Food storing does not affect hippocampal volume in experienced adult willow tits

Understanding hippocampal neural plasticity in captivity: Unique contributions of spatial specialists

Neural plasticity in the hippocampus has been studied in a wide variety of model systems, including in avian species where the hippocampus underlies specialized spatial behaviours. Examples of such behaviours include navigating to a home roost over long distances by homing pigeons or returning to a potential nest site for egg deposit by brood parasites. The best studied example, however, is food storing in parids and the interaction between this behaviour and changes in hippocampus volume and neurogenesis. However, understanding the interaction between brain and behaviour necessitates research that includes studies with at least some form of captivity, which may itself affect hippocampal plasticity. Captivity might particularly affect spatial specialists where free-ranging movement on a large scale is especially important in daily, and seasonal, behaviours. This review examines how captivity might affect hippocampal plasticity in avian spatial specialists and specifically food-storing parids, and also considers how the effects of captivity may be mitigated by researchers studying hippocampal plasticity when the goal is understanding the relationship between behaviour and hippocampal change.

Seasonal changes in the hippocampal formation of hoarding and non-hoarding tits

The hippocampal formation (HF) processes spatial memories for cache locations in food-hoarding birds. Hoarding is a seasonal behavior, and seasonal changes in the HF have been described in some studies, but not in others. One potential reason is that birds may have been sampled during the seasonal hoarding peak in some studies, but not in others. In this study, we investigate the seasonal changes in hoarding and HF in willow tits (Poecile montanus). We compare this to seasonal changes in HF in a closely related non-hoarding bird, the great tit (Parus major). Willow tits near Oulu, Finland, show a seasonal hoarding peak in September and both HF volume and neuron number show a similar peak. HF neuronal density also increases in September, but then remains the same throughout winter. Unexpectedly, the great tit HF also changes seasonally, although in a different pattern: the great tit telencephalon increases in volume from July to August and decreases again in November. Great tit HF volume follows suit, but with a delay. Great tit HF neuron number and density also increase from August to September and stay high throughout winter. We hypothesize that seasonal changes in hoarding birds’ HF are driven by food-hoarding experience (e.g., the formation of thousands of memories). The seasonal changes in great tit brains may also be due to experience-dependent plasticity, responding to changes in the social and spatial environment. Large-scale experience-dependent neural plasticity is therefore probably not an adaptation of food-hoarding birds, but a general property of the avian HF and telencephalon.

Seasonal change in the avian hippocampus

The hippocampus plays an important role in cognitive processes, including memory and spatial orientation, in birds. The hippocampus undergoes seasonal change in food-storing birds and brood parasites, there are changes in the hippocampus during breeding, and further changes occur in some species in association with migration. In food-storing birds, seasonal change in the hippocampus occurs in fall and winter when the cognitively demanding behaviour of caching and retrieving food occurs. The timing of annual change in the hippocampus of food-storing birds is quite variable, however, and appears not to be under photoperiod control. A variety of factors, including cognitive performance, exercise, and stress may all influence seasonal change in the avian hippocampus. The causal processes underlying seasonal change in the avian hippocampus have not been extensively examined and the more fully described hormonal influences on the mammalian hippocampus may provide hypotheses for investigating the control of hippocampal seasonality in birds.

The history of scatter hoarding studies

In this review, I will present an overview of the development of the field of scatter hoarding studies. Scatter hoarding is a conspicuous behaviour and it has been observed by humans for a long time. Apart from an exceptional experimental study already published in 1720, it started with observational field studies of scatter hoarding birds in the 1940s. Driven by a general interest in birds, several ornithologists made large-scale studies of hoarding behaviour in species such as nutcrackers and boreal titmice. Scatter hoarding birds seem to remember caching locations accurately, and it was shown in the 1960s that successful retrieval is dependent on a specific part of the brain, the hippocampus. The study of scatter hoarding, spatial memory and the hippocampus has since then developed into a study system for evolutionary studies of spatial memory. In 1978, a game theoretical paper started the era of modern studies by establishing that a recovery advantage is necessary for individual hoarders for the evolution of a hoarding strategy. The same year, a combined theoretical and empirical study on scatter hoarding squirrels investigated how caches should be spaced out in order to minimize cache loss, a phenomenon sometimes called optimal cache density theory. Since then, the scatter hoarding paradigm has branched into a number of different fields: (i) theoretical and empirical studies of the evolution of hoarding, (ii) field studies with modern sampling methods, (iii) studies of the precise nature of the caching memory, (iv) a variety of studies of caching memory and its relationship to the hippocampus. Scatter hoarding has also been the subject of studies of (v) coevolution between scatter hoarding animals and the plants that are dispersed by these.

