Reading: Summary, Note & Table Completion: Question 1

Across the salt pans and gravel plains of the Sahara, the desert ant Cataglyphis fortis forages for the bodies of insects killed by the extreme midday heat, often venturing more than a hundred metres from its nest along a winding, indirect route. Yet when a forager has found food, it does not retrace its outward path; instead it turns and walks in a single straight line directly back to a nest entrance no wider than a few millimetres, in a landscape almost entirely without visible landmarks. This remarkable feat of orientation, known as path integration, has made Cataglyphis one of the most closely studied navigators in the animal kingdom.

Path integration requires an animal to continuously track two things: the direction it has travelled and the distance covered, so that at any moment it can calculate a direct vector back to its starting point. Researchers established that the ant's sense of direction depends on a specialised region of its compound eye that detects the pattern of polarised light scattered across the sky, a pattern invisible to the human eye but which shifts predictably with the sun's position. Even under a mostly overcast sky, a small patch of blue is often enough for the ant to maintain an accurate heading.

Measuring distance proved harder to explain until a set of experiments manipulated the ants' legs directly. Foragers were captured after reaching a feeder, and their legs were either shortened using fine scissors or extended by gluing on stiff bristles, before the ants were allowed to walk home. Ants on stilts consistently overshot the nest entrance, while ants on stumps stopped short of it, in direct proportion to the change in leg length. The conclusion drawn from this work was that Cataglyphis measures distance by counting its own steps, effectively functioning as a built-in pedometer rather than relying on any external cue such as energy expended or time elapsed.

What continues to intrigue biologists is how the ant's tiny brain, containing fewer than a million neurons, integrates these two independent streams of information, compass heading and step count, into a single continuously updated vector, recalculated with every step of a foraging trip that may last less than ten minutes in total.

Complete the summary below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

The Saharan ant Cataglyphis fortis forages along an indirect route but returns to its nest via a single straight line, a navigational skill called (1) ____________________. To determine direction, the ant relies on a part of its compound eye that detects (2) ____________________ in the sky, a pattern that shifts with the sun's position. To measure distance travelled, researchers experimentally altered the ants' leg length by attaching (3) ____________________ or by shortening the legs with scissors. Ants fitted with longer legs consistently (4) ____________________ the nest entrance, showing that distance is measured by counting its own (5) ____________________ rather than through cues such as energy expended. This step-counting mechanism is often compared to a built-in (6) ____________________. Overall, the ant's brain, which contains fewer than a million neurons, must combine compass heading with step count into a single vector, recalculated throughout a foraging trip lasting under (7) ____________________.