Animal learning and memory: an integration of cognition and ecology

Long-term memory in wild falcons

Long-term memory - information retention over long timescales - can allow animals to retain foraging skills and efficiently respond to seasonally available resources and changing environments1. Most long-term memory research is with captive species, focusing on spatial, individual or object recognition, with less known about wild species and the retention of motor task abilities, as in the case of complex foraging skills2,3. We have examined whether wild striated caracaras (Phalcoboenus australis), recently shown to rapidly and flexibly innovate with an eight-task puzzle box4, retain task memories one year later. We found that, despite no reinforcement, caracaras repeated motor techniques that led to their most recent success on tasks the year prior, solving nearly twice as fast as a naïve control group and four times faster than when naïve. Our results suggest long-term memory may be important for non-migratory opportunistic generalists, particularly in remote island environments with seasonally available resources, and further highlight how striated caracaras are promising candidates for avian cognitive studies.

Linking pollen limitation and seed dispersal effectiveness

Seed production and dispersal are crucial ecological processes impacting plant demography, species distributions and community assembly. Plant-animal interactions commonly mediate both seed production and seed dispersal, but current research often examines pollination and seed dispersal separately, which hinders our understanding of how pollination services affect downstream dispersal services. To fill this gap, we propose a conceptual framework exploring how pollen limitation can impact the effectiveness of seed dispersal for endozoochorous and myrmecochorous plant species. We summarize the quantitative and qualitative effects of pollen limitation on plant reproduction and use Optimal Foraging Theory to predict its impact on the foraging behaviour of seed dispersers. In doing so, we offer a new framework that poses numerous hypotheses and empirical tests to investigate links between pollen limitation and seed dispersal effectiveness and, consequently, post-dispersal ecological processes occurring at different levels of biological organization. Finally, considering the importance of pollination and seed dispersal outcomes to plant eco-evolutionary dynamics, we discussed the implications of our framework for future studies exploring the demographic and evolutionary impacts of pollen limitation for animal-dispersed plants.

Decision-making at the time of parasitism: cowbirds prefer to peck eggs with weaker shells

Interspecific avian brood parasites, like cuckoos and cowbirds, lay their eggs in nests of other species, the hosts, which take over the entire parental care of the parasite's eggs and chicks. This breeding strategy requires decisions that may affect the parasite's reproductive success. During the breeding season, cowbirds search for host nests and revisit them to monitor its progress and parasitize at the time host laying begins. When visiting hosts nests, they repeatedly peck the nest contents trying to destroy one or more eggs. This behaviour favours parasite's offspring by reducing the competition for food with nestmates. We evaluated if the egg-pecking behaviour of female shiny (Molothrus bonariensis) and screaming (M. rufoaxillaris) cowbirds is affected by the strength and the size of the eggs they find in the nest. We presented to wild-caught females artificial clutches with two natural eggs that differ in size and shell strength. We found that female shiny and screaming cowbirds adjusted egg-pecking behaviour based on the strength but not on the size of the eggs. When differences in strength between eggs were high, both cowbird species pecked more frequently the egg with the weaker shell, increasing the probability of a successful puncture. Our results indicate that female cowbirds can discriminate eggs through the strength of the shell, and by choosing the weaker egg to peck, they increase the probability of puncturing.

Poor adult nutrition impairs learning and memory in a parasitoid wasp

Animals have evolved cognitive abilities whose impairment can incur dramatic fitness costs. While malnutrition is known to impact brain development and cognitive functions in vertebrates, little is known in insects whose small brain appears particularly vulnerable to environmental stressors. Here, we investigated the influence of diet quality on learning and memory in the parasitoid wasp Venturia canescens. Newly emerged adults were exposed for 24 h to either honey, 20% sucrose solution, 10% sucrose solution, or water, before being conditioned in an olfactory associative learning task in which an odor was associated to a host larvae (reward). Honey fed wasps showed 3.5 times higher learning performances and 1.5 times longer memory retention than wasps fed sucrose solutions or water. Poor diets also reduced longevity and fecundity. Our results demonstrate the importance of early adult nutrition for optimal cognitive function in these parasitoid wasps that must quickly develop long-term olfactory memories for searching suitable hosts for their progeny.

