Jack describes Nostrovites as 'carnivorous alien shape-shifters with a taste for human flesh'. They hunt in pairs and mate for life. They are most notable for their method of reproduction; after fertilisation, the female passes her eggs to the male, who stores them in a protective pouch in the throat, where they can be passed into a suitable host through a bite. The eggs are incubated within the host until they are ready to hatch; the female then tracks down the host and tears them open to free the offspring. Female Nostrovites are also resilient to injury, surviving being shot repeatedly. They only appear to attack living prey. Undead Owen Harper does not seem to palatable to them. The female adapts to pose as a woman in a black dress, Rhys's mother Brenda and Jack, after only brief encounters with the latter two, suggesting some slight psychic power (The female also attempted to use Gwen's attraction to Jack to put her off-guard by posing as him). However, they are unable to imitate smells; Gwen recognises Rhys' real mother by her disgusting perfume. When they are revealed as Nostrovites, their eyes turn red and their teeth become elongated and grey.