Party in the Park

Summer’s here and the time is near, for long days, sunshine (if we’re lucky) and, of course, picnics in the park.  But with picnics come the challenges of an encounter with one of the world’s most efficient hunter-gatherers.  Ants.

Have you ever noticed that ants never seem to take long to find you and your picnic?   Not only that, but once they’ve tracked you down, it’s not long before a single ant becomes a huge stream of hungry and fearless food gatherers.  Although they can be a nuisance, they deserve to be treated with respect.  Here’s a quick look at one of the most amazing family of animals on Earth.

Ancient Ants

Ants have been around for a long time, always a mark of a successful group of animals.  The oldest preserved ant is around 80 million years old, trapped, in some amber (fossilized plant sap).

Latest research suggests that the ants, whose closest relatives are wasps, have been roaming the earth for at least 100 million years.  This explains how ants all over the world are so similar; they’ve been around since the time when the separate continents we see today were joined together in one single landmass.

Whether you’ve stumbled across an African Driver ant (the world’s biggest ant – queens are up to 5cm in length) or a worker ant from a colony of Oligomyrmex bruni from Sri Lanka (the world’s smallest – less than a millimetre long) you’ll recognise their body shape as “ant-like”.

Ants, Ants, Everywhere

There are 8,800 different species of ants in the world that have been named, with more being discovered all the time.  Estimates of how many may be waiting to be discovered vary, but most researchers seem to agree on roughly 20,000 species in total.  Estimates vary, but at any one time, there are thought to be around 10 quintillion (that’s a million-billion, or 10 with 15 zeros after it) on the planet surface.

You can find ants in surprising places.  Today, you can stumble across ants everywhere from hot deserts to the cold arctic.  There are ants that survive in temperatures as low as –40C, and not one, but 14 species that make their homes on land too cold for trees to grow on.  At the other extreme, there are some that can forage in the midday sun of the world’s hottest deserts, running across the sand in temperatures that would cook an egg.  The majority of ants, however, live in the tropics, the hot, fertile land north and south of the equator.  In the cooler, temperate regions to the north and south of the tropics, there are 760 different species.  In the UK we have 42.

So, what is the secret of all this success?

It could be strength; ants regularly carry 5 times their own body weight and can drag 25 times their weight.  Impressive, but there are stronger insects that haven’t conquered the world.

It could be their stings, as they all have the ability to make acid (formic acid), which they can inject when they bite.  Unpleasant, but many ants don’t bite, choosing instead to live a more peaceful life.

It certainly isn’t their choice of food, as this is as varied as their choice of living conditions.  Some ants are mainly scavengers, who pick up what they can find, others are aggressive hunters, some, even farm fungus to eat, growing it in their nests on special gardens of leaves which they collect for the purpose.

The key to their success is actually organisation.

Success in Numbers.

Ants are part of a larger family of insects, the Hymentoptera, often referred to as the ‘social’ insects.  This large group also includes the bees and wasps.   With a few exceptions, all members of this family live in colonies where every member is related to the others.  A single queen starts the colony and then spends the rest of her life laying eggs, allowing the colony to grow.  Because every member of the colony shares the same parents they don’t compete with each other for food and space but instead work together.  For this reason, ants and their relatives are often referred to as ‘super-organisms’ because they act together to ensure the survival of the colony just as the different parts of your and my bodies work together to keep you alive.  Just as your body parts are specialized to do different jobs: heart for pumping blood, brain for thinking, muscles for moving things, the different members of a colony of bees, wasps or ants are specialized to do different jobs that allow the colony to function.  Of all the social insects, the ants are the ones that show the highest degree of specialization.

Caste of Thousands

The different specialist members of an ant colony, are referred to as castes.  Basically, there are workers, queens, gynes (young fertile females, potential queens of the future) and males.

The Queen can live for twenty years and lays the eggs that hatch and grow to form the colony.  With chemicals she produces (pheremones), she also, to an extent, controls the behaviour of the other colony members.

Some species of ants have a single queen in their colonies whilst others have a number.  The multi-queen species have the advantage that their colonies can survive the death of a single queen.  One colony of Sphecomyrma freyi,  for years thought to be the largest in the world, in Japan has 1,080,000 queens and 306,000,000 workers spread between 45 connected nests, spread over 26km of tunnels.  Although there is still argument about this, a new, potentially larger super-colony has recently been discovered, a lot closer to home, in Europe.  This super-colony stretches 6,000km, from northern Italy, through the south of France to the Atlantic coast.  The ants, called Linepithema humile, are all related to each other and are invaders from Argentina.  They were introduced to Europe around 80 years ago and have spread with incredible speed across the continent, pushing out 30 or so native species of ant.  Because they have spread so far, so fast, their numbers have yet to rise to the levels where different colonies would be competing for the same food.  It is thought that when their numbers rise high enough, their behaviour will change and individual colonies will start to become aggressive toward each other – as they are in their native Argentina.  Until then, they’re all on the same team, a team with several billion members.  At the other end of the scale, nests of the British ant Leptothorax tuberointeruptus on the British coast can consist of just 50 workers and a single queen.  These nests can fit into a space equivalent to the area between 2 one-penny pieces separated by a few grains of sand.

Queens only mate once in their lives, and most of the eggs they lay will grow into sterile, female, worker ants, incapable of breeding.  At certain times however, a queen produces eggs that will hatch into gynes, capable of becoming queens, and males.  Although there are exceptions, gynes typically have wings, and take to the air, pursued by the males.  The males who fly fastest get to mate with the gyne.

These ‘nuptial flights’ are often synchronized, with many nests in a given area taking to the air at the same time.  In Southern England, the common black ants, Lasius niger, all come out together, providing a feast for predatory birds and often blackening pavements with their bodies.  After the flight, the males die; the mated female, now a queen, looks for somewhere suitable to settle down.