Long-term memory of relative reward values

Long-term memory can be adaptive as it allows animals to retain information that is crucial for survival, such as the appearance and location of key resources. This is generally examined by comparing choices of stimuli that have value to the animal with those that do not; however, in nature choices are rarely so clear cut. Animals are able to assess the relative value of a resource via direct comparison, but it remains unclear whether they are able to retain this information for a biologically meaningful amount of time. To test this, captive red-footed tortoises (Chelonoidis carbonaria) were first trained to associate visual cues with specific qualities and quantities of food, and their preferences for the different reward values determined. They were then retested after an interval of 18 months. We found that the tortoises were able to retain the information they had learned about the cues as indicators of relative reward values over this interval, demonstrating a memory for the relative quantity and quality of food over an extended period of time. This is likely to impact directly on an animal's foraging decisions, such as the exploitation of seasonally varying resources, with obvious fitness implications for the individual; however, the implications may also extend to the ecological interactions in which the animal is involved, affecting processes such as herbivory and seed dispersal.

The effect of brumation on memory retention

Long-term torpor is an adaptive strategy that allows animals to survive harsh winter conditions. However, the impact that prolonged torpor has on cognitive function is poorly understood. Hibernation causes reduced synaptic activity and experiments with mammals reveal that this can have adverse effects on memories formed prior to hibernation. The impact of brumation, the winter dormancy that is observed in ectotherms, on memory remains unknown. The aim of this study was to examine whether an amphibian, the fire salamander (Salamandra salamandra), was able to retain learned spatial information after a period of brumation. Twelve fire salamanders were trained to make a simple spatial discrimination using a T-maze. All subjects learned the initial task. Upon reaching criterion, half of the subjects were placed into brumation for 100 days while the other half served as controls and were maintained under normal conditions. A post-brumation memory retention test revealed that animals from both conditions retained the learned response. Control tests showed that they solved the task using learned information and not olfactory cues. This finding contrasts with much of the mammalian research and suggests that the processes involved in prolonged torpor may have a fundamentally different impact on memory in mammals and amphibians.

Similarities in spatial cognition in sister species of the striped mouse Rhabdomys originating from different ecological contexts

Variation in spatial cognition is correlated with differences in the environments where animals originate, such that different environments might select for different cognitive ability. We investigated whether three sister species of the striped mouse genus Rhabdomys differed in their spatial cognition. The species originated from three locations across a rainfall gradient in southern Africa, which vary in habitat complexity. We tested individuals in a modified Barnes maze and asked whether the species had different spatial memory and navigation and whether these differences were related to their geographic location. We showed that the species had similar spatial memory and cue use, differing only when external cues were initially removed and during the first probe test of spatial memory. The similarities suggest that the environment of origin is not associated with spatial cognition in Rhabdomys, and that spatial cognition is phylogenetically constrained or there might be similar selection pressures across the distribution.

Ecophysiology of cognition: How do environmentally induced changes in physiology affect cognitive performance?

Cognitive performance is based on brain functions, which have energetic demands and are modulated by physiological parameters such as metabolic hormones. As both environmental demands and environmental energy availability change seasonally, we propose that cognitive performance in free-living animals might also change seasonally due to phenotypic plasticity. This is part of an emerging research field, the 'ecophysiology of cognition': environmentally induced changes in physiological traits, such as blood glucose and hormone levels, are predicted to influence cognitive performance in free-living animals. Energy availability for the brain might change, and as such cognition, with changing energetic demands (e.g. reproduction) and changes of energy availability in the environment (e.g. winter, drought). Individuals spending more energy than they can currently obtain from their environment (allostatic overload type I) are expected to trade off energy investment between cognition and other life-sustaining processes or even reproduction. Environmental changes reducing energy availability might thus impair cognition. However, selection pressures such as predation risk, mate choice or social demands may act on the trade-off between energy saving and cognition. We assume that different environmental conditions can lead to three different trade-off outcomes: cognitive impairment, resilience or enhancement. Currently we cannot understand these trade-offs, because we lack information about changes in cognitive performance due to seasonal changes in energy availability and both the resulting changes in homeostasis (for example, blood glucose levels) and the associated changes in the mechanisms of allostasis (for example, hormone levels). Additionally, so far we know little about the fitness consequences of individual variation in cognitive performance. General cognitive abilities, such as attention and associative learning, might be more important in determining fitness than complex and specialized cognitive abilities, and easier to use for comparative study in a large number of species. We propose to study seasonal changes in cognitive performance depending on energy availability in populations facing different predation risks, and the resulting fitness consequences.

