Evolution of spatial memory and the hippocampus in the black-capped chickadee

Our first large-scale comparison used 5 populations of black-capped chickadees along the south-north gradient of environmemntal severity (Roth & Pravosudov 2009).	The initial study (Pravosudov & Clayton 2002) compared birds from just two populations - Alaska (Anchorage) and Colorado (Fort Collins). Chickadees were trapped, brough in laboratory, and compared in food caching rates, memory in a cache-retrieval task and in a one-trial associative learning task.
Chickadees in more harsh environments have larger hippocampi and more hippocampal neurons
Next, we compared additional populations mainly along lattitudinal gradient, so that we can test whether winter climate severity has an effect on the hippocampus independnetly of day length (Roth et al. 2011). Our locations around Seattle, Minneapolis and Presque Isle have approximately the same day length during the winter, but winter conditions are more severe in Maine and Minnesota.	In one-trail associative learning task we used both spatial and non-spatial versions. The differences were present only in the spatial task.
Chickadees in Maine and Minnesota have larger hippocampi and more hippocampal neurons than chickadees from Washington State or Iowa. In fact, chickadees from Maine and Minnesota are similar to those from Alaska.
How do we know whether these large differences in hippocampal morphology are inherited or experience-based? We conducted a common garden experiment in which we took 10-day old chicks from natural nests and reared them in the same laboratory conditions (Roth et al. 2012).
Hippocampal volume was not different in the common garden birds even though it was different in wild-sampled birds. This result suggests that differences in hippocampal volume observed in wild birds are likely result of plasticity.
Total number of hippocampal neurons was different between the two populations, both in the common garden experiment and in wild sampled birds. Moreover, the were no differences between common garden and wild-sampled birds from the same populations. This result strongly suggests that (1) differences in the number of hippocampal neurons are inherited and (2) impoverished captive environment has no effect on the number of hippocampal neurons.
We used doublecortin to measure hippocampal neurogenesis. Doublecortin is expressed only in immature neurons and so the number of doublecortin (DCX)-stained neurons represents a populations of new neurons between 0 and ca. 30 days of age. This measure gives an estimate of combined neuron production and neuron survival rates. The number of new immature neurons were different between our two populations in both common garden and wild-sampled birds suggesting strong role of inheritance. Interestingly, there were no significant differences between captive common garden birds and wild sample birds, which suggests that importance of experience in neurogenesis is not as important as it has been widely assumed. Our results suggest that there may be a minimum threshold that is probably never reached with lab mice or rats above which experience has little effect on neurogenesis.