Migrating hummingbirds put a new twist on torpor

Abstract

Hummingbirds are some of the most metabolically active birds, beating their wings 50 times per second while sipping nectar from flowers and, as a consequence, they live their lives on a metabolic knife edge. For some species – such as ruby-throated hummingbirds (Archilochus colubris) – these energetic challenges are made worse by the need to engage in other taxing activities such as a 5000 km autumn migration from Canada to Costa Rica. So it makes sense that these birds would take advantage of any physiological tools available to ensure that they have sufficient metabolic fuel in their daily struggle to survive. One important tool that hummingbirds use is torpor, an energy-saving strategy where metabolism, activity and body temperature are suppressed for short periods of time. These birds generally use torpor to make it through cold nights when their fat stores are low. Although this behaviour is well understood in many contexts, Erich Eberts from the University of Toronto, Canada, and colleagues from the same institution and the University of Western Ontario, Canada, wanted to understand if these birds could alternatively use torpor as a way to maximize their fat stores as they entered their migration season.The scientists caught hummingbirds in London, Canada, and housed them at the University of Western Ontario, simulating the daylight conditions that the birds would experience during the transition from the midsummer breeding season (15 h light:9 h dark) to the migratory season (12 h light:12 h dark). During this period, the researchers tracked changes in fat content and body mass of individual birds to determine when they began preparing for migration. The team also measured the metabolic rate of these same birds as they slept to determine when and if these birds reduced their metabolism and switched into torpor.During the midsummer, the thinner hummingbirds with lower fat stores dipped into torpor earlier and remained torpid for longer – confirming that these birds use torpor when they had the least energy available. But as these birds readied themselves for migration – as days became shorter – their entry into torpor became more predictable and fatter birds went into torpor more often. By abandoning the ‘old rules’ surrounding the use of torpor as a survival strategy, hummingbirds are able to reduce their metabolism to stock up on fuel to ensure that they can maximize their energy stores for migration.Life for hummingbirds involves a constant battle to ensure that they have plenty of metabolic fuel. Eberts and colleagues have shown that these birds can change the rules that govern the use of torpor based on their environmental conditions. This means that torpor is something of a metabolic Swiss Army knife – a multifunctional tool that animals can repurpose to increase their odds of survival. This research opens up new and exciting questions about how hummingbirds control this switch in torpor use and whether other animals are similarly capable of bending the rules.