Poor body condition is associated with lower hippocampal plasticity and higher gut methanogen abundance in adult laying hens from two housing systems.

It is still unclear which commercial housing system provides the best quality of life for laying hens. In addition, there are large individual differences in stress levels within a system. Hippocampal neurogenesis or plasticity may provide an integrated biomarker of the stressors experienced by an individual. We selected 12 adult hens each with good and poor body condition (based on body size, degree of feather cover and redness of the comb) from a multi-tier free range system containing H&N strain hens, and from an enriched cage system containing Hy-Line hens (n = 48 total). Immature neurons expressing doublecortin (DCX) were quantified in the hippocampus, contents of the caecal microbiome were sequenced, and expression of inflammatory cytokines was measured in the spleen. DCX+ cell densities did not differ between the housing systems. In both systems, poor condition hens had lower DCX+ cell densities, exhibited elevated splenic expression of interleukin-6 (IL6) mRNA, and had a higher relative caecal abundance of methanogenic archea Methanomethylophilaceae. The findings suggest poor body condition is an indicator that individual hens have experienced a comparatively greater degree of cumulative chronic stress, and that a survey of the proportion of hens with poor body conditions might be one way to evaluate the impact of housing systems on hen welfare.

Keel bone fractures induce a depressive-like state in laying hens.

In commercial flocks of laying hens, keel bone fractures (KBFs) are prevalent and associated with behavioural indicators of pain. However, whether their impact is severe enough to induce a depressive-like state of chronic stress is unknown. As chronic stress downregulates adult hippocampal neurogenesis (AHN) in mammals and birds, we employ this measure as a neural biomarker of subjective welfare state. Radiographs obtained longitudinally from Lohmann Brown laying hens housed in a commercial multi-tier aviary were used to score the severity of naturally-occurring KBFs between the ages of 21-62 weeks. Individual birds' transitions between aviary zones were also recorded. Focal hens with severe KBFs at 3-4 weeks prior to sampling (n = 15) had lower densities of immature doublecortin-positive (DCX+) multipolar and bipolar neurons in the hippocampal formation than focal hens with minimal fractures (n = 9). KBF severity scores at this time also negatively predicted DCX+ cell numbers on an individual level, while hens that acquired fractures earlier in their lives had fewer DCX+ neurons in the caudal hippocampal formation. Activity levels 3-4 weeks prior to sampling were not associated with AHN. KBFs thus lead to a negative affective state lasting at least 3-4 weeks, and management steps to reduce their occurrence are likely to have significant welfare benefits.

Unpredictable Chronic Mild Stress Suppresses the Incorporation of New Neurons at the Caudal Pole of the Chicken Hippocampal Formation.

In the mammalian brain, adult hippocampal neurogenesis (AHN) is suppressed by chronic stress, primarily at the ventral pole of the hippocampus. Based upon anatomy, we hypothesise that the caudal pole of the avian Hippocampal Formation (HF) presents a homologous subregion. We thus investigated whether AHN is preferentially suppressed in the caudal chicken HF by unpredictable chronic mild stress (UCMS). Adult hens were kept in control conditions or exposed to UCMS for 8 weeks. Hens experiencing UCMS had significantly fewer doublecortin-positive multipolar neurons (p < 0.001) and beaded axons (p = 0.021) at the caudal pole of the HF than controls. UCMS birds also had smaller spleens and lower baseline plasma corticosterone levels compared to controls. There were no differences in AHN at the rostral pole, nor were there differences in expression of genetic mediators of the HPA stress response in the pituitary or adrenal glands. Duration of tonic immobility and heterophil/lymphocyte (H/L) ratios were also not responsive to our UCMS treatment. These results support the hypothesised homology of the caudal pole of the avian HF to the ventral pole of the rodent hippocampus. Furthermore, quantifying neurogenesis in the caudal HF post-mortem may provide an objective, integrative measure of welfare in poultry, which may be more sensitive than current welfare measures.

Hypothalamic agouti-related protein expression is affected by both acute and chronic experience of food restriction and re-feeding in chickens.

The central melanocortin system is conserved across vertebrates. However, in birds, little is known about how energy balance influences orexigenic agouti-related protein (AGRP) and anorexigenic pro-opiomelanocortin (POMC) expression, despite the fact that commercial food restriction is critical to the efficient production of poultry meat. To enable contrasts to be made, in broiler-breeder chickens, between levels of food restriction, between birds with the same body weight but different feeding experience, and between birds moved from restricted feeding to ad lib. feeding for different periods, five groups of hens were established between 6 and 12 weeks of age with different combinations of food restriction and release from restriction. AGRP and neuropeptide Y expression in the basal hypothalamus was significantly increased by chronic restriction but only AGRP mRNA levels reflected recent feeding experience: hens at the same body weight that had recently been on ad lib. feeding showed lower expression than restricted birds. AGRP expression also distinguished between hens released from restriction to ad lib. feeding for different periods. By contrast, POMC and cocaine- and amphetamine-regulated transcript mRNA levels were not different. These results showed that AGRP mRNA not only reflected differences between a bird's weight and its potential weight or set point, but also discriminated between differing feeding histories of birds at the same body weight. Therefore, AGRP expression potentially provides an integrated measure of food intake experience and an objective tool to assess a bird's perception of satiety in feeding regimes for improved poultry welfare.

