Pheromonally mediated colonization patterns in the velvet worm Euperipatoides rowelli (Onychophora)

We demonstrate that the males of the peripatopsid Euperipatoides rowelli se crete a pheromone from crural papillae, which acts as an attractant to both males and females of the species. Patterns of colonization of decomposing logs and differences in sex ratio between incipient vs established populati ons indicate that males are the initial dispersers and colonizers, finding suitable log habitats in an exploratory fashion, while females subsequently colonize logs. This results in a disproportionately high frequency of male s in newly colonized logs, followed by a gradual increase in female proport ion. We argue that females use the aggregating pheromone secreted by males to target appropriate rotting log microhabitats, resulting in a clumped dis tribution of females compared to a more random distribution for males among recently colonized logs. This mode of colonization reduces the time that a nimals, especially females, spend outside the safety of suitable logs, and the risk-taking strategy of male exploration may explain the marked sex rat io bias in favour of females in the population. Rapid and non-random disper sal may also account for the contradictory evidence of unexpectedly high ra tes of colonization of new logs, yet a paucity of animals in leaf litter sa mples. The possibility that pheromone trails play a part in the dispersal p rocess is discussed.