GAMES of THRONES

Wood ants live in vast su­per-so­ci­eties led by pow­er­ful queens. Ross Piper en­ters their hid­den world.

BBC Wildlife Magazine - - Analysis Agenda -

Asub­ter­ranean city, a labyrinth of tun­nels and cham­bers, home to some 300,000 in­hab­i­tants. At its centre, an ageing queen sur­rounded by daugh­ters, her pro­tec­tors, for­agers and nurse­maids. In­side and out, plun­der­ers and preda­tors lurk. No, this isn’t TV block­buster Game of Thrones, but some­thing even more ex­tra­or­di­nary. It’s the re­al­ity of life for some of our most fas­ci­nat­ing in­sects.

Red wood ants are fa­mil­iar to many of us, with six closely re­lated species in Europe’s Formica rufa group, four of which are na­tive to Bri­tain (though one may now be ex­tinct here). Take a walk in a good bit of wood­land that’s home to th­ese ants and you will be hard-pressed to miss them. They’re of­ten present in mas­sive num­bers and their nest mounds can be as much as 2m high. In Bri­tain, some of the best ex­am­ples can be seen in the Scot­tish High­lands.

Look closely at one of th­ese mounds and around your feet and you’ll see thou­sands of scut­tling ants. Many will have taken of­fence to your pres­ence and be stand­ing with jaws agape, or spray­ing formic acid in your gen­eral di­rec­tion, lend­ing the air the dis­tinc­tive odour of th­ese in­sects. Cov­er­ing the mound is a seem­ingly un­tidy thatch of pine nee­dles and other plant ma­te­rial, but it is ac­tu­ally con­structed in such a way as to make the most of the sun’s rays to warm the nest as well as pro­tect­ing it from rain.

A FE­MALE WORKER ANT IS MORE CLOSELY RE­LATED TO HER SIS­TERS IN THE NEST THAN TO HER OWN OFF­SPRING.

The loose thatch of the mound also be­lies the com­plex­ity of the nest be­neath, which is noth­ing short of a marvel of arthro­pod ar­chi­tec­ture. The sub­ter­ranean part is of­ten big­ger than the mound you can see above ground, giv­ing you a good idea of the size of th­ese struc­tures. Shielded by the thatch is the rot­ting tree stump in which the nest was es­tab­lished, cham­bers for the queen, nurs­eries for the brood in vary­ing stages of de­vel­op­ment – all the way from egg to pupa – not to men­tion food stores and rub­bish tips.

HID­DEN WORLDS

Get­ting to grips with the nest’s struc­ture and trac­ing its ev­ery tun­nel and cham­ber is next to im­pos­si­ble with­out dis­turb­ing it in some way. In­deed, the chal­lenges of work­ing out what’s go­ing on in a nest with­out up­set­ting it is one rea­son why we still have plenty to learn about wood ants. The in­ter­nal ar­chi­tec­ture of the nest and the ac­tiv­ity therein is one thing, but the over-arch­ing so­cial struc­ture of th­ese ants in the land­scape is also re­mark­able.

The story of a nest can be acted out in a num­ber of ways, but the be­gin­ning is al­ways the same: young lar­vae in the dark­ness of a ma­ture, healthy nest des­tined to be queens, which are fed more and bet­ter qual­ity food than the rest of the brood. Once adult, the young queens mate in the nest or leave it to mate nearby, either on the ground or in the air – a so-called nup­tial flight.

Hav­ing mated, the queens have to es­tab­lish a nest, and the place they of­ten choose is a cosy re­treat in a rot­ting tree stump. But the queens of many wood ant species are com­pletely in­ca­pable of do­ing this on their own. A young red wood ant queen, for ex­am­ple, be­comes a par­a­site in the nest of a re­lated ant species, re­ly­ing on the host work­ers to do all the hard work for her.

A queen that mates un­der­ground in her birth nest em­ploys a dif­fer­ent strat­egy. In­stead of turn­ing par­a­site she es­tab­lishes her nest very close to the birth nest with the help of a splin­ter group of work­ers. This process, known as bud­ding, is the rea­son why in some pop­u­la­tions of th­ese ants you find nests very close to­gether form­ing a su­per-colony, while in other ar­eas the nest mounds are more widely scat­tered.