Failure to detect seasonal changes in the song system nuclei of the black-capped chickadee (Poecile atricapillus).

Most temperate songbird species sing seasonally, and the brain areas involved in producing song (the song system) vary in size alongside the changes in behavior. Black-capped chickadees (Poecile atricapillus) also sing seasonally, and we find that there are changes in the stereotypy and the length of the fee-bee song from the nonbreeding to the breeding season. Yet despite these changes, we fail to find any evidence of seasonal changes in the song system. The song system of males is larger than that of females, as is typical in songbirds, but the ratio between the sexes is small compared to other species. We suggest three hypotheses to explain our failure to find seasonal variation in the chickadee song system.

A framework for integrating the songbird brain.

Biological systems by default involve complex components with complex relationships. To decipher how biological systems work, we assume that one needs to integrate information over multiple levels of complexity. The songbird vocal communication system is ideal for such integration due to many years of ethological investigation and a discreet dedicated brain network. Here we announce the beginnings of a songbird brain integrative project that involves high-throughput, molecular, anatomical, electrophysiological and behavioral levels of analysis. We first formed a rationale for inclusion of specific biological levels of analysis, then developed high-throughput molecular technologies on songbird brains, developed technologies for combined analysis of electrophysiological activity and gene regulation in awake behaving animals, and developed bioinformatic tools that predict causal interactions within and between biological levels of organization. This integrative brain project is fitting for the interdisciplinary approaches taken in the current songbird issue of the Journal of Comparative Physiology A and is expected to be conducive to deciphering how brains generate and perceive complex behaviors.

Expression of immediate early genes in the hippocampal formation of the black-capped chickadee (Poecile atricapillus) during a food-hoarding task.

Black-capped chickadees store food in many different locations in their home range and are able to accurately remember these locations. We measured the number of cells immunopositive for three different Immediate Early Gene products (Fra-1, c-Fos and ZENK) to map neuronal activity in the chickadee Hippocampal Formation (HF) during food storing and retrieval. Fra-1-like immunoreactivity is downregulated in the dorsal HF of both storing and retrieving chickadees compared to controls. In retrieving birds, the number of Fos-like immunoreactive neurons relates to the number of items remembered, while the number of ZENK-like immunoreactive neurons in the HF may be related to the accuracy of cache retrieval. These results imply that the brain might process complex information by recruiting more neurons into the network of active neurons. Thus, our results could help explain why food-hoarding birds have more HF neurons than non-hoarders, and why this number increases in autumn when large numbers of food items are cached.

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.

Effects of captivity and testosterone on the volumes of four brain regions in the dark-eyed junco (Junco hyemalis).

This study investigates the effects of captivity and testosterone treatment on the volumes of brain regions involved in processing visual and spatial information in adult dark-eyed juncos (Junco hyemalis). We treated captive and free-living male juncos with either testosterone-filled or empty implants. Captive juncos had a smaller hippocampal formation (HF) (both in absolute volume and relative to telencephalon) than free-living birds, regardless of hormone treatment. Testosterone-treated males (both captive and free-living) had a smaller telencephalon and nucleus rotundus, but not a smaller HF or ectostriatum, than controls. We found that free-living testosterone-treated males had larger home ranges than free-living controls in agreement with earlier experiments, but we found no corresponding difference in HF volume. We discuss the implications of the effect of captivity on HF volume for past and future laboratory experiments.

A game theoretical model of the evolution of food hoarding: applications to the Paridae.

A game against the field is proposed that models the evolution of food-hoarding behavior in a group-living species, like many members of the family Paridae (Aves, Passeriformes). The model predicts that no special retrieval mechanisms (e.g., memory) are necessary for food-hoarding individuals to invade a population of nonhoarders, as long as the winters are very severe. Once food hoarding is established in a population, having smaller groups and separating foraging niches between group members prevent cheaters from benefiting from other individuals' caches. A scenario is proposed for the evolution of hoarding in the Paridae.

Seasonal variation in hippocampal volume in a food-storing bird, the black-capped chickadee.

Black-capped chickadees (Parus atricapillus) in upstate New York show a peak in food-hoarding intensity in October. We caught chickadees at six different times of the year and measured the volume of several brain structures. We found that the hippocampal formation, which is involved in spatial memory for cached food items, has a larger volume, relative to the rest of the brain, in October than at any other time of the year. We conclude that there is an association between the intensity of food hoarding and the volume of the hippocampal formation and suggest that the enhanced anatomy might be caused by the increased use of spatial memory.