The gatekeeper or hedge brown (Pyronia tithonus) is a species of butterfly found across Europe. Given its preference for warmer weather, the restriction of range expansion can be assumed to be due to climate. Colonies vary in size depending on the available habitat, and can range from a few dozen to several thousand butterflies. Named for its rigorous patrol of hedges and woodland rides, the gatekeeper butterfly is a prime pollinator. The caterpillars primarily feed on fine grasses such as fescues (Festuca) and bents (Agrostis), which are common in meadows and roadside verges.
Similar species and subspecies
It is a member of the subfamily Satyrinae in the family Nymphalidae. A similar species is the meadow brown; the two species can be difficult to distinguish with closed wings, since the underwing markings are very similar. However, the gatekeeper tends to rest with its wings open, whereas the meadow brown usually rests with its wings closed. The gatekeeper is also smaller and more orange than the meadow brown and has double pupils on its eyespots.
Two other similar species of Pyronia are found in southern Europe, the southern gatekeeper (P. cecilia) and the Spanish gatekeeper (P. bathsheba).
P. tithonus has two known subspecies. P. t. ssp. britanniae, defined by Ruggero Verity in 1915, is represented in the British Isles. P. t. ssp. tithonus, defined by Carl Linnaeus in 1771, is not found in the British Isles. Instead, this subspecies is seen in central and southern Europe except southern Italy and in the Mediterranean islands except for southern Corsica and Sardinia.
Physical appearance
The gatekeeper is orange with two large brown spots on its wings and a brown pattern on the edge of its wings. The eyespots on the fore wings most likely reduce or deflect bird attacks, which would explain why the gatekeeper is often seen resting with its wings open. A large number of aberrant forms are known, such as excessa, where specimens have two to four extra spots on the fore wing upperside. The number of spots on the hind wing underside also varies.
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Pyronia tithonus MHNT CUT 2013 3 32 Balma male dorsal.jpg|Male
Pyronia tithonus MHNT CUT 2013 3 32 Balma male ventral.jpg|Male underside
Pyronia tithonus MHNT CUT 2013 3 32 Balma female dorsal.jpg|Female
Pyronia tithonus MHNT CUT 2013 3 32 Balma female ventral.jpg|Female underside
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Gatekeeper butterfly (Pyronia tithonus) male.jpg|Male
Gatekeeper butterfly (Pyronia tithonus) male underside.jpg|Male underside<br/>six spots on hind wing
Pyronia tithonus1.jpg|Female
Gatekeeper (Pyronia tithonus) female ab. excessa.jpg|Female<br/>aberration excessa
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Sexual dimorphism
The male has a dark patch on the upper side of the fore wing that contains scent-producing scales known as the androconia. This is most likely for courtship purposes. Androconia have evolved through sexual selection for the purpose of releasing pheromones for attracting mates. Little is known about how androconia actually function during courtship, and the chemical composition of the pheromones is unknown.
Females typically have more spots than males. Males have more costally placed eyespots, compared to the females, whose eyespots are more spread over the wing margin. In fact, the population contracted before re-expanding beginning in the 1940s. Over the past three decades, the flight range of the gatekeeper has extended northwards in Britain. Furthermore, the length of the flight period has been observed to be significantly shorter close to the edge of the range, suggesting that the extension of flight period and expansion of range are likely to be related. However, the mean flight date and length of flight period are not related. Larger individuals have been found to cover longer distances, and this recent expansion of the gatekeeper may explain the larger size of recent populations.
Food
P. tithonus is a characteristic field-margin species; it feeds on grasses as larvae and nectar as adults. The larvae of the Satyrinae all feed on grasses, such as rough meadowgrass (Poa trivialis), smooth meadow grass (Poa pratensis), and sheep's fescue (Festuca ovina); they are usually green or brown. The pupae are a flimsy chrysalis either hanging upside down or lying in grass. The adults are often found around blackberry plants. The adult butterfly has a short proboscis and the shallow flowers of the blackberry provide an excellent nectar source. The gatekeeper is a relatively active butterfly, but not very mobile, as seen when comparing it to a similar species, Maniola jurtina. Mobility in butterflies refers to the distance covered from flying, while activity refers to how often they are in flight. In an experiment assessing wing damage, P. tithonus showed faster wing damage as a result of their increased activity, and these results showed that activity levels do not necessarily correlate with mobility. Their low mobility may also explain why they can be very abundant at one site, but not at a similar habitat only a few kilometres away. This weather trend may explain why P. tithonus numbers have been low in northern Britain because of the cooler summers and that range expansion has resulted from climate change. Weather as a cause for changes in relative abundance has been supported in other ways, as well. Changes have also been synchronous between species including P. tithonus, with weather being a potential explanation. Based on these findings on the impact of climate, their abundance is expected to become 50% greater by 2080 given a large climate change.
Reproduction
One generation of gatekeeper butterflies occurs each year, with adults emerging in July and peaking in early August, and only a few adults remain at the end of the month. No specific courtship ritual is known, but the male scent spots most likely play a role. Males set up small territories and actively seek out a mate.