Intraspecific differences

In all interspecific comparisons employed to test the adaptive specialization hypothesis, the main underlying assumption was that the biggest difference between the species lies in their food-caching behavior (Krebs et al. 1989; Sherry et al. 1989; Brodin and Lundborg 2003a; Lucas et al. 2004; Chapter 2). However, it is possible that there are other differences between species, unrelated to caching behavior, that could have resulted in differential selection pressure on both spatial memory and the hippocampus. Comparing different populations within the same species might provide an opportunity to control for such differences.

Many food-caching Parids occupy extremely wide ranges, extending from south to north (Harrap and Quinn 1995) with greatly varying environmental conditions, especially during the winter. The range of black-capped chickadees, for example, extends from New Mexico in the south to Alaska in the north (Harrap and Quinn 1995). Compared to more southerly populations, Alaskan black-capped chickadees regularly experience longer winters with lower ambient temperatures, deeper and longer lasting snow cover, and, most importantly, much shorter day-length available for foraging during winter (Pravosudov and Clayton 2002). Obtaining enough food to build energy reserves sufficient to survive the night is crucial for survival in these small birds

(e.g. Pravosudov and Lucas 2001) and, in December, Alaskan chickadees have only about 5 hours of daylight to obtain enough energy reserves to survive about 19 hours of night. In Colorado, on the other hand, these birds have almost twice as much daylight hours to complete the same foraging task while probably requiring less energy due to generally warmer night-time temperatures (Pravosudov and Clayton 2002). In both environments, caches should provide a more predictable food source than naturally available food. Having sufficient food caches, and the capacity to successfully find them when needed should thus have much larger survival consequences for chickadees in energetically demanding Alaskan conditions. Such differences in the significance of cached food between southern and northern populations are likely to provide higher selection pressure in northern populations for more intense food caching, better spatial memory, and larger hippocampi.

When black-capped chickadees from Alaska and Colorado were acclimatized for 45 days and then tested in identical laboratory conditions, birds from Alaska cached significantly more food and performed significantly better on a spatial version of a one-trial associative learning task (Fig. 3.1;

Figure 3.1 Mean number of seeds consumed (fully or partially) and cached during 20-mln food-caching trials (a) and memory performance In a one-trial associative learning task (b) by black-capped chickadees from Alaska (black bars) and Colorado (hatched bars) (redrawn from Pravosudov and Clayton 2002). Memory performance was measured as the number of sites inspected (= number of looks) in order to find the site previously containing food among either 70 or 15 available sites. Fewer looks indicate better memory performance. A color version of a one-trial associative learning task was used to investigate possible motivational differences between the groups. If one group were less motivated to search for food, it would show similar poor performance on both spatial and color versions of the task. If there were differences in spatial memory between the groups, there would be differences in performance on spatial but not on color version of a one-trial associative learning task.

Figure 3.1 Mean number of seeds consumed (fully or partially) and cached during 20-mln food-caching trials (a) and memory performance In a one-trial associative learning task (b) by black-capped chickadees from Alaska (black bars) and Colorado (hatched bars) (redrawn from Pravosudov and Clayton 2002). Memory performance was measured as the number of sites inspected (= number of looks) in order to find the site previously containing food among either 70 or 15 available sites. Fewer looks indicate better memory performance. A color version of a one-trial associative learning task was used to investigate possible motivational differences between the groups. If one group were less motivated to search for food, it would show similar poor performance on both spatial and color versions of the task. If there were differences in spatial memory between the groups, there would be differences in performance on spatial but not on color version of a one-trial associative learning task.

Pravosudov and Clayton 2002). Interestingly, Alaskan chickadees were significantly smaller, with smaller brains, than chickadees from Colorado (Fig. 3.2; Pravosudov and Clayton 2002). Compared to Colorado chickadees, however, individuals from Alaska had significantly larger hippocampi with more neurons in both absolute terms and relative to telencephalon volume (Fig. 3.2; Pravosudov and Clayton 2002). These results support the adaptive specialization hypothesis; because Alaskan chickadees exist in a more energetically demanding environment, they cache more food, and have better spatial memory—which may be associated with their larger hippocampi containing more neurons—compared to conspecifics from Colorado (Pravosudov and Clayton 2002).

Based on the above data, however, it was not possible to unambiguously conclude whether the differences in caching, spatial memory, and hip-pocampal volume were genetic or experience based. Birds from both Alaska and Colorado were maintained and tested in identical laboratory environments, suggesting that the differences found between these groups were not related to the immediate environment. Alaskan chickadees cached more food and thus they had more food-caching experience in the laboratory. A separate analysis of a control group of birds, which did not participate in the experiment while being maintained in the laboratory for the same duration of time, revealed no differences in the hippocampal volume or neuron numbers between the individuals that cached food and the ones that did not cache food (Pravosudov and Clayton 2002). At the same time, the differences between Alaska and Colorado remained even in individuals that did not cache food during the experiment, thus ruling out a possibility that larger hippocampus with more neurons in Alaskan chickadees resulted from more caching during the experiment. It remains possible, however, that differences between Alaska and Colorado chickadees in food caching and cache retrieval experience prior to capture produced long-term changes in memory and the hippocampus and thus further testing is necessary to differentiate between a genetic versus environmental basis of population differences.

Irrespective of the basis of these differences between Alaskan and Colorado black-capped chickadees, more food caching, better spatial memory, and an enlarged hippocampus with more neurons in Alaskan individuals appear to be highly adaptive to life in extremely energetically

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200 220 240 260 280 300 320 340 360 380 400 Volume of the remainder of telencephalon (mm3)

200 220 240 260 280 300 320 340 360 380 400 Volume of the remainder of telencephalon (mm3)

Figure 3.2 The relationship between the hippocampal volume and the volume of the rest of the telencephalon in black-capped chickadees from Alaska (open circles) and Colorado (filled circles) (redrawn from Pravosudov and Clayton 2002).

challenging environments. More research is needed to ascertain whether these differences have a genetic basis. Similar intraspecific studies may also be possible with tufted titmice, Carolina chickadees, Siberian and willow tits, or high-elevation populations of mountain chickadees, which would test the generality of environmental selection on caching behavior across species.

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