rudolfensis. Nonetheless, an ancestral-descendant notion of their evolution influences expectations around the anatomy of missing elements, especially the foot. Weak support for long-held notions of postcranial modernity in H. erectus raises the possibility of alternative drivers of dispersal. New observations suggest that the dearth of faces in later H. erectus may mask taxonomic diversity in Asia and suggest various later mid-Pleistocene populations could derive from either Asia or Africa. Future advances will rest on the development of nuanced ways to affiliate fossils, greater transparency of implicit assumptions, and attention to detailed life history information for comparative collections; all critical pursuits for future research given the great potential they have to enrich our evolutionary reconstructions for the next fifty years and beyond.

Interactions between native and invasive species: A systematic review of the red squirrel-gray squirrel paradigm

The eastern gray squirrel (Sciurus carolinensis) has been labeled as one of the 100 worst invasive alien species by the IUCN. In Europe, the species has been introduced to Britain, Ireland and Italy, and its subsequent spread has resulted in wide-scale extinction of native Eurasian red squirrels (Sciurus vulgaris) from the areas colonized by the gray squirrel. This replacement of a native by an alien competitor is one of the best documented cases of the devastating effects of biological invasions on native fauna. To understand how this replacement occurs, we present a systematic review of the literature on competition and interactions between red and gray squirrels. We describe the patterns of red and gray squirrel distribution in those parts of Europe where gray squirrels occur and summarize the evidence on the different processes and mechanisms determining the outcome of competition between the native and alien species including the influence of predators and pathogens. Some of the drivers behind the demise of the red squirrel have been intensively studied and documented in the past 30 years, but recent field studies and mathematical models revealed that the mechanisms underlying the red-gray paradigm are more complex than previously thought and affected by landscape-level processes. Therefore, we consider habitat type and multi-species interactions, including host-parasite and predator-prey relationships, to determine the outcome of the interaction between the two species and to better address gray squirrel control efforts.

Can we build a neuroecology of innovativeness similar to that pioneered by David Sherry for spatial memory?

David Sherry's pioneering work on the neuroecology of spatial memory has three characteristics that could inspire studies on other cognitive processes: it was grounded in a robust prior literature in psychology and neuroscience; it identified several natural history contexts in which repeated independent evolution of spatial memory differences had occurred in different clades; it involved a precise cognitive ability with a precise neural substrate. We discuss the application of these three principles to a more domain-general trait-innovation. We argue that targeting the caudolateral nidopallium and its connected areas, favoring problem-solving over reversal learning as an experimental assay, and focusing on situations that involve environmental change, such as urbanization and invasion, can help the study of innovation progress, like the field of spatial memory has since 1989.

Characteristics of urban environments and novel problem-solving performance in Eurasian red squirrels

Urban environments can be deemed 'harsh' for some wildlife species, but individuals frequently show behavioural flexibility to cope with challenges and demands posed by life in the city. For example, urban animals often show better performance in solving novel problems than rural conspecifics, which helps when using novel resources under human-modified environments. However, which characteristics of urban environments fine-tune novel problem-solving performance, and their relative importance, remain unclear. Here, we examined how four urban environmental characteristics (direct human disturbance, indirect human disturbance, size of green coverage and squirrel population size) may potentially influence novel problem-solving performance of a successful 'urban dweller', the Eurasian red squirrel, by presenting them with a novel food-extraction problem. We found that increased direct human disturbance, indirect human disturbance and a higher squirrel population size decreased the proportion of solving success at the population level. At the individual level, an increase in squirrel population size decreased the latency to successfully solve the novel problem the first time. More importantly, increased direct human disturbance, squirrel population size and experience with the novel problem decreased problem-solving time over time. These findings highlight that some urban environmental characteristics shape two phenotypic extremes in the behaviour-flexibility spectrum: individuals either demonstrated enhanced learning or they failed to solve the novel problem.

Animal cognition in an urbanised world

Explaining how animals respond to an increasingly urbanised world is a major challenge for evolutionary biologists. Urban environments often present animals with novel problems that differ from those encountered in their evolutionary past. To navigate these rapidly changing habitats successfully, animals may need to adjust their behaviour flexibly over relatively short timescales. These behavioural changes, in turn, may be facilitated by an ability to acquire, store and process information from the environment. The question of how cognitive abilities allow animals to avoid threats and exploit resources (or constrain their ability to do so) is attracting increasing research interest, with a growing number of studies investigating cognitive and behavioural differences between urban-dwelling animals and their non-urban counterparts. In this review we consider why such differences might arise, focusing on the informational challenges faced by animals living in urban environments, and how different cognitive abilities can assist in overcoming these challenges. We focus largely on birds, as avian taxa have been the subject of most research to date, but discuss work in other species where relevant. We also address the potential consequences of cognitive variation at the individual and species level. For instance, do urban environments select for, or influence the development of, particular cognitive abilities? Are individuals or species with particular cognitive phenotypes more likely to become established in urban habitats? How do other factors, such as social behaviour and individual personality, interact with cognition to influence behaviour in urban environments? The aim of this review is to synthesise current knowledge and identify key avenues for future research, in order to improve our understanding of the ecological and evolutionary consequences of urbanisation.

