Cohesiveness reduces foraging efficiency in a social herbivore

For social foragers, movement as a group could increase foraging efficiency through collective discovery of high-quality food sources. This would require an efficient mechanism for transferring information about food quality between individuals. Conversely, the constraints of foraging as a cohesive group could decrease efficiency; grouping may persist to serve other functions such as protection from predators. To test what drives cohesion in herbivores, we manipulated patch shape and within-patch pattern of food quality and quantified the effects on group level diet selection by a social herbivore, the fallow deer, Dama dama. We arranged feeders containing fodder in lines or blocks, and manipulated the pattern of food quality within patches by adding tannin, a plant secondary compound that decreases palatability. We quantified the relative consumption of low- and high-tannin food to compare diet selectivity at the group level between patch treatments. If group foraging evolved to increase foraging efficiency, altering the spatial arrangement of food should not affect diet selectivity because information about food location and quality is shared. We found, however, that the herd expressed different levels of selectivity between both patch shapes and food quality patterns. Deer selected better diets in blocks than lines. In lines, the herd selected better diets when quality varied between alternate feeders rather than between the two halves of the patch, suggesting a reliance on personal rather than group information. Deer consumed the most at patch centres in all treatments except in blocks with high-tannin centres, but diet selection was poorer in the latter compared to blocks with low-tannin centres. Aggregation at the centre of patches appears to have restricted exploitation of the best food. Predation pressure and/or resource variability may have favoured the evolution of a foraging strategy that prioritizes social cohesion over effective diet selection.