Is your life full on? Well, try keeping up with a hummingbird. If the agile aeronauts aren't constantly on the go searching for high-energy nectar during they day, they run out of carbohydrate fuel rapidly and have to switch to burning fat, which they don't carry much of. And if the temperature drops or the diminutive birds settle down to sleep, they simply don't have enough fuel on board to maintain their body temperature, so the tiny creatures slip into daily torpor – a type of short-term hibernation – when their body temperature plumets from 39°C to below 10°C, to conserve energy. Tiny mammals also use a low-temperature hibernation strategy to overwinter when food is scarce. They turn down the mitochondria – the cell's power packs – which reduces the amount of ATP available to fuel warmth and activity, allowing their body temperature to fall quickly. But no one knew whether hummingbirds use the same mitochondria-supressing strategy to slip into torpor. Hibernation experts Jim Staples and Amalie Hutchinson from The University of Western Ontario, Canada, teamed up with migration specialist Chris Guglielmo to find out whether ruby throated hummingbirds (Archilochus colubris) also turn down their mitochondria to conserve energy.
‘Ruby-throated hummingbirds are seasonal long-distance migrants that inhabit eastern North America, and winter in South and Central America’, explains Hutchinson, who captured 17 of the birds during their Canadian summer sojourn. The researchers then encouraged some of the birds to slip into torpor by turning the lights out, removing their nectar supply and chilling the room to 10°C over night. Once the hummingbirds’ body temperature had fallen, the team recorded their oxygen consumption and carbon dioxide production to confirm that they were truly torpid. Then the team collected portions of the tiny birds’ pectoral muscles, which comprise one-third of their body mass and generate warmth when the animals are active, to check on the muscle's mitochondria to find out whether the chilly hummingbirds had essentially turned down their mitochondrial power packs.
However, after months of meticulous analysis, it was clear that the hummingbirds were not turning down their mitochondria to conserve energy when the temperature dropped and they were unable to forage in the dark. The mitochondria of the torpid birds were every bit as active as the mitochondria of lively ruby-throated hummingbirds as they flew around in search of nectar to sip. So, the birds do not use the same energy-conserving strategy as tiny mammals to reduce their energy consumption when the temperature falls and food is scarce.
Having ruled out mitochondria as the source of the mechanism that allows hummingbirds to reduce energy use and become torpid overnight, Hutchinson and colleagues suggest that the birds may instead reduce ATP use by their muscles to conserve energy. Alternatively, their diminutive body size, and relatively large surface-to-body-size ratio, could allow their body temperature to decline rapidly when they stop moving, so that their metabolism might slow naturally. Either way, ruby-throated hummingbirds use a completely different tactic from hibernating mammals to reduce their energy consumption, and Hutchinson and colleagues are keen to find out precisely how they conserve energy when it's too cold to keep warm.