Cats are sit-and-wait predators, says Steve Harris. They pin small mammals to the ground, then often use them as playthings before carrying them away in their mouths. As a result, the victims may suffer internal injuries or puncture wounds, though these are usually fatal.
Since the paralysis was temporary, your voles and shrews may be affected by capture myopathy. This condition is usually associated with extreme exertion, but can also occur in animals experiencing fear or anxiety.
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Victims lose the use of their hind limbs, either dragging them or lying stiffly on their sides. They may appear to recover after a while, but the prognosis is poor and they generally die within a few days.
Another possible explanation is some form of catalepsy, when animals freeze in response to being snared by a predator.
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Mice, for instance, often enter a brief period of cataleptic-like immobility when picked up by the scruff of the neck (the most common way a cat carries its quarry). However, if otherwise uninjured, they have a reasonable chance of making a full recovery.