Once she has found a suitable site, the queen loses her wings and her body re-absorbs her flight muscles.  Her flying days are over, from now on, she’ll stick to laying eggs.  This is a priority, because until she can produce some worker ants to bring her food, she has got to survive on her internal food stores.

All work and no play

Because of her egg-laying role, the queen is often called the ‘most important’ member of an ant colony.  However, without workers, there would be no colony.  The workers are the ones who, you guessed it, do all the work!  They are also the ones that display the highest levels of specialization for their tasks.  This is what really set the ants apart from the bees and wasps, whose workers are all very similar.

The degree of variation between the different types of worker varies between species.  Typically, there are major, medium and minor workers.  The major workers are usually responsible for defending the colony.  Typically, they have huge jaws, sometimes so large that they can’t feed themselves and have to be fed by other workers.  Some species of ants have major workers specialized for shifting obstacles, such as marauder ants: Pheidologeton diversius has major workers whose heads are 2.5mm across, ten times the size of the other workers.

Perhaps the biggest difference in worker types is shown by the leaf-cutter ants.  The largest are the heavily armoured and massive jawed soldiers who defend the colony (sometimes classed not as major workers but as a separate caste of ant entirely), next come the strong major workers who cut leaves from bushes and trees and carry them back to the nest.  Within the nest there are medium workers who have blunt mouthparts adapted to chewing the leaves into pulp so they can be added to the fungus garden that grows within the nest.  The fungus garden is tended by a class of minor workers who keep everything growing well.  Tiny, even more adapted minor workers travel on the heads of the foraging ants to defend them against parasitic wasps.  These wasps, unless fought away, would lay their eggs inside the forager’s heads (where they hatch into maggots that would eat the ants alive from the inside out – gross).

Finding Food

Hunting and foraging ants use their jaws, their formic acid attack, surprise and pursuit, brute force and some even have a venomous sting in their tails.   The range of techniques and strategies is truly amazing.

Perhaps the most feared ants in the world are the army ants of Africa and South America.  They can live in huge colonies of up to 20 million individuals.  Famously, these ants don’t make permanent nests but have nomadic habits.  When they are on the move, carrying eggs and larvae as they travel, they can form columns which are 20 metres across.  They have a reputation for killing and consuming anything that lies in their path.  However, although they are ferocious hunters, capable of capturing in the region of 100,000 prey items a day, they are not the ‘lightning fast’ eaters of human flesh that the movies would have us believe they are.

If army ants have the simplest strategy for finding food, summarized by the rule ‘If it’s in your way eat it.  If you can’t eat it, go over it!’, then the most complex feeding strategy is probably that used by the leaf-cutter ants with their fungus gardens.  In between, there are a huge variety of hunting and foraging strategies.

Aphids, insects (bugs to be precise) that tap into the sap-stream of plants are a rich source of food for many ants: for example, they are incredibly important to British black, red and wood ant species.  The ants don’t eat the bugs though, they milk them!  As they consume sap for the sugar it contains, aphids take on a lot of water.  This would make them explode if they didn’t get rid of it, so they excrete it.  Along with the excess water, they also lose some sugar.  This sweet ‘honey-dew’ is a fantastic source of energy for the foraging ants who go from one aphid to the next, ‘milking’ them.  Some ants build shelters for their aphids to protect them from predators; others actually stand guard over their aphids, defending them from attack.

Foragers try to bring back excess food to the nest.  Once there, they regurgitate it and pass it to the hungry workers, larvae and queen(s) that await them.  This passing of food from mouth to mouth also transfers the common scent and the pheromone signals from the queen throughout the colony.

How do they find their way around?

Wood Ants in Britain use ‘motorways’, established, well-trodden tracks that they can find easily.  These take the ants from the nest, to their feeding grounds.  Once there, they can spread out to hunt/forage.  Once successful, or when they need to head back to the nest to get some food from another ant, they retrace their steps until they rejoin the motorway.  Leaf-cutting ants have paths into the forest that they keep permanently free of debris so that their harvested leaves can be passed back to their nest with the minimum of effort.  On well-worn paths like these, the sheer number of ants, visual cues from landmarks (for those ants that can see) and smell (for all ants) help them work out which way to go.  This is great for ants that are literally surrounded by food, but what about the more opportunistic species that need to vary their searches constantly to take advantage of new or constantly changing food sources?

I started this quick dip into the world of ants with a single question: how does one ant on a picnic turn into tens, even hundreds in such a short space of time?  After zipping round the world and marvelling at the variety, complexity and success of the ant family I still haven’t come up with the answer.

Well, it’s all due to smell, some simple logic and something called positive feedback.  Ants recognise each other using scent, they identify food using scent, the queen sends messages using scent and, it’s how they find the fastest route to food.

When Common black ants (the most likely picnic invaders in the UK) forage, they start off by following a random search pattern, leaving a scent-trail as they go.  They will keep this up until they find food, at which point they head straight back to the nest in a straight line, or come across the trail of another ant from their colony.  They work out the direct route home by remembering the twists and turns they have made since leaving the nest.

If they find another scent trail they will follow it to see where it leads.  If it leads to nothing, they go back to random searching.  However, if the trail leads to food, they grab some and then take the direct route home, just like the ant that found the food first.  Although the ants that discover a trail that ends in food and join it in this way will all follow slightly different routes towards the food, they will all take the direct route home.

Each ant that travels this return path, adds to the scent trail (this is the positive feedback bit).  Providing the food doesn’t run out, the scent along the direct route from nest to food keeps getting stronger and easier to follow, leading ever larger numbers of ants along it until the food supply is exhausted

Simple, beautiful, Perfectly efficient – just like an ant.