Stress hormone-mediated antipredator morphology improves escape performance in amphibian tadpoles

Complete functional descriptions of the induction sequences of phenotypically plastic traits (perception to physiological regulation to response to outcome) should help us to clarify how plastic responses develop and operate. Ranid tadpoles express several plastic antipredator traits mediated by the stress hormone corticosterone, but how they influence outcomes remains uncertain. We investigated how predator-induced changes in the tail morphology of wood frog (Rana sylvatica) tadpoles influenced their escape performance over a sequence of time points when attacked by larval dragonflies (Anax junius). Tadpoles were raised with no predator exposure, chemical cues of dragonflies added once per day, or constant exposure to caged dragonflies crossed with no exogenous hormone added (vehicle control only), exogenous corticosterone, or metyrapone (a corticosteroid synthesis inhibitor). During predation trials, we detected no differences after four days, but after eight days, tadpoles exposed to larval dragonflies and exogenous corticosterone had developed deeper tail muscles and exhibited improved escape performance compared to controls. Treatment with metyrapone blocked the development of a deeper tail muscle and resulted in no difference in escape success. Our findings further link the predator-induced physiological stress response of ranid tadpoles to the development of an antipredator tail morphology that confers performance benefits.

Partial submergence: An undescribed behavioral adjustment for thermoregulation at high ambient temperature in Aeshnidae

Many insects including odonates thermoregulate using a combination of behavioral and physiological mechanisms. At high ambient temperature (Ta), these mechanisms include decreased heat production and increased heat loss. Heat production can be reduced by decreasing activity. Heat loss can be enhanced by perching in a shaded microhabitat where temperature is cooler than in the surrounding environment. Aeshnids, which are intermittent endotherms, increase heat loss at high Ta; also by increasing hemolymph circulation from the thorax, where most metabolic heat is produced, to the abdomen, where it dissipates to the environment by convection. While studying two aeshnid species (Anax junius and Rhionaeschna multicolor) at a Sonoran Desert (Arizona, USA) stream, I observed partially submerged mature individuals of both sexes of these species. This heretofore undescribed behavior was seen only at Ta; ≥ 43 °C and almost exclusively during the hottest part of the day (15:00–17:00 hr), when the daily difference between Ta; and water temperature (Tw) was, on average, largest. A cooling effect of partial submergence behavior on body temperature would, therefore, presumably be most effective also during this period. Several percher species of libellulids were present at the study site. These dragonflies are not known to use endothermy for thermoregulation or to increase hemolymph circulation to the abdomen to dissipate heat at high Ta;, and none was ever observed to partially submerge. It is suggested in aeshnids that partial submergence at high Ta; serves a thermoregulatory function by facilitating body heat dissipation from the abdomen.

Radio-tracking reveals how wind and temperature influence the pace of daytime insect migration

Insects represent the most diverse and functionally important group of flying migratory animals around the globe, yet their small size makes tracking even large migratory species challenging. We attached miniaturized radio transmitters (less than 300 mg) to monarch butterflies ( Danaus plexippus ) and common green darner dragonflies ( Anax junius ) and tracked their autumn migratory movements through southern Ontario, Canada and into the United States using an automated array of over 100 telemetry towers. The farthest estimated distance a monarch travelled in a single day was 143 km at a wind-assisted groundspeed of 31 km h −1 (8.7 m s −1 ) and the farthest estimated distance a green darner travelled in a single day was 122 km with a wind-assisted groundspeed of up to 77 km h −1 (21.5 m s −1 ). For both species, increased temperature and wind assistance positively influenced the pace of migration, but there was no effect of precipitation. While limitations to tracking such small animals remain, our approach and results represent a fundamental advance in understanding the natural history of insect migration and environmental factors that govern their movements.

Consumption of the eggs, hatchlings, and tadpoles of Green Frogs (Lithobates clamitans) by native and non-native predators

Predation by native and non-native predators on the eggs, embryos, and early stage tadpoles can affect the recruitment of offspring into a population. We examined the effects of native (Little Brown Mudbugs, Cambarus thomai; overwintered Rana tadpoles; Common Green Darner, Anax junius, larvae) and non-native (Western Mosquitofish, Gambusia affinis) potential predators on the eggs, hatchlings, and early tadpoles of the Green Frog (Lithobates clamitans). The predators had no effect on survivorship or hatching of L. clamitans eggs. However, tadpole survivorship was significantly reduced by dragonfly larvae and crayfish, but not G. affinis or the overwintered ranid tadpoles. Our observation that invertebrates consumed Green Frog tadpoles while vertebrates did not is consistent with palatability contributing to the tadpoles’ susceptibility to different predators. Our results therefore suggest Green Frog tadpoles, but not eggs or embryos, from some populations may be subject to differential predation by invertebrate and vertebrate predators.

Tracking dragons: stable isotopes reveal the annual cycle of a long-distance migratory insect

Insect migration is globally ubiquitous and can involve continental-scale movements and complex life histories. Apart from select species of migratory moths and butterflies, little is known about the structure of the annual cycle for migratory insects. Using stable-hydrogen isotope analysis of 852 wing samples from eight countries spanning 140 years, combined with 21 years of citizen science data, we determined the full annual cycle of a large migratory dragonfly, the common green darner ( Anax junius ). We demonstrate that darners undertake complex long-distance annual migrations governed largely by temperature that involve at least three generations. In spring, the first generation makes a long-distance northbound movement (further than 650 km) from southern to northern range limits, lays eggs and dies. A second generation emerges and returns south (further than 680 km), where they lay eggs and die. Finally, a third resident generation emerges, reproducing locally and giving rise to the cohort that migrates north the following spring. Since migration timing and nymph development are highly dependent on temperature, continued climate change could lead to fundamental changes in the biology for this and similar migratory insects.

L’anax précoce au Québec : une libellule migratrice

Dans le cadre des travaux du Migratory Dragonfly Partnership, les limites nord des aires de dispersion et de reproduction de l’anax précoce (Anax junius) au Québec ont été mises à jour à partir de la documentation scientifique et des données inédites des participants à l’Initiative pour un atlas des libellules du Québec. Chaque année, dans les basses terres de l’Outaouais et du Saint-Laurent, les premiers adultes immigrant des États-Unis se montrent en mai, exceptionnellement à la fin avril. L’espèce est occasionnellement rapportée dans les contreforts laurentiens et appalachiens de même que dans les régions habitées situées plus au nord du 47e parallèle, sans apparemment dépasser le 50e parallèle le long des côtes de l’estuaire maritime et du golfe du Saint-Laurent. L’émergence de populations estivantes de naïades se produit régulièrement dans la zone tempérée feuillue entre la mi-août et le début octobre tandis que l’émergence de populations hivernantes de naïades s’observe rarement en juin ou au début juillet dans l’extrême sud du Québec. Dans la zone tempérée mixte, des émergences peuvent se produire dans l’enclave bioclimatique du Témiscouata et fort possiblement dans celle du Saguenay–Lac-Saint-Jean. D’après le modèle climatique de Gérardin et McKenney (2001), la limite nord de reproduction de l’anax précoce pourrait atteindre le 49e parallèle, suivant l’isotherme de 15,7 °C pour les 3 mois les plus chauds. Il semble peu probable que l’anax précoce puisse compléter un cycle vital dans la zone boréale, par exemple à l’île d’Anticosti. L’impact des changements climatiques devrait se manifester sur la précocité de l’arrivée printanière des adultes immigrants et sur l’augmentation de la fréquence des émergences de populations hivernantes et estivantes de naïades.