Field notes on things that run themselves
Sitting on Them Would Be Cheaper
A malleefowl does not sit on its eggs. It buries them in a heap of rotting leaves and sand in the mallee scrub of southern Australia and spends most of a year holding that heap near 33 °C. The obvious reading is that the bird has found a way out of the work — let the microbes do the incubating. When somebody costed it, the answer came back inverted. Building and tending a mound across a five- or six-month incubation runs to about twice the total energy an ordinary bird spends by simply sitting down.
The heap starts in winter. The pair scrape out a pit, rake in leaf litter, and cover it with sand; a finished mound can be five metres across and a metre high, and making one means shifting tonnes of dirt. Then nothing happens, because a pile of dry leaves in a dry country is inert. It needs winter rain: wet litter decomposes, and decomposing litter gives off heat. In one six-season camera study, the year of lowest rainfall produced no breeding at any monitored mound.
When the chamber comes up to temperature the female begins to lay — eggs a tenth of her own weight, roughly one a week, often fifteen in a season. Before each, the mound is dug open and the temperature checked — not the tidy gesture of the fact sheets. The bird pushes its head deep into the exposed chamber and holds it there for two or three seconds. Which tissue does the measuring has never been established; the tongue is the leading guess. Germans call it the Thermometerhuhn — an instrument nobody has found.
Then comes the part that makes this a control problem rather than a warm pile. The heat source does not merely fluctuate. It reverses.
In spring the compost is at full output and the mound runs hot, so the bird’s work is to shed heat: open the mound at dawn while the air is cold, let the excess go, rake cool sand back. By late summer the litter has dried and largely spent itself, and the sun has become the main supply — so the work inverts. Now it spreads sand thin to warm in the daylight and shovels it back over the chamber to carry that heat down. The target never moves. The correct action reverses halfway through.
Worth separating from a termite mound (No. 60), which looks adjacent and isn’t. A termite mound rectifies a daily swing it does not control, with no individual assigned to the job; the geometry does the work. Here there is a set point, an error, and one animal measuring the error and correcting it. It is a controller, and the controller has to walk there.
Now the awkward part. “Held to within a degree of 33 °C” is repeated almost everywhere, and the only continuous record from inside natural megapode mounds doesn’t support it. Researchers taped data loggers to freshly laid brush-turkey eggs and left them for the whole incubation. The mound alongside the eggs averaged 33.8 °C, with a standard deviation of 1.9 and a range from 24.5 to 37.5. Heavy rain knocked mounds down four to ten degrees for up to ten days. Embryos spent twelve-hour stretches six degrees above, or nine below, the optimum. They hatched anyway.
Which moves the achievement elsewhere. In an ordinary bird, prolonged incubation a few degrees off optimum yields deformity rather than a chick; megapode embryos turn out to tolerate roughly what a buried reptile egg does. The bird is a worse thermostat than advertised and the egg is a far better subject. Compare No. 69, where a shell’s gas conductance is fixed at laying and adjusted by nobody, so everything inside bends around a boundary that will not move. This is the mirror image: the egg is passive and the environment is driven, by a parent that never touches it.
There is a small loop inside the loop. In the last third of incubation the embryo’s own metabolic heat lifts the egg several degrees above the material around it. The thing being warmed joins the warming.
So what does the doubled cost buy? Two things brooding cannot. A sitting bird is limited to the clutch it can cover; a malleefowl lays for months. And the chicks get nothing — no feeding, no guarding, no parent. A chick claws up through as much as a metre of sand, a climb of hours, surfaces alone, and can fly inside a day.
Elsewhere in the family, the maleo of Sulawesi skips the apparatus entirely: it buries one egg in beach sand or volcanic ground already warmed by the sun or the earth beneath, and leaves. It found a place already at temperature. The malleefowl lives where no such place exists, so it builds one and cannot go anywhere. The mound is not a nest it made. It is a nest for exactly as long as it is being tended.
One loop I’m watching
Next: a star burning hydrogen through carbon that is never used up. In the CNO cycle, a carbon nucleus takes on a proton, becomes nitrogen, then oxygen, and after six steps hands back a helium nucleus and comes out as carbon again, unchanged and ready to start over. The carbon is not fuel; it is the machinery, and the same few nuclei run the reaction for the life of the star. It supplies only about one per cent of the Sun’s output — the Sun is too cool for it, and it takes over in stars above roughly 1.3 solar masses — and nobody confirmed directly that it was running down there at all until a detector under an Italian mountain caught its neutrinos in 2020.
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