Territorial defence and its seasonal decline in the speckled wood butterfly ( Pararge aegeria)

Abstract

By means of analysis of films (24 frames/s) of male-male interactions we show that male Pararge aegeria butterflies fight aggressively over ownership of sunspot territories in the ground layer of woodland. The duration of these encounters decreases from a mean of 50 s and 85 s in the first two days to a mean of 10 s after a week's activity. This decline is best correlated with the progression of the season but also with the daily maximum temperature and the density of males, measured as number of encounters per h. Releases of virgin females suggest that the sunspots are used as mating rendezvous sites. If so, sunspots might be more attractive to both males and females when ambient temperatures are low, so that the importance of sunspots as mating rendezvous sites (and concomitantly, the benefit of territory ownership) decreases as the embient temperatures increase with the progression of the season. Our results are largely consonant with those of Davies (1978b) except for the observation that prior ownership of sunspots does not function as a cue for settling territorial conflicts in our study. We observed 17 instances in which the original territory owner regained ownership after a few minutes' absence, during which time a newcomer had landed in the sunspot territory. Possible explanations for the discrepancy between Davies' and our results are discussed.