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(c) Gus Jones

Narrow-headed ant (Formica exsecta) © (c) Gus Jones

Narrow-headed ant

(c) Gus Jones

Fast facts

  • Latin name: Formica exsecta
  • Notable feature: Deep notch at the back of its head.
  • Rarity in UK: Rare / Common

The Narrow-headed ant (Formica exsecta) is a very rare wood ant. In the UK, it is restricted to the Scottish Highlands and an isolated population in Devon. However, it is important for the entire ecosystem – in particular, the seeds of many plants are dependent upon them in areas of newly regenerating forest! With this species declining due primarily to habitat loss, the remaining nests are becoming isolated and are in danger of being lost themselves.

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Although there are other species of wood ants – all of which are a reddish colour and similar size (queens are around 12mm long, and workers 10mm) – you can identify the Narrow-headed ant by the deep notch at the back of its head.

The Narrow-headed ant lives at woodland edges and open areas within forests, and is associated with areas of forest regeneration. This ant performs an important role in this regeneration process, helping establish plants whose seeds are dependent upon ants (myrmecochorous plants), such as the rare small cow wheat (Melampyrum sylvaticum). It is also an important prey species for the endangered Capercaillie (Tetrao urogallus), and also helps manage the lekking habitat for this species.

Woodants

A number if different species of wood ants live in our heathland and pine forests. They are all very territorial, and will attack other species. Indeed, Narrow-headed ant workers have been known to climb on to the backs of larger wood ants to decapitate them with their powerful mandibles! However, they all have slightly different habitat preferences – or niches – allowing them to coexist. For example, while Narrow-headed ants live in open areas, the Scottish wood ant (Formica aquilonia) lives in more mature, shaded areas. Therefore, maintaining natural dynamic woodland with a mosaic of different ages of forest is essential for the conservation of this species, and all other wood ants!

High society

Like all ants, Narrow-headed wood ants live in colonies, with complex societies. Different casts of ants – queens, workers and males – perform different roles in the colony. The workers forage for food, following established trails under and overground, and through trees, and live for about a year. In the meantime, the queens produce eggs to replace workers. In spring, new queens and males are raised. In possession of wings, they leave the nests and mate. The fertilised female then returns either to her home nest, another nearby or may establish a new one close-by. Shedding her wings, she settles down to lay eggs, and may live for up to 15 years! On the other hand, the males, with their job done, die shortly after mating.

One nest may contain many queens, and around 1000 workers!

Masters of eco architecture

Narrow-headed ants build giant nest structures – much like massive human buildings! What’s more, they are masterfully designed to take maximum advantage of renewable energy and local resources!

The nests are normally a dome, around 25cm in diameter and 30cm tall. They build the nest around a tussock of grass, soil or another feature. This provides not only a foundation for the nest, but also a source of heat from decaying vegetation. Although at first glance the nest may appear symmetrical, it’s not. If you look closely, you will notice that the south facing side is larger and flatter than the other sides. This maximises the amount of sunlight hitting the nest, and provides solar-powered heating! The workers will also ‘sunbathe’, and then return to the brood chambers, where the eggs and young are. Radiating the heat gathered while basking, the workers act just like storage heaters!

The final, and very impressive touch is the ‘thatched roof’ that the ants make. This covers the entire nest, and is made from grass, heather and pine needles. This serves as fantastic insulation, and conserves the heat within the nest!

These nests provide habitat for other species as well, including the Shining guest ant (Formicoxenus nitidulus).

The Narrow-headed ant is truly a master of eco architecture and sustainable design!

Hunter-gatherers with a secret weapon

Narrow-headed ants stalk other invertebrates, and will cooperate to take prey larger than themselves. When they attack their prey or are themselves threatened, they may use their secret weapon – acid! A gland in their abdomen produces formic acid, which they can fire up to 10cm, with great accuracy.

They also ‘milk’ aphids for honeydew – which is very nutritious. As aphids feed on tree sap, which is rich in sugars, salts and other nutrients, but low on protein, they drink a lot and excrete the excess nutrients as honeydew. The ants, in return, protect the aphids from predators and other competition from other sap-sucking insects.

National decline

This ant once lived throughout the UK, including the New Forest, Bournemouth, the Isle of Wight and Devon in England. Now, only one site remains at Devon, and it is feared this wonderful animal will soon become extinct in England, making the Scottish stronghold all the more important!

This decline is thought to be the result of a combination of factors, including the dramatic loss of heathland habitat in England, disturbance through human activities (such as dirt-biking and pony grazing), and commercial forestry which destroys the habitat structure and is far too dense and dark for these ants. These same factors threaten the remaining populations, while fragmentation is also a serious concern. Narrow-headed ants cannot disperse far – they can’t travel over unsuitable habitat to establish new colonies, or to mate. This results in inbreeding, as all of the colonies in one area are closely related and no queens are arriving from more distantly related populations. It also means that there are areas of suitable habitat that are unoccupied by the ants, as they just can’t get there!

The core population is now found in the Caledonian Pine Forest, at Abernethy, Glenmore and Rothiemurchus. It is listed as endangered in the UK Red Data Book, and is both a UK Biodiversity Action Plan (UK BAP) Priority Species and listed on the Scottish Biodiversity List. Two important areas in Scotland are receiving protection and conservation management – one as a Site of Special Scientific Interest (SSSI) and RSPB reserve, and the other a Forestry Commission Scotland forest where foresters are proactively helping to protect and enhance populations of the ant. The Scottish Wildlife Trust has also campaigned successfully for the better protection of this ant over the last decade and continues to provide management advice to those landowners with ants on their land. The last remaining population in England also occurs on an SSSI.

A few remote colonies have also recently been found at the fringes of their range. These are at Mar Lodge (Deeside) and Rannoch Moor (Perthshire). However, they are entirely isolated, and very small populations. While the core sites at Abernethy and Glenmore hold around 150 and 120 nests respectively, Mar Lodge only had 16 nests, but three of these have since been lost. Urgent conservation action is required if these sites are to continue to support these fantastic ants!

 

(c) Chris Catherine

Narrow-headed ants nest © (c) Chris Catherine

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