Like the caterpillars of several Nymphalid butterflies, the caterpillars feed on stinging nettles (Urtica dioica) and small nettle, Urtica urens. Adults feed on nectar. The species has one of the longest seasons of any Eurasian butterfly, extending from early spring to late autumn. Adults overwinter in hibernation, emerging on the first warm sunny days of the year to mate and breed. In southern parts of the range there may be two broods each year, but northern insects are inhibited by long length of summer days from breeding a second time. Hatching Tortoiseshell butterflies usually began to emerge from their pupa from mid June into August. They began hibernation sometime in October and immediately show territorial behavior after hibernation. The Tortoiseshell Butterflies that are found in the north usually have one brood a season, whereas further south these butterflies can have two broods. The ability to go through three generations of butterflies in a year is due to the Tortoiseshell Butterflies' decreased thermal requirement. The larvae of this butterfly are social. These larvae can be found on Urtica diocia, which have a high nitrogen content and lots of water in the leaves. Hibernation The Small Tortoiseshell Butterfly tends to enter hibernation by mid to late September. Typically this butterfly will try to hibernate in dark sheltered locations. Because of this hibernation, they need to accumulate a lot of fat to survive the winter. The tortoiseshell needs at least 20% of its body weight in lipids in order to survive, making them much slower. Towards the end of their foraging for hibernation, they are much more susceptible to attacks by birds because of their low muscle to body mass ratio. During the first few weeks of hibernation, Tortoiseshell Butterflies are very susceptible to predator attacks. Up to 50% of the population hibernating in any given area can be eaten. The butterflies that hibernate in areas containing more light, and that are accessible to rodents who can climb, are the most susceptible to this type of predation. During hibernation Tortoiseshell Butterflies are able to supercool in order to keep from freezing. In sheltered areas, these butterflies can stand up to -21 degrees celsius without freezing. However, they experience rapid weight loss during unusually mild winters.

• published: 28 Jan 2014
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