Damselfly projects (PA)

Calopteryx aequabilis (Odonata: Caloperygidae): The River Jewelwing

  • The River Jewelwing (Calopteryx maculata) has lighter wings (with varying degrees of melanization) and is found primarily along large streams and rivers in the northeastern United States and Canada.
  • The Ebony Jewelwing (Calopteryx maculata) has dark wings and is found along various streams and rivers all along the eastern portions of the United States.
  • Tom Castle, an undergraduate, conducting field research on interspecific male aggression among two species of Calopteryx damselflies along a stream in upstate New York.

 

Sexual selection is defined as the differential reproductive success due to competition over access to mates, and it can be divided into categories of intersexual selection (female choice) and intrasexual selection (male competition).  Within species, sexual selection can lead to exaggerated male traits used in attracting females or intrasexual combat. Between species, it can maintain separate species through reproductive isolation even in areas where both species occur. We are currently interested in precopulatory forces that prevent interspecific matings – specifically, we studied intrasexual aggression by territorial males to gain insight into reproductive isolation between the damselfly species Calopteryx aequabilis (The River Jewelwing) and Calopteryx maculata (The Ebony Jewelwing). The importance of intrasexual selection is supported by male wing spot variation in both damselflies – the wings look similar in allopatric parts of their ranges (places where only one species occurs), whereas C. maculata wings are darker and C. aequabilis wing spots are smaller in areas of sympatry (places where the species co-occur). This character displacement (divergent shift in character associated with species recognition) is likely due to selection on one or both species to reduce courting errors or misplace intrasexual aggression.

 

We found two field sites near Ithaca, NY that were similar in appearance and abiotic factors, differing only in relative abundance of C. aequabilis and C. maculata. We caught, marked, measured (spot size), and released males of both species to be used as focal territorial individuals in trials where we tested levels of intrasexual aggression toward conspecifics and heterospecifics. We found the C. maculata males were more aggressive towards large-spotted (vs. small-spotted) C. aequabilis males, thus providing evidence that elevated intrasexual aggression from C. maculata males toward melanized males is driving character displacement in C. aequabilis in areas of sympatry. We presented these results at the Animal Behavior Society meeting in 2013, and our results were published in Behavioral Ecology and Sociobiology in 2014 (PDF).  We are currently interested in looking at intraspecific factors that may affect spot size in C. aequabilis, as well as abiotic factors that may affect the relative abundance and distribution of both Calopteryx species, and are in the process of scouting local sites in PA.