Shreeve

The Shreeves are a sapient species of shapeshifters which live incognito among aliens and feed upon electrical energy. A Shreeve called San recalls that she was sent to Earth along with twenty brothers and sisters, dropped from a mothership that cruises the galaxy constantly searching for suitable planets.

Once infiltrated, the Shreeve instinctively assumes the appearance of a rather ordinary individual of the planet's dominant species. They can also assume the appearance of a specific individual if they wish to, but they generally don't.

Culture and society
They're usually a harmless and nonchalant species, neither beneficial nor parasitic to the planets they infiltrate in. Instead, they seem to operate on what seems to be a sort of commensalism, disguising themselves as natives in order to live a safe and comfortable life. They tend to choose an unremarkable appearance and generally avoid drawing attention, as they're not interested in sex, romance or friendships with the planet's native inhabitants.

Instead, all they wish for is a warm, humid environment and some source of electricity to feed upon. They will usually find an ordinary job to pay for a place to live and spend their free time happily resting on a carpet, absorbing energy from nearby electrical appliances and communicating telepathically with the other Shreeves on the planet. They only need to be careful to avoid being discovered, as both their shapeshifting and their ability to absorb even huge quantities of electricity unharmed could potentially lead them to their exposure as aliens.

The Doctor is familiar with this species and doesn't consider them a threat, claiming that they mean no harm and have the right to live their lives as they please.

Appearances

 * Big Finish Doctor Who audio releases - Short Trips: Volume 1 (2010) - "Police and Shreeves", by Adam Smith