Age-Related Variation in Body Temperature, Thermoregulation and Activity in A Thermally Polymorphic Dragonfly Available to Purchase

Thoracic temperatures (T_th) of Libellula pulchella dragonflies during activity in the field were compared between age classes and with laboratory measures of optimal thoracic temperature for flight performance (T_th,opt; a trait that varies during adult maturation in this species). Newly emerged adults (tenerals) had mean T_th values during flight (34.5 °C; range 29–40 °C) that did not differ from their mean T_th,opt (34.6 °C; range 28.5–43.8 °C). Mature adults had higher and more precisely regulated thoracic temperatures (mean T_th 41.7 °C; range 37.5–45.2 °C), which were somewhat lower than their mean T_th,opt (43.6 °C; range 38.7–49.9 °C). Among matures, behaviors requiring the highest levels of flight exertion (aerial copulation; mate guarding; escalated territorial contests) caused an elevation of T_th above that of concurrently sampled individuals engaged in routine flight (mean T_th difference 1.3 °C), which raised mean T_th to a level that was not significantly different from T_th,opt (42.5 versus 43.5 °C). Compared with tenerals, matures spent more time flying, made longer-duration flights and showed a more restricted pattern of daily activity. Sympatric Anax junius dragonflies that regulate T_th endothermically had a uniform 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 elevated body temperatures that occur during exercise and that the ecological benefits of an expanded niche are a secondary benefit rather than a primary selective force during the evolution of homeothermy and high body temperatures.