Intraspecific variability in associative learning in the parasitic wasp Nasonia vitripennis

The ability to learn is key to behavioral adaptation to changing environments. Yet, learning rate and memory retention can vary greatly across or even within species. While interspecific differences have been attributed to ecological context or life history constraints, intraspecific variability in learning behavior is rarely studied and more often, ignored: inferences of the cognitive abilities of a species are most of the time made from experiments using individuals of a single population. Here, we show that learning of host-associated cues in the parasitic wasp Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) shows considerable interpopulation variability, which is at least partly, genetically determined. The strengths of the learning response differed predictably between populations and also varied with the rewarding stimulus. We tested memory retention in a genetically diverse strain and in an iso-female line, bearing a low genetic variability. In addition, we compared our findings with published studies on a third strain using a meta-analytical approach. Our findings suggest that all three strains differ in memory formation from each other. We conclude that, even though the associative learning of host cues is most likely under strong natural selection in parasitoid wasps, considerable genetic variability is maintained at the population as well as at the species level in N. vitripennis.

Long-term fidelity of foraging techniques in common marmosets (Callithrix jacchus)

The formation of behavioral traditions has been considered as one of the main building blocks of culture. Numerous studies on social learning in different animal species provide evidence for their capability of successful transmission of information. However, questions concerning the memory and maintenance of this information have received comparably little attention. After the innovation and initial spread of a novel behavior, the behavior should stabilize and be maintained over time. Otherwise, the behavioral pattern might collapse and no tradition formation would be possible. The aim of this study was to investigate long-term preferences in a two-action manipulation task in common marmosets (Callithrix jacchus). Three captive family groups (23 individuals in total) were trained on one of two possible techniques to open a wooden box and gain access to a food reward, by either pulling or pushing a flap door. The training phase took place in a family group setting, while the test phase was conducted individually. Although the subjects could experience the alternative technique during the test sessions, the majority preferentially used the technique learned in the group setting. Moreover, the subjects were re-tested six times over a period of more than four years, in order to examine the fidelity of their preferences. The longest break without exposure the task lasted for 3.5 years. In all tests, the marmosets showed a similar preference as in the first test block shortly after the training. To our knowledge, this is the first lab study that experimentally demonstrates memory and fidelity of experimentally seeded information in a manipulation task over a time period of several years, supporting the assumption that socially learned foraging techniques can lead to relatively stable behavioral traditions.

Does spatial learning ability of common voles (Microtus arvalis) and bank voles (Myodes glareolus) constrain foraging efficiency?

Place learning abilities represent adaptations that contribute also to foraging efficiency under given spatio-temporal conditions. We investigated if this ability in turn constrains decision making in two sympatric vole species: while the herbivorous common vole (Microtus arvalis) feeds on spatio-temporally predictable food resources (e.g. roots, tubers and shoots of plant tubers), the omnivorous bank vole (Myodes glareolus) additionally subsists on temporally unpredictable food resources (e.g. insects and seeds). Here, we compare the spatial reference memory and working memory of the two species. In an automated operant home cage with eight water places, female voles either had to learn the fixed position of non-depletable places (reference memory task) or learn and avoid previously visited water places depleted in a single visit (win-shift task). In the reference memory task, Microtus females required significantly more choices to find all water places, initially performed slightly worse than Myodes females, and displayed slightly lower asymptotic performance. Both species were highly similar in new learning of the same task. In the more complex win-shift task, asymptotic performance was significantly lower in Microtus (72% correct) than in Myodes (79%). Our results suggest that both vole species resemble each other in their efficiency to exploit habitats with low spatio-temporal complexity but may differ in their efficiency at exploiting habitats with temporally changing spatial food distributions. The results imply that spatial ability adjusted to specific food distributions may impair flexible use of habitats that differ in their food distribution and therefore, decrease a species' chances of survival in highly dynamic environments.