Is bigger always better? A critical appraisal of the use of volumetric analysis in the study of the hippocampus

A well-developed spatial memory is important for many animals, but appears especially important for scatter-hoarding species. Consequently, the scatter-hoarding system provides an excellent paradigm in which to study the integrative aspects of memory use within an ecological and evolutionary framework. One of the main tenets of this paradigm is that selection for enhanced spatial memory for cache locations should specialize the brain areas involved in memory. One such brain area is the hippocampus (Hp). Many studies have examined this adaptive specialization hypothesis, typically relating spatial memory to Hp volume. However, it is unclear how the volume of the Hp is related to its function for spatial memory. Thus, the goal of this article is to evaluate volume as a main measurement of the degree of morphological and physiological adaptation of the Hp as it relates to memory. We will briefly review the evidence for the specialization of memory in food-hoarding animals and discuss the philosophy behind volume as the main currency. We will then examine the problems associated with this approach, attempting to understand the advantages and limitations of using volume and discuss alternatives that might yield more specific hypotheses. Overall, there is strong evidence that the Hp is involved in the specialization of spatial memory in scatter-hoarding animals. However, volume may be only a coarse proxy for more relevant and subtle changes in the structure of the brain underlying changes in behaviour. To better understand the nature of this brain/memory relationship, we suggest focusing on more specific and relevant features of the Hp, such as the number or size of neurons, variation in connectivity depending on dendritic and axonal arborization and the number of synapses. These should generate more specific hypotheses derived from a solid theoretical background and should provide a better understanding of both neural mechanisms of memory and their evolution.

Captivity reduces hippocampal volume but not survival of new cells in a food-storing bird

In many naturalistic studies of the hippocampus wild animals are held in captivity. To test if captivity itself affects hippocampal integrity, adult black-capped chickadees (Poecile atricapilla) were caught in the fall, injected with bromodeoxyuridine to mark neurogenesis, and alternately released to the wild or held in captivity. The wild birds were recaptured after 4-6 weeks and perfused simultaneously with their captive counterparts. The hippocampus of captive birds was 23% smaller than wild birds, with no hemispheric differences in volume within groups. Between groups there was no statistically significant difference in the size of the telencephalon, or in the number and density of surviving new cells. Proximate causes of the reduced hippocampal volume could include stress, lack of exercise, diminished social interaction, or limited caching opportunity-a hippocampal-dependent activity. The results suggest the avian hippocampus-a structure essential for rapid, complex relational and spatial learning-is both plastic and sensitive, much as in mammals, including humans.

Sex differences in the effects of captivity on hippocampus size in brown-headed cowbirds (Molothrus ater obscurus)

In brood parasitic cowbirds, hippocampus (Hp) size is correlated with environmental spatial memory demands. Searching for host nests is the presumed causal factor influencing cowbird Hp size, because Hp volumes vary across species, sexes, and seasons according to nest-searching participation. Brown-headed cowbirds have female-only nest searching and, at least in the eastern subspecies, a larger Hp in females than in males, suggesting that nest searching influences cowbird Hp size. We predicted that female brown-headed cowbirds housed in aviaries lacking host nests would have a smaller Hp than wild-caught females whereas males would be unaffected. We found that the Hp was smaller in captive females, but not males, compared to their wild-caught counterparts. This did not appear to be due to general effects of an impoverished environment on all brain regions. Our results imply that interruption of nest searching in cowbirds prevents seasonal increase in Hp size in females. Future studies should isolate which behavioral differences between wild and captive birds contributed to captivity-induced changes in Hp volume in females while not affecting males.

