Reading: Sentence Completion: Question 1

The duck-billed platypus, found only in the rivers and streams of eastern Australia, is one of just two mammal groups that lay eggs, yet its most extraordinary adaptation has nothing to do with reproduction. When a platypus dives to hunt along a riverbed, folds of skin seal shut over its eyes, ears and nostrils, leaving the animal effectively blind, deaf and unable to smell for the length of a typical dive, which lasts around ninety seconds. Deprived of its ordinary senses, the platypus instead relies almost entirely on its bill, a soft, flexible structure covered in roughly forty thousand tiny sensory receptors.

These receptors are of two distinct kinds. Mechanoreceptors detect the faint pressure waves created by a moving shrimp or insect larva pushing water aside, while a second, rarer type, electroreceptors, can sense the weak electrical field generated whenever a prey animal's muscles contract. By comparing the tiny delay between the mechanical and electrical signals reaching receptors on either side of its bill, the platypus can calculate not just the direction of its prey but roughly how far away it is, a technique researchers have compared to a simple form of biological triangulation.

Electroreception of this kind is rare among mammals; it is far more commonly associated with sharks and rays, whose electroreceptive organs, called ampullae of Lorenzini, evolved entirely independently. The platypus and its close relative the echidna appear to have developed the ability through a completely separate evolutionary pathway, which is why biologists regard the platypus bill as a striking case of convergent evolution rather than a shared inherited trait. Laboratory studies in the 1980s first confirmed the bill's electrical sensitivity by recording nerve responses to a weak artificial current, overturning the earlier assumption that the organ served only a mechanical function. Because a hunting platypus swims with its eyes closed, sweeping its bill from side to side in a continuous searching motion, researchers now describe the behaviour less as swimming than as a form of underwater scanning.

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

1. While diving with its eyes, ears and nostrils sealed shut, a platypus hunts primarily using its ____________________.
2. A typical platypus hunting dive lasts about ____________________.
3. The platypus's bill is covered in around ____________________ tiny sensory receptors.
4. Receptors that detect pressure waves from a moving shrimp or insect larva are called ____________________.
5. Electroreceptors sense the weak electrical field produced when a prey animal's ____________________ contract.
6. In sharks and rays, the organs responsible for electroreception are called the ____________________ of Lorenzini.
7. Because the platypus and its relatives evolved electroreception separately from sharks and rays, biologists treat the platypus bill as an example of ____________________ evolution.
8. Nerve-response studies conducted in the ____________________ first confirmed that the platypus bill could detect electrical signals.