A different explanation is implied by young giraffes being far more vulnerable to predation than adults: more than half of all giraffe calves die within a year, and giraffe mothers hide their calves, which spend much of the time lying down in cover while their mothers are away feeding. Since the presence of a mother nearby does not affect survival, it is argued that young giraffes must be very well camouflaged; this is supported by coat markings being strongly inherited. The possibility of camouflage in plants has been little studied until the late 20th century. Leaf variegation with white spots may serve as camouflage in forest understory plants, where there is a dappled background; leaf mottling is correlated with closed habitats. Disruptive camouflage would have a clear evolutionary advantage in plants: they would tend to escape from being eaten by herbivores. Another possibility is that some plants have leaves differently coloured on upper and lower surfaces or on parts such as veins and stalks to make green-camouflaged insects conspicuous, and thus benefit the plants by favouring the removal of herbivores by carnivores. These hypotheses are testable. Some animals, such as the horned lizards of North America, have evolved elaborate measures to eliminate shadow. Their bodies are flattened, with the sides thinning to an edge; the animals habitually press their bodies to the ground; and their sides are fringed with white scales which effectively hide and disrupt any remaining areas of shadow there may be under the edge of the body. The theory that the body shape of the horned lizards which live in open desert is adapted to minimise shadow is supported by the one species which lacks fringe scales, the roundtail horned lizard, which lives in rocky areas and resembles a rock. When this species is threatened, it makes itself look as much like a rock as possible by curving its back, emphasizing its three-dimensional shape. Some species of butterflies, such as the speckled wood, "Pararge aegeria", minimise their shadows when perched by closing the wings over their backs, aligning their bodies with the sun, and tilting to one side towards the sun, so that the shadow becomes a thin inconspicuous line rather than a broad patch. Similarly, some ground-nesting birds, including the European nightjar, select a resting position facing the sun. Eliminating shadow was identified as a principle of military camouflage during the Second World War. Many prey animals have conspicuous high-contrast markings which paradoxically attract the predator's gaze.