Phenotype dependent breeding dispersal in male blue dasher dragonflies (Pachydiplax longipennis)

Background/Question/Methods

Individual traits can affect the risks, costs, and benefits of dispersal.  Therefore, dispersers will frequently be a non-random fraction of the population.  Individuals that disperse may be those which incur lower risks or costs from dispersal, or those which have the greatest potential fitness gains from dispersing.  However, for most taxa few data exist on the characteristics that distinguish dispersers and non-dispersers within a population.   The goals of this study were to determine 1) whether dispersers differed from non-dispersers, and 2) what behavioral and morphological characteristics were associated with dispersal probability and/or dispersal distance.  To address these issues, I conducted a mark-release-resight study of individually marked male blue dasher dragonflies (Pachydiplax longipennis).  Males were captured, measured, and marked during three time periods across the flight season.  All marking was done at one pond and then four ponds were monitored for marked individuals.  Each pond was divided into 30m sections and the position within the pond was recorded for all re-sighted individuals.  These data allowed me to examine whether there were differences between dispersers and non-dispersers in both body size and within-pond site fidelity.  
Results/Conclusions

Dispersers from the first two marking periods were a non-random fraction of the population but the characteristics distinguishing dispersers from non-dispersers changed across the season in association with a population level decline in body size.  This decline is common in temperate zone odonates but has not previously been examined as a factor that may affect the dispersal decisions of individuals as they encounter different social contexts across the season.  Among individuals from the first period, dispersers were smaller than non-dispersers.  Dispersing males from the middle marking period had lower within pond site fidelity than non-dispersers but did not differ in body size.  Dispersers and non-dispersers from the last marking period did not differ in either character.  Among those individuals that dispersed there was a negative association between site fidelity at the pond and dispersal distance.  The characteristics that distinguished dispersers from non-dispersers in the first two time periods are also associated with decreased territorial success suggesting that competitively inferior males may be more likely to disperse and may travel further in their search for potential breeding sites than males which have higher territorial success.