Abstract: With their vertically elongated body form, giraffes generally feed above the level of other browsers within the savanna browsing guild, despite having access to foliage at lower levels. They ingest more leaf mass per bite when foraging high in the tree, perhaps because smaller, more selective browsers deplete shoots at lower levels or because trees differentially allocate resources to promote shoot growth in the upper canopy. We erected exclosures around individual Acacia nigrescens trees in the greater Kruger ecosystem, South Africa. After a complete growing season, we found no differences in leaf biomass per shoot across height zones in excluded trees but significant differences in control trees. We conclude that giraffes preferentially browse at high levels in the canopy to avoid competition with smaller browsers. Our findings are analogous with those from studies of grazing guilds and demonstrate that resource partitioning can be driven by competition when smaller foragers displace larger foragers from shared resources. This provides the first experimental support for the classic evolutionary hypothesis that vertical elongation of the giraffe body is an outcome of competition within the browsing ungulate guild.

Abstract: An animal's within-group spatial position has several important fitness consequences. Risk of predation, time spent engaging in antipredatory behavior and feeding competition can all vary with respect to spatial position. Previous research has found evidence that feeding rates are higher at the group edge in many species, but these studies have not represented the entire breadth of dietary diversity and ecological situations faced by many animals. In particular the presence of concentrated, defendable food patches can lead to increased feeding rates by dominants in the center of the group that are able to monopolize or defend these areas. To fully understand the tradeoffs of within-group spatial position in relation to a variety of factors, it is important to be able to predict where individuals should preferably position themselves in relation to feeding rates and food competition. A qualitative model is presented here to predict how food depletion time, abundance of food patches within a group, and the presence of prior knowledge of feeding sites affect the payoffs of different within-group spatial positions for dominant and subordinate animals. In general, when feeding on small abundant food items, individuals at the front edge of the group should have higher foraging success. When feeding on slowly depleted, rare food items, dominants will often have the highest feeding rates in the center of the group. Between these two extreme points of a continuum, an individual's optimal spatial position is predicted to be influenced by an additional combination of factors, such as group size, group spread, satiation rates, and the presence of producer-scrounger tactics.

Abstract: Food caching birds hide food and recover the caches when supplies are less abundant. There is, however, a risk to this strategy because the caches are susceptible to pilfering by others. Corvids use a number of different strategies to reduce possible cache theft. Scrub-jays with previous experience of pilfering other's caches cached worms in two visuospatially distinct caching trays either in private or in the presence of a conspecific. When these storers had cached in private, they subsequently observed both trays out of reach of a conspecific. When these storers had cached in the presence of a conspecific, they subsequently watched the observer pilfering from one of the trays while the other tray was placed in full view, but out of reach. The storers were then allowed to recover the remaining caches 3 h later. Jays cached more worms when they were observed during caching. At the time of recovery, they re-cached more than if they had cached in private, selectively re-caching outside of the trays in sites unbeknown to potential thieves. In addition, after a single pilfering trial, the jays switched their recovery strategy from predominantly checking their caches (i.e. returning to a cache site to see whether the food remained there) to predominantly eating them. Re-caching remained constant across the three trials. These results suggest that scrub-jays use flexible, cognitive caching and recovery strategies to aid in reducing potential future pilfering of caches by conspecifics.