Food hoarding: Memory and social conditions - an evolutionary approach

Abstract: Food hoarding is a widespread behavior among a large number of animals, and it exists in several different varieties, all of them used by the animals as a method of having a steady supply of food during periods of low food abundance such as the winter. This thesis concentrates mainly on two aspects of scatter food hoarding behavior, using small birds as study and model animals. Scatter hoarding is common in many bird species, and it means that the hoarding animal stores only one or just a few food items in each of its caches. These caches can be very numerous and spread out over a large area, making them hard to defend but also hard to find. It is generally believed that birds use spatial memory as an aid in recovering scatter-hoarded food. Lately, many studies have been made, trying to find out whether the degree of hoarding in birds is correlated with the volume of the hippocampus – a region of the avian brain thought to play an important role for spatial memory. This thesis concludes that a correlation between hippocampal size and degree of food hoarding probably not exists, and that food hoarding alone cannot be the cause of different hippocampus size in the studied species. Some food hoarding bird species share their territories with conspecifics during the hoarding season and the following winter, and this creates a social environment that affects food hoarding decisions. Social dominance in a winter flock is studied theoretically and predicted to affect food storing strategies of both dominant birds and subordinate ones. Subordinate birds are predicted to always store more than dominant birds, but this difference should even out if winter conditions become predictably worse, with the dominant bird increasing its hoarding effort. If better winter conditions are expected, dominants should instead spend more time pilfering from subordinates. And, if no selfish recovery advantage for hoarders exists, dominants should refrain from hoarding entirely. The theoretical predictions were tested in two field studies and we found that subordinate birds always stored more than dominants, that both ranks stored more food when winter conditions got worse, and that the differences between the ranks became smaller during bad conditions.