Draglines and Assessment of Fighting Ability in Cannibalistic Jumping Spiders

The frequency of injury and death during female-female aggression varies in the jumping spider genus Portia, with interactions being more violent (likely to end in death or injury of one of the combatants) in P. labiata (from Sri Lanka) than in another two species (P. fimbriata from Australia and P. schultzi from Kenya). To investigate the role of draglines in the assessment of fighting ability, two types of tests were carried out: 1) dragline discrimination and 2) mirror image response (Portia's reaction to mirror images is comparable to interaction with conspecific rivals). For both types of testing, triplets of equal-size conspecific females were used: one female (the test spider) was exposed to draglines of two equal-size conspecific females they had not encountered before (donor spiders). The fighting abilities of donor spiders were determined directly by staging intraspecific contests between them. In dragline-discrimination tests (spider placed in petri dish containing draglines from two conspecific females with different fighting ability), females of P. labiata, but not the other two species, avoided draglines of the superior fighter (i.e., they spent the majority of their time on draglines of donor spiders with lesser fighting ability). For mirror-image testing, the test spider was placed in a petri dish containing a mirror and draglines. Each test spider was tested on two successive days, with donor draglines in the two tests coming from conspecific females with different fighting ability. In these tests, females of P. labiata (but not the other two species) spent less time embracing (each spider pressing its forelegs, palps and front of body against the other spider) and more time in a part of the petri dish where view of the mirror was obstructed when on the draglines of donor spiders with greater fighting ability than when on the other conspecific's draglines. Findings from this study suggest that P. labiata females use signpost cues associated with draglines to assess the relative fighting abilities of unknown opponents.