Food-caching Mountain Chickadees Research Articles

Elevation-related differences in memory and the hippocampus in mountain chickadees, Poecile gambeli

Harsh environments may lead to increased demands on memory in animals that rely on memory for survival. We previously showed that winter severity is associated with non-experience-based differences in memory and the hippocampus over a large continental scale in food-caching black-capped chickadees, Poecile atricapillus. However, large climatic differences also occur along steep elevational gradients in montane environments over a small geographical scale. Here, we demonstrate for the first time that large differences in memory and the hippocampus exist over extremely short distances (10 km) along the elevation gradient. We found that food-caching mountain chickadees (P. gambeli) from the highest elevations in the Sierra Nevada Mountains had significantly better spatial memory associated with larger hippocampi, with almost twice the number of hippocampal neurons, than individuals only 600 m lower in elevation. We found similarly large differences in hippocampal neurogenesis rates as indicated by the total number of immature neurons. Our study therefore suggests that climate-related environmental differences can produce dramatic differences in memory and the hippocampus in animals within close proximity on small spatial scales and that currently observed trends in global climate may have significant effects on cognition and the brain.

Effects of captivity and memory-based experiences on the hippocampus in mountain chickadees.

The complexity of an animal's physical environment is known to affect the hippocampus. Captivity may affect hippocampal anatomy and this may be attributable to the limited opportunities for memory-based experiences. This has tangential support, in that differential demands on memory can mediate changes in the hippocampus. What remains unclear is whether captivity directly affects hippocampal architecture and whether providing memory-based experiences in captivity can maintain hippocampal attributes comparable to wild-caught conspecifics. Using food-caching mountain chickadees (Poecile gambeli), we found that wild-caught individuals had larger hippocampal volumes relative to the rest of the telencephalon than captive birds with or without memory-based food-caching experiences, whereas there were no differences in neuron numbers or telencephalon volume. Also, there were no significant differences in relative hippocampal volume or neuron numbers between the captive birds with or without memory-based experiences. Our results demonstrate that captivity reduces hippocampal volume relative to the remainder of the telencephalon, but not at the expense of neuron numbers. Further, memory-based experiences in captivity may not be sufficient to maintain hippocampal volume comparable to wild-caught counterparts.

Effects of demanding foraging conditions on cache retrival accuracy in food-caching mountain chickadees (Poecile gambeli).

Birds rely, at least in part, on spatial memory for recovering previously hidden caches but accurate cache recovery may be more critical for birds that forage in harsh conditions where the food supply is limited and unpredictable. Failure to find caches in these conditions may potentially result in death from starvation. In order to test this hypothesis we compared the cache recovery behaviour of 24 wild-caught mountain chickadees (Poecile gambeli), half of which were maintained on a limited and unpredictable food supply while the rest were maintained on an ad libitum food supply for 60 days. We then tested their cache retrieval accuracy by allowing birds from both groups to cache seeds in the experimental room and recover them 5 hours later. Our results showed that birds maintained on a limited and unpredictable food supply made significantly fewer visits to non-cache sites when recovering their caches compared to birds maintained on ad libitum food. We found the same difference in performance in two versions of a one-trial associative learning task in which the birds had to rely on memory to find previously encountered hidden food. In a non-spatial memory version of the task, in which the baited feeder was clearly marked, there were no significant differences between the two groups. We therefore concluded that the two groups differed in their efficiency at cache retrieval. We suggest that this difference is more likely to be attributable to a difference in memory (encoding or recall) than to a difference in their motivation to search for hidden food, although the possibility of some motivational differences still exists. Overall, our results suggest that demanding foraging conditions favour more accurate cache retrieval in food-caching birds.