A Multi-disciplinary Approach to Understanding Hippocampal Function in Foodhoarding Birds

Spatial memory and the hippocampal formation (HF) of food-hoarding birds have been put forward as a prime example of how natural selection has shaped a cognitive system and its neural underpinnings. Here, I review what we know about the HF of hoarding birds and lay out the work that is currently underway to use this system to obtain a better understanding of hippocampal function in general. This interdisciplinary programme includes evolutionary, ecological, psychological, ethological, and neuroscientific approaches to the study of behaviour and cognition. Firstly, we need to understand the behaviour of the birds in their natural environment, and identify the aspects of cognition and behaviour that may be especially valuable for the species under study. Secondly, these cognitive and behavioural traits are compared to closely-related non-hoarding species. Thirdly, we also compare HF anatomy between closely-related hoarding and non-hoarding species, identifying possible neural mechanisms underlying behavioural differences. Finally, behavioural and neuroscientific approaches are combined in experiments directly investigating the involvement of the HF or any of its anatomical and physiological aspects in the behaviours under study. This process loops back upon itself in many different ways, with all the different approaches informing each other. In this way we are making progress in understanding the functioning of the HF, not only in food-hoarding birds, but in all vertebrates.

Does hippocampal size correlate with the degree of caching specialization?

A correlation between the degree of specialization for food hoarding and the volume of the hippocampal formation in passerine birds has been accepted for over a decade. The relationship was first demonstrated in family-level comparisons, and subsequently in species comparisons within two families containing a large number of hoarding species, the Corvidae and the Paridae. Recently, this approach has been criticized as invalid and excessively adaptationist. A recent test of the predicted trends with data pooled from previous studies found no evidence for such a correlation in either of these two families. This result has been interpreted as support for the critique. Here we reanalyse the original dataset and also include additional new data on several parid species. Our results show a surprising difference between continents, with North American species possessing significantly smaller hippocampi than Eurasian ones. Controlling for the continent effect makes the hoarding capacity/hippocampal formation correlation clearly significant in both families. We discuss possible reasons for the continent effect.

Dominance-related changes in spatial memory are associated with changes in hippocampal cell proliferation rates in mountain chickadees

It is well established that spatial memory is dependent on the hippocampus in both mammals and birds. As memory capacity can fluctuate on a temporal basis, it is important to understand the mechanisms mediating such changes. It is known that early memory-dependent experiences in young animals result in hippocampal enlargement and in increased neurogenesis, including cell proliferation and neuron survival. It is less clear, however, whether temporal changes in spatial memory are also associated with changes in hippocampal anatomy and cell proliferation in fully grown and experienced adult animals. In a previous study, we experimentally demonstrated that socially subordinate mountain chickadees (Poecile gambeli) showed inferior spatial memory performance compared to their dominant group mates, in the absence of significant differences in baseline corticosterone levels. Here we investigated whether these differences in memory between dominant and subordinate birds were associated with changes in the hippocampus. Following memory tests, chickadees were injected with 5-bromo-2'-deoxyuridine to label dividing cells and sacrificed 2 days after the injections. We found no significant differences in volume or the total number of neurons in the hippocampal formation between dominant and subordinate chickadees, but subordinate birds had significantly lower cell proliferation rates in the ventricular zone adjacent to both the hippocampus and mesopallium compared to the dominants. Individuals, which performed better on spatial memory tests tended to have higher levels of cell proliferation. These results suggest that social status can affect cell proliferation rates in the ventricular zone and support the hypothesis that neurogenesis might be involved in memory function in adult animals.

Septum volume and food-storing behavior are related in parids

The hippocampal formation (HF) of food-storing birds is larger than non-storing species, and the size of the HF in food-storing Black-Capped Chickadees (Poecile atricapillus) varies seasonally. We examined whether the volume of the septum, a medial forebrain structure that shares reciprocal connections with the HF, demonstrates the same species and seasonal variation as has been shown in the HF. We compared septum volume in three parid species; non-storing Blue Tits (Parus caeruleus) and Great Tits (Parus major), and food-storing Black-Capped Chickadees. We found the relative septum volume to be larger in chickadees than in the non-storing species. We also compared septum and nucleus of the diagonal band (NDB) volume of Black-Capped Chickadees at different times of the year. We found that the relative septum volume varies seasonally in food-storing birds. The volume of the NDB does not vary seasonally. Due to the observed species and seasonal variation, the septum, like the hippocampal formation of food-storing birds, may be specialized for some aspects of food-storing and spatial memory.