Th­ese ‘bud­ded’ colonies re­tain links to the ‘mother’ colony, and if they are too far from food sources they do not make any for­ag­ing trails, in­stead re­ly­ing on food from other parts of the su­per-colony’s em­pire. Some of th­ese non­for­ag­ing nests may even­tu­ally form for­ag­ing trails and are then fully in­te­grated into the su­per-colony, while oth­ers do not and are aban­doned.

Es­tab­lish­ing a nest de­pends on the queen be­ing able to pro­duce many daugh­ters that will main­tain and ex­pand the nest, tend to the brood, find food and re­pel en­e­mies. This abil­ity to pro­duce an army of loyal daugh­ters hinges on the mind-bend­ing na­ture of re­pro­duc­tion in th­ese an­i­mals. The queen ant can con­trol the sex of her off­spring by us­ing the sperm she stored from her sin­gle dal­liance vary spar­ingly.

If a daugh­ter is needed, the queen re­leases a tiny amount of sperm to fer­tilise the egg as it is be­ing laid. If the queen with­holds the sperm, the egg is not fer­tilised and it will de­velop into a son. This quirk is at the root of the suc­cess of ants be­cause it means that a fe­male worker is more closely re­lated to her sis­ters than her own off­spring, so it pays for her to look af­ter her sis­ters and the nest rather than strike out on her own.

With a nest well and truly es­tab­lished, this sis­ter­hood be­gins to ex­ert a pow­er­ful in­flu­ence on its habi­tat. The driv­ing pre­oc­cu­pa­tion of the mul­ti­tude of ant work­ers is sourc­ing food for them­selves and their nest-mates, in­clud­ing the brood, which are par­tic­u­larly rav­en­ous. If you watch a nest – or a for­ag­ing trail lead­ing to it – for any length of time, you’ll see bits and pieces of var­i­ous in­sects and spi­ders that have been butchered and are be­ing trans­ported back to the nest to be fed to the brood.

Any arthro­pods are fair game. Cater­pil­lars, bee­tles or spi­ders that find them­selves on the wrong side of a for­ag­ing party will be quickly sub­dued and dis­mem­bered. Al­though th­ese ants are very ca­pa­ble preda­tors and scav­engers, their most im­por­tant food source, mak­ing up 90 per cent of their diet, is the sweet liq­uid – hon­ey­dew – that oozes from the back-ends of aphids high in the trees that sur­round the nest.

SUP­PLY AND DE­MAND

An ant del­i­cately strokes an aphid with its an­ten­nae to elicit a droplet of hon­ey­dew, which it greed­ily con­sumes – a process called milk­ing. Like minia­ture herders tend­ing their live­stock, the ants are very pro­tec­tive of the aphids and will drive off the many species for whom aphids are a favourite snack. They’ll even move the aphids to bet­ter feed­ing grounds to in­crease the pro­duc­tion of hon­ey­dew. Sat­is­fied ants then head back to their nest and re­gur­gi­tate the hon­ey­dew for the queen and other work­ers.

In tend­ing and pro­tect­ing aphids, th­ese ants have a con­sid­er­able im­pact on tree growth and the struc­ture of the for­est. They also play a cru­cial role in nu­tri­ent cy­cling, tak­ing hon­ey­dew and arthro­pod prey from the canopy and sur­round­ing for­est down into their sub­ter­ranean cham­bers. Th­ese out­side nu­tri­ents, the waste ma­te­rial pro­duced in the nest, and the con­stant dig­ging and tweak­ing of the nest, to­gether help to en­rich the soil for fur­ther plant growth.

In con­struct­ing their elab­o­rate nest mounds the ants move lots of plant ma­te­rial, in­clud­ing seeds: they’ve been shown to be im­por­tant seed dis­persers for many plants, in­clud­ing rar­i­ties such as small cow-wheat. Even though the nest mound is pro­tected by hordes of fierce, acid-squirt­ing work­ers, it is noth­ing more than a pile of food to many larger an­i­mals, such as badgers, ca­per­cail­lies and green wood­peck­ers. To them, the brood deep in the nest are the real prize – they’re less ‘bitey’ than the work­ers.

ECOSYS­TEM EN­GI­NEERS

The in­ter­nal com­plex­ity of the nest, its be­nign con­di­tions and the bounty of food and pro­tec­tion it of­fers, has also been a draw for a ver­i­ta­ble menagerie of other species, some of which are com­pletely de­pen­dent on the ants. As an en­to­mol­o­gist, it is this as­pect of life in­side the nest that re­ally ex­cites me, as the biology of many of th­ese an­tas­so­ci­ates is shrouded in mys­tery.

