Life duration and turnover of foragers in the antCataglyphis Bicolor (Hymenoptera, Formicidae)

Abstract

1) Foraging workers of nests ofCataglyphis bicolor Fabricius in a study area in Southern Tunisia, were captured, individually marked, and released. Subsequent censuses showed a constant daily loss of 16.4% in the number of individuals. Evidence is given that the losses are due to high mortality risk which the foragers may incur, particularly a substantial predation pressure (mainly by spiders and robber-flies). Therefore, foraging workers experience an expected half life time of 4.2 days and a life expectancy of 6.1 days. 2) In a field experiment, workers of different ontogenetic stages (callows, repletes, interior workers, and foragers) were collected from the secondary nest of a polydomous colony, individually marked and then introduced to the nearby undisturbed main nest of this colony. The foraging behaviour of these individuals, after their appearance as foragers, was then recorded during two months. The introduced callows remained in the nest for approximately four weeks (range 10 to 43 days) before they started to forage. From the sequence of appearance of the various introduced groups, it is concluded that workers may first serve as repletes, and then seem to carry out other interior duties before they finally appear outside the nest and start to forage. 3) The total number of foragers of the main nest did not change significantly during the experiment (averaging approximately 290 foragers during the summer months). The experiment thus suggests the occurrence of a high turnover of workers that eventually leads to the replacement of foragers by callow production. The number of ants foraging on a census day could be predicted by a model taking into account the date of appearance of replacement workers and a constant daily forager mortality. 4) The observation of individually known ants showed that the number of foraging runs/individual/day does not increase with the number of days the ant has already foraged. On the other hand, the duration of runs increased considerably (from 23 min in the first few runs to 62 min a week later). Similarly, the foragers are more likely to find a food item in a run with increasing foraging are (finds in 25% of the number of runs at the beginning of a foraging career, 80% after a week). 5) This short life duration of foragers could impose a major constraint on opportunities to learn about the environment, e.g. to sample the locations of food. In the foraging system ofCataglyphis the rapid replacement of foragers might also be important for the number of ants, foraging in any one direction from the nest, to be adjusted according to changes in food availability.