Cognitive Performance of Wild Eastern Gray Squirrels (Sciurus carolinensis) in Rural and Urban, Native, and Non-native Environments

Enhanced cognitive ability has been shown to impart fitness advantages to some species by facilitating establishment in new environments. However, the cause of such enhancement remains enigmatic. Enhanced cognitive ability may be an adaptation occurring during the establishment process in response to new environments or, alternatively, such ‘enhancement’ may merely reflect a species’ characteristic. Based on previous findings that have shown ‘enhanced’ cognitive ability (i.e., higher success rate in solving novel food-extraction problems or, ‘innovation’) in Eastern gray squirrels (Sciurus carolinensis), a successful mammalian invader and urban dweller, we used an intraspecific comparative paradigm to examine the cause of their ‘enhanced’ cognitive ability. We conducted a field study to compare cognitive performance of free-ranging squirrels residing in rural and urban habitats in native (United States) and non-native environments (United Kingdom). By using established tasks, we examined squirrels’ performance in easy and difficult, novel food-extraction problems (innovation), a motor memory recall test of the difficult problem, and a spatial learning task. We found that the four groups of squirrels showed comparable performance in most measures. However, we also found that the native urban squirrels showed: (1) higher success rate on the first visit for the difficult problem than the non-native urban squirrels; (2) some evidence for higher recall latency for the difficult problem after an extended period than the non-native rural squirrels; and (3) learning when encountering the same difficult problem. These results suggest that the previously reported ‘enhanced’ performance is likely to be a general characteristic and thus, a pre-adaptive phenotypic trait that brings fitness advantages to this species in a new environment. Despite this, some cognitive abilities in gray squirrels such as solving novel problems has undergone mild variation during the adaptive process in new environments.

Behavioral flexibility: A review, a model, and some exploratory tests

This paper aimed to explore and clarify the concept of behavioral flexibility. A selective literature review explored how the concept of behavioral flexibility has been used in ways that range from acknowledging the fact that animals’ behavior is not always bounded by instinctual constraints, to describing the variation between species in their capacity for innovative foraging, a capacity that has repeatedly been linked to having a brain larger than would be predicted from body size. This wide range of usages of a single term has led to some conceptual confusion. We sought to find a more precise meaning for behavioral flexibility by representing it within a simple formal model of problem solving. The key to our model is to distinguish between an animal’s state of knowledge about the world and its observable behavior, using a construct of response strength to represent that underlying knowledge. We modelled behavioral flexibility as a parameter in the function that transforms response strengths into observable response probabilities. We tested this model in simulations based on some recent experimental work on animal problem solving. Initial results showed that parametric manipulation can mimic some of the behavioral effects that have been attributed to flexibility.

Innovation in a native Australian rodent, the fawn-footed mosaic-tailed rat (Melomys cervinipes)

Innovation is the ability to use a new behaviour, or use an existing behaviour in a new context. Innovation, as an aspect of behavioural flexibility, could be important for allowing animals to cope with rapid environmental changes. Surprisingly, few studies have focused on how innovation ability is affected by task complexity. We investigated innovation ability across multiple tasks of varying complexity in a native Australian rodent, the fawn-footed mosaic-tailed rat (Melomys cervinipes). We predicted that mosaic-tailed rats would be capable of innovating because they live in complex habitats and can exploit disturbed and changing environments. However, we also predicted that the success rate of innovating would decrease as task complexity increased. Mosaic-tailed rats were exposed to six novel problems: cylinder, matchbox, obstruction test, pillar, tile and lever (the last three presented in a Trixie dog activity board), which represented increasing complexity. We counted the number of individuals that could solve at least one task, compared individuals for solving efficiency and latency to solve, and compared the solving success of each task. All mosaic-tailed rats could innovate. However, solving success differed between individuals, with some solving every task and others only solving one. Solving success rate was significantly higher in the simplest task (pillar) compared to the most complicated task (lever). There was no effect of sex or sampling condition on innovation. This study is the first to demonstrate innovation ability across task complexity in an Australian rodent and provides promising avenues for future studies of innovation.