AGE-RELATED VARIATION IN BODY-TEMPERATURE, THERMOREGULATION AND ACTIVITY IN A THERMALLY POLYMORPHIC DRAGONFLY

Thoracic temperatures (T-th) Of Libellula pulchella dragonflies during activity in the field were compared between age classes and with labo ratory measures of optimal thoracic temperature for flight performance (T-th,T-opt; a trait that varies during adult maturation in this spec ies). Newly emerged adults (tenerals) had mean T-th values during flig ht (34.5 degrees C; range 29-40 degrees C) that did not differ from th eir mean T-th,T-opt (34.6 degrees C; range 28.5-43.8 degrees C). Matur e adults had higher and more precisely regulated thoracic temperatures (mean T-th 41.7 degrees C; range 37.5-45.2 degrees C), which were som ewhat lower than their mean T-th,T-opt (43.6 degrees C; range 38.7-49. 9 degrees C). Among matures, behaviors requiring the highest levels of flight exertion (aerial copulation; mate guarding; escalated territor ial contests) caused an elevation of T-th above that of concurrently s ampled individuals engaged in routine flight (mean T-th difference 1.3 degrees C), which raised mean T-th to a level that was not significan tly different from T-th,T-opt (42.5 versus 43.5 degrees C). Compared w ith tenerals, matures spent more time flying, made longer-duration fli ghts and showed a more restricted pattern of daily activity. Sympatric Anax junius dragonflies that regulate T-th endothermically had a unif orm pattern of activity across the entire day, i.e. occupied a broader -ecological niche than that of L. pulchella. These results support the hypotheses that optimal body temperature evolves to match the elevate d body temperatures that occur during exercise and that the ecological benefits of an expanded niche are a secondary benefit rather than a p rimary selective force during the evolution of homeothermy and high bo dy temperatures.