One-hun­dred and twenty-five species of arthro­pod are known from in or around Euro­pean red wood ant nests. There are 52 bee­tle species, as well as 28 mites, 15 other ants and wasps, 10 flies, eight true bugs, six spi­ders and even a moth. Some live in close prox­im­ity to a nest, oth­ers in­side it; some are not tol­er­ated if dis­cov­ered and oth­ers fool the ants into ac­cept­ing them. It is among the many bee­tle species where we see the most in­trigu­ing ant guests.

Take the rove bee­tle, Lomechusa pu­bi­col­lis, which as a larva lives deep in­side the nest of red wood ants and dupes its hosts into groom­ing, feed­ing and car­ry­ing it by tapping into their odour-dom­i­nated mes­sag­ing sys­tem. In ef­fect, the ants treat the bee­tle larva as one of their own brood. In re­turn, the young bee­tle tucks into the de­vel­op­ing sib­lings of its car­ers. Af­ter pu­pa­tion, the adult bee­tle begs it car­ers for food one more time and heads out­side to seek the nest of another ant species in which to see out the win­ter. As the warmth of spring ar­rives, the bee­tle slips back into another red wood ant nest, where it will breed to com­plete its life­cy­cle.

A strange species of leaf bee­tle, Cly­tra quadripunc­tata, is another spe­cial­ist in the nests of red wood ants. From a leafy perch, the fe­male Cly­tra deftly en­cases each of her eggs in fae­ces be­fore flick­ing them to the ground. Some of th­ese eggs are picked up and car­ried by work­ers into the nest. The bee­tle lar­vae hatch but re­tain their egg case as a mo­bile re­treat, re­coil­ing into it at the first sign of dan­ger, their tough lit­tle heads mak­ing a per­fect plug. They add to the case with their own drop­pings and soil as they grow, even­tu­ally form­ing a per­fect lit­tle pot.

Trundling around the ant’s nest, th­ese lar­vae feed on or­ganic mat­ter and de­tri­tus and are im­per­vi­ous to at­tack thanks to their pro­tec­tive pots. This is not the case for the adult bee­tle when it emerges, and it must run a gaunt­let of an­gry worker ants to es­cape the nest.

We still have a huge amount to learn about the in­ner work­ings of th­ese very fa­mil­iar ants and their com­plex in­ter­ac­tions with other species. We know they are a cru­cial el­e­ment of forests around the world and that they’re ex­tremely vul­ner­a­ble to hu­man ac­tiv­i­ties, which have caused the de­cline and ex­tinc­tion of some species. But only by study­ing them will we fully ap­pre­ci­ate their eco­log­i­cal sig­nif­i­cance and fragility.

ONE-HUN­DRED AND TWENTY-FIVE SPECIES OF ARTHRO­POD ARE KNOWN FROM IN OR AROUND EURO­PEAN RED WOOD ANT NESTS.

Red wood ants are hy­per­so­cial in­sects that live in colonies of up to half a mil­lion in­di­vid­u­als, most of which are work­ers.

Red wood ants carry a cater­pil­lar back to their nest for the colony to eat.

Wood ant su­per-colonies oc­cu­py­ing many sep­a­rate but con­nected nests may have hun­dreds of queens like this one.

Above left: a worker milks black bean aphids, stroking them with its an­ten­nae to draw out drops of hon­ey­dew. Above: another worker car­ries an ant co­coon over the wood­land floor, pos­si­bly mov­ing it be­tween bud­ded and mother nests. Below: all work­ers be­long to an army of loyal daugh­ters.

Above: wood ants in the Formica

rufa group are pro­tected by law in many Euro­pean coun­tries. The in­sects play vi­tal eco­log­i­cal roles.

Above: wood ants cre­ate ven­ti­la­tion holes in their mas­sive nests to deal with the heat of sum­mer. Below: the larva of a leaf bee­tle de­pen­dent on ants, pic­tured in its pro­tec­tive egg case.

ROSS PIPER is an en­to­mol­o­gist and author, and was a pre­sen­ter on the BBC Two se­ries Wild Burma.

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