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In-flight meal

During migration the ruby-throated hummingbird (Archilochus colbris) tanks up with a few drops of nectar for the last time on the northern shores of the Gulf of Mexico. It then flies non-stop for at least 800 kilometres to reach the shores of the southern Gulf. Can anyone calculate the metabolic fuel efficiency of these birds that fly so far on so little, and how does this compare to a human? (Continued)

• The basal metabolic rate is the measurement of how much oxygen an organism uses when at rest. Just the fact that the hummingbird has a very high body mass-to-surface area ratio gives it a basal metabolic rate that is 12 times as high as a pigeon’s and 100 times that of an elephant.

The metabolism of the ruby-throated hummingbird is much lower when in torpor than in flight. To go from torpor to an active state takes it about an hour. The heart rate rises from 50 beats per minute to 500, and its temperature from 10 °C to 40 °C. When in full flight its heart pumps at 1260 beats per minute and its wings beat 50 to 200 times per second. And this is still more energy-efficient than a human taking a walk.

Ruby-throated hummingbirds belong to a group of birds known as passerines, which have three toes facing forward and one toe back. Passerines tend to have a metabolic rate that is as much as 70 per cent higher than either non-passerine birds or mammals. Their muscles are made up of about 35 per cent mitochondria with densely packed cristae – infoldings of their inner membrane – which makes them at least twice as efficient as human mitochondria.

To achieve the same efficiency, humans would have to have muscles composed of 70 per cent mitochondria. And even then the muscles could not work because there would be too few myofibrils in them.

While wintering in Mexico, the bird doubles its weight by building up considerable fat reserves to use on its long journey north. In fact, most ruby-throated hummingbirds travel along the edge of the Gulf of Mexico eating a little along the way, but some do take the short cut from Yucatan to Florida using their fat reserves and catching gnats along the way.

A number of key factors make the ruby-throated hummingbird so efficient. Its pectoral muscles are red meat, which is rich in the oxygen-carrying protein myoglobin; the muscle has a high capillary-to-fibre ratio, giving it a good blood supply; it has an energy-rich diet of nectar that is stored as fat; it can eat nectar from any species of flower so it does not waste time looking around for a particular source, and will also eat any insects; its tongue is fringed so that the nectar is effortlessly drawn up by capillary action; and it operates at high temperatures to make its metabolic reactions more efficient.

It has been calculated that this species eats as much as three times its own weight in a day, during which it is awake for 16 hours on average. This is the equivalent of an 83-kilogram human eating 125 kilograms of hamburgers every day, or 1,335,000 kilojoules.

Mike Ball

Gorinchem, Netherlands

Double chin

My son-in-law discovered a strange property of the remote control that switches his car alarm on and off. By pressing it firmly to his chin, he can increase considerably the range at which it works. Not believing him, we experimented with other chins. It works, and in some cases the range is doubled depending on the type of car alarm. How does this effect occur?

• Your son’s car remote control has a miniature radio transmitter that is built into the key ring and which sends a coded message to a receiver in the car. You don’t have to point the device directly at the receiver in the car, but it will help to increase the range if you press the remote control against your body. In fact, any part of the body will do, not just your chin. Arms and legs work too. And you can have endless fun pointing your bum at your car while pressing the key ring against your buttocks.

The range over which the remote control operates nearly doubles when you involve your body because your body picks up the radio signal and acts like a large aerial, transmitting it more effectively to the car.

The key and the body act like the two plates of a capacitor separated by an electrical insulator. When an electric charge flows into or out of one of the plates, the electrostatic effect drives a similar charge into or out of the other. Although no charge actually crosses the insulator, a current appears to flow for a while until the capacitor is fully charged. Applying a varying current to one plate means that the capacitor never gets fully charged and a current appears to flow indefinitely through the second plate.

This phenomenon, which is called capacitive coupling, works best with rapidly varying currents, which is exactly what the radio transmitter in the key ring produces.

Because it operates at a frequency of 433 megahertz, the current from the key can cross the insulating barriers of your clothing and skin to reach the conducting interior of your body. Your body then acts as a giant aerial when you press your remote control against it – Ed.

This week’s questions

Water substitutes

Is water the ideal liquid in which to swim? Assuming there are no ill effects to your health, would a different liquid that was either denser and more viscous, or had some other property, enable one to swim faster or with less effort?

Robert Laing

London, UK

Toxic tatties

When I was a child my grandmother told me that I should never eat the green areas of skin on old or damaged potatoes. I’ve since learned that this contains a toxin similar to that found in deadly nightshade. But how much green potato skin would I have to eat before falling ill, and what would the toxin actually do to me? Do similar problems lurk in species that are related to potatoes, such as yams or aubergines?

Emily Jane Horseman

Buxton, Derbyshire, UK