Changes in spatial memory mediated by experimental variation in food supply do not affect hippocampal anatomy in mountain chickadees (Poecile gambeli)

Earlier reports suggested that seasonal variation in food-caching behavior (caching intensity and cache retrieval accuracy) might correlate with morphological changes in the hippocampal formation, a brain structure thought to play a role in remembering cache locations. We demonstrated that changes in cache retrieval accuracy can also be triggered by experimental variation in food supply: captive mountain chickadees (Poecile gambeli) maintained on limited and unpredictable food supply were more accurate at recovering their caches and performed better on spatial memory tests than birds maintained on ad libitum food. In this study, we investigated whether these two treatment groups also differed in the volume and neuron number of the hippocampal formation. If variation in memory for food caches correlates with hippocampal size, then our birds with enhanced cache recovery and spatial memory performance should have larger hippocampal volumes and total neuron numbers. Contrary to this prediction we found no significant differences in volume or total neuron number of the hippocampal formation between the two treatment groups. Our results therefore indicate that changes in food-caching behavior and spatial memory performance, as mediated by experimental variations in food supply, are not necessarily accompanied by morphological changes in volume or neuron number of the hippocampal formation in fully developed, experienced food-caching birds.

Hippocampal growth and maintenance depend on food-caching experience in juvenile mountain chickadees (Poecile gambeli)

This experiment investigated the development of caching behavior and the hippocampus (HF) in postfledging mountain chickadees (Poecile gambeli). From Days 35 to 53, the number of seeds stored increased but the proportion recovered did not. Birds that stored and recovered during 3 or more trials had significantly enlarged HF but not telencephalon volumes (experienced) compared with those that stored but did not recover (store only) and those deprived of caching experience altogether (deprived). HF size did not increase linearly with the number of experience trials. Birds that received less than 3 experience trials did not differ from deprived birds in HF size, suggesting a threshold effect. Experienced birds prevented from caching for 1 month had significantly smaller HF volumes than those examined immediately after caching experience and did not differ from deprived birds. Experience of both storing and recovery is required to initiate growth and maintain HF size.

Seasonal changes in neuron numbers in the hippocampal formation of a food-hoarding bird: the black-capped chickadee

The volume of the hippocampal formation (HF) in black-capped chickadees (Poecile atricapillus) varies across the seasons, in parallel with the seasonal cycle in food hoarding. In this study, we estimate cell density and total cell number in the HF across seasons in both juveniles and adults. We find that the seasonal variation in volume is due to an increase in the number of small and large cells (principally neurons) in the fall. Adults also have lower neuron densities than juveniles. Both juveniles and adults show an increase in cell density in the rostral part of the HF in August and a subsequent decrease toward October. This suggests that the net cell addition to the HF may already start in August. We discuss the implications of this early start with respect to the possibility that the seasonal change in HF volume is driven by the experience of food hoarding. We also speculate on the functional significance of the addition of neurons to the HF in the fall.

NEUROETHOLOGY OF SPATIAL LEARNING: The Birds and the Bees

The discipline of neuroethology integrates perspectives from neuroscience, ethology, and evolutionary biology to investigate the mechanisms underlying the behavior of animals performing ecologically relevant tasks. One goal is to determine if common organizational principles are shared between nervous systems in diverse taxa. This chapter selectively reviews the evidence that particular brain regions subserve behaviors that require spatial learning in nature. Recent evidence suggests that the insect brain regions known as the mushroom bodies may function similarly to the avian and mammalian hippocampus. Volume changes in these brain regions during the life of an individual may reflect both developmental and phylogenetic trends. These patterns may reveal important structure-function relationships in the nervous system.