The Nile crocodile, Crocodylus niloticus, is among the more serious-minded of the world's large reptiles.

It does not hurry. It does not improvise. It conducts its affairs with the measured, unhurried gravity of a senior civil servant who has outlasted every administration since the Cretaceous and has formed settled views about the reliability of schedules. A crocodile with a GPS tracking collar approaches its research obligations in the same spirit: it lies in the water, occasionally surfaces, occasionally does not, and transmits its coordinates with the reliable monotony of an animal that has agreed, however reluctantly, to be useful to science.

It does not, under any circumstances, travel overland at forty kilometres per hour.

This was the detail that first troubled the researchers.

The study had been running for several months with the productive boredom that characterises good field science — a steady accumulation of data points, a gratifying regularity of movement patterns, the comfortable confirmation that the crocodile was doing what crocodiles do, which is to say, very little, very slowly, in shallow water. The team had given it a name, as researchers invariably do, which invests the data with a spurious personality and makes the grant reports more readable.

Then, one morning, it disappeared.

Not the crocodile specifically — the signal remained, active and transmitting, conscientiously reporting its coordinates with the punctual reliability the team had come to expect. What disappeared was any correspondence between those coordinates and the behaviour of an animal that weighs several hundred kilograms and prefers to conduct its life in a horizontal position adjacent to water.

The signal was moving. Rapidly. Across land.

The researchers gathered around their monitors with the particular expression of scientists confronting data that does not fit the hypothesis — a look combining professional scepticism with the dawning, deeply unwelcome suspicion that something has happened which the methodology did not anticipate and the grant proposal did not cover.

Crocodiles, the team confirmed among themselves, do not jog.

The investigation that followed covered several weeks and produced, before its conclusion, a quantity of alternative theories that speaks well of the team's imagination if somewhat less well of their immediate instincts. A vehicle had perhaps struck the crocodile and carried the collar on its bodywork. The collar had malfunctioned. A local fisherman had found it. The river had done something unprecedented.

None of these theories survived contact with the actual explanation, which was provided eventually by a ranger who had been following the signal on foot through the bush with the stoic professionalism of a man who has agreed to track a crocodile and is not yet aware that this is no longer what he is doing.

He found the hyena on a Tuesday evening.

The hyena was going about its business — that particular mixture of purposeful trotting and opportunistic assessment that constitutes a hyena's standard operational mode — entirely unaware that it had, for the past several weeks, been the most productive member of a crocodile research team operating out of a field station forty kilometres to the south.

The collar, recovered eventually for examination, told the story with the impersonal clarity of tracking data, which does not editorialize. The crocodile had died. The hyena had encountered the carcass in the manner of its profession. The collar — robust, waterproof, and designed to survive the considerable indignities of a large reptile's daily life — had survived digestion with rather more composure than its previous host, and the hyena had worn it, internally, ever since.

The researchers sat with this information for some time.