Discover the amazing anthills at Coombe Hill

The anthills consist of earthy mounds, covered in vegetation, that reach up to 50cm above the weathered chalk rock beneath (F).  Under each dome-shaped mound is the nest, which can extend up to a metre below the surface. The mound (D) helps to regulate temperature and humidity in the nest, and the roots of plants (C) that grow on the mound provide food for aphids (A), which in turn provide food for the ants.

The nests are highly intricate, with numerous fine channels (G) made in the soil; the whole structure is reinforced by the roots and rhizomes of plants, the growth of which is encouraged by the workers, who defecate into crevices in the soil, providing the plants with valuable nutrients. The mounds usually have one flatter face which is oriented towards the south-east; this maximising the warming effects of the early morning sunshine.

Like all species of ants, the yellow meadow ant lives in organised social colonies, consisting of a reproductive female known as the queen (7–9 mm), a few males (3–4 mm), and a large number of workers (2–4 mm), which are non-sexual females. Their colour varies from yellow to brown, with queen and males being slightly darker. The 'workers' are mainly active underground, so they are seldom seen unless the nests are disturbed. The ants have poor eyesight so they hunt and navigate largely by smell.

Yellow meadow ants are passive and rarely bite. If their nests are attacked they normally withdraw into the safety of the anthill’s underground chambers. If they do have to defend themselves they bite the victim’s skin and squirt a little formic acid from their abdomen into the wound, which can be quite painful.

Lifecycle

During warm, humid days in July and August, alates (winged male and female ants) create holes in the earth mounds, climb out, and swarm in their mating flight. After mating, each female lands on the ground, sheds her wings, and searches for a suitable place to establish a new colony. She makes a new chamber in the ground where she stays throughout the winter. The new queen will then begin to lay eggs (B) in the spring.

Yellow ants as ‘livestock farmers’

Yellow meadow ants feed on the honeydew from aphids that feed on plant roots. The aphids are bred by the ants in the nests (A). Honeydew is a sugar-rich sticky liquid, secreted by the aphids as they feed on plant sap. During winter, the aphids themselves are sometimes eaten by the ants. Occasionally, the ants may forage outside the nests for other insects that are found in grass, including fly larvae, wireworms, woodlice and springtails.

Ants help to protect our butterflies

At Coombe Hill, yellow meadow ants have a special relationship with the caterpillars of chalkhill blue butterflies. The ants are partial to the substances, similar to aphid honeydew, secreted by the butterfly’s caterpillars, so the worker ants carefully bury the butterfly larvae, and in doing so they unintentionally protect them from predators. This is a good example of symbiosis in nature.

Ants add to biodiversity

The mounded nests of yellow meadow ants create numerous small scale micro-sites within grassland ecosystems, each with small differences in soil nutrients, soil temperature, humidity, drainage and sunlight (E). This allows a much greater range of grasses, herbs and other flowering plants to thrive. So the activities of ants help to increase the diversity of plants species at Coombe Hill. The most notable plants to be found are rock rose, marjoram, basil and thyme. These in turn attract other insects, birds and mammals to the chalk grassland.

Managing for yellow meadow ants

If the vegetation at Coombe Hill were to be allowed to grow higher, the sun would no longer warm the anthills, and the ants would abandon them. In the past, grazing sheep and rabbits would have kept the grass low. These days, Coombe Hill is grazed in the summer by cattle and a small population of rabbits. This has to be supplemented by careful removal of scrub and small trees by National Trust rangers, contractors and volunteers in the winter months.

Other National Trust Chiltern Countryside sites with chalk grassland with anthills of yellow meadow include Small Dean Bank (near Bradenham) and on Watlington Hill.