Payoffs of the two alternative nesting tactics in the African dung beetle, Scarabaeus catenatus


HiroakiSato Biological Laboratory, Nara University of Education, Nara, 630


1. Scarabaeus catenatus is a ball-rolling scarab in the subfamily Scarabaeinae. This species, however, makes use of two tactics for nest building: rolling and tunnelling. The tunnelling tactic differs substantially from the rolling tactic in that (1) it always involves repeated movements to and from the dung source and the nest, whereas rolling does not, and (2) it involves a shorter distance between the two sites.

2. Brood-nest founders were usually males and less often females, with about 25% adopting the rolling tactic and 75% adopting the tunnelling tactic. During nest building, the founder paired off with a scarab of the opposite sex, and they co-operated in the work. The female made one to four brood balls from the dung in the nest, each of which contained one egg.

3. Each scarab seemed to be able to employ both tactics. The tactic employed was independent of an individual's status, e.g. body size and timing of nest founding.

4. The rolling tactic offered only male founders a greater nest-defence success than the tunnelling tactic due to a lower intrusion into the rolled nest and a higher intensity of male–male fighting. The tunnelling tactic offered both male and female founders a larger number of brood balls than the rolling tactic because it enabled scarabs to take a larger amount of dung into the nest.

5. The reproductive success for the two tactics was estimated from the product of nest-defence success and the number of brood balls. As a result, the two tactics had equal fitness payoffs for males, but unequal payoffs for females.

6. The results suggest that male alternation of tactics is controlled by a mixed strategy. Female alternation, however, cannot be explained by mixed strategy, alternative strategies or conditional strategy.