Nature is so often, to borrow Tennyson’s phrase, ‘red in tooth and claw’ – from a prairie falcon breaking a mallard’s neck in a 100-mph fly-by to the parasitic tongue-eating sea louse, whose antics speak for themselves. And yet there are just as many relationships in nature where everyone just gets along.
Mutualism is a type of symbiosis – a close association between two (or more) species – where both species in the partnership benefit. This is in contrast to other symbiotic associations, like parasitism, where the parasite benefits and the host is harmed, and commensalism, where one species benefits and the other is unaffected – like barnacles living on the skin of a humpback whale. The barnacles get a free ride to plankton-rich waters, and the whale is unharmed by its hitchhikers.
But they are more than just interesting, transient interactions between species. Mutualism intertwines the evolutionary fates of species and has created life as we know it.
Bacterial beginnings
Both mitochondria (the organelles in our cells which generate most of the chemical energy needed to power their biochemical reactions) and chloroplasts (the site of photosynthesis in plants and algae) descended from specialised bacteria that somehow survived being engulfed by another more complex cell. This is why our mitochondria has its own DNA, which is ring-shaped like that of bacteria, as well as its own transcriptional and translational machinery for reading and coding proteins. In other words, one of our soupy ancestors gobbled up