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Volume 72, Issue 4 p. 1408-1419
Article

Learning, Memory, and Foraging Efficiency in Two Species of Desert Seed-Harvester Ants

First published: 01 August 1991
Citations: 52

Abstract

I examined seed learning and memory in the desert seed—harvester ants Messor pergandei and Pogonomyrmex rugosus to determine (1) which aspects of handling and search efficiency are learned, (2) how fast this information is learned and then forgotten, and (3) how learning and memory affect foraging efficiency. Individuals of both species learned a suite of behaviors that enhanced foraging efficiency on a novel seed species under natural conditions. Handling time per seed and return time (travel time to and from the nest and time inside the nest) decreased for both species. Each species also used specific harvest techniques that enhanced foraging efficiency. Individuals of the larger P. rugosus increased efficiency by increasing the number of seeds harvested per trip. As a result, the percentage of individuals that carried > 1 seed per trip increased over time. The smaller M. pergandei decreased the number of seeds handled before harvesting one, and still harvested heavier than average seeds because they handled but did not harvest lightweight seeds. Messor pergandei also varied the number of seeds handled before harvesting one, such that the fewest number of seeds was handled at low natural seed densities. Overall, foraging efficiency of M. pergandei and P. rugosus individuals, measured as seed mass harvested per unit time, increased 1.9— and 3.85—fold, respectively, within one observation period, even excluding the effect of harvesting heavier than average seeds. The difference between species arose because individuals of P. rugosus could carry twice as many of the test seeds as M. pergandei. Memory was clearly an important component of foraging efficiency for both species, as all foraging efficiency indices fell much more slowly than they rose. Tactile—associated behaviors, such as handling time per seed and number of seeds handled, were lost quickly compared to olfactory—associated behaviors such as seed recognition and acceptance.