Age-specific reproductive success: evidence for the selection hypothesis.

Age-specific reproductive success has been demonstrated in many species. Three hypotheses have been raised to explain this general phenomenon: the experience hypothesis based on age-specific reproductive experience, the effort hypothesis based on age-specific reproductive effort, and the selection hypothesis based on progressive disappearance of phenotypes due to variation in individual productivity and survival. We used data from a long-term study of Leach's storm-petrels (Oceanodroma leucorhoa) to present a single test of mutually exclusive predictions about the relationship between early breeding success and longevity. There should be no correlation between early breeding success and longevity under the experience hypothesis, a negative correlation under the effort hypothesis, and a positive correlation under the selection hypothesis. We found a significant (P < 0.0001) positive relationship between success in the first two breeding attempts and longevity in this population of long-lived seabirds, strongly suggesting that low-productivity parents were also less likely to survive early breeding. These data provide some of the strongest support to date for the selection hypothesis.

Relative hippocampal volume in relation to food-storing behavior in four species of woodpeckers.

Previous studies have shown that those food-storing birds of the order Passeriformes that remember the locations of their caches have relatively larger hippocampal complexes than do non-storing passerines. Woodpeckers constitute a different avian order (Piciformes), which also includes some food-storing species. We compared hippocampal volume, relative to the volume of the rest of the telencephalon, across four species of woodpeckers with disparate caching behavior. Red-bellied woodpeckers (Melanerpes carolinus) are "scatter hoarders'. During the fall and winter they cache acorns or beechnuts in dispersed sites throughout a large territory. Red-headed woodpeckers (Melanerpes erythrocephalus) also store nuts but in central "larders' on their small territories which they fiercely defend. Caching is absent or much reduced in hairy woodpeckers (Picoides villosus) and downy woodpeckers (Picoides pubescens), both of which forage on a variety of foods within large winter home ranges. The relative volume of the hippocampal complex in the scatter hoarder was larger than in the larder hoarder, suggesting that red-bellied woodpeckers, like passerine scatter hoarders, rely on memory to recover their caches. Surprisingly, the relative hippocampal volumes in the two non-storing Picoides woodpeckers were most similar to the scatter hoarder of the other genus. In passerine birds, hippocampal volume and telencephalon volume are highly correlated in storing species but not in non-storers. We found that the volumes of these two brain areas were highly correlated in both Melanerpes species, uncorrelated in the hairy woodpeckers, and more weakly correlated in the downy woodpeckers. The unexpectedly large hippocampal complexes in the Picoides species suggests they may engage in some behavior, other than food-storing, that selects for this trait. Conversely, our results concerning the relationship between hippocampal and telencephalon volumes may indicate that a weak correlation is associated with a less specialized hippocampus, independent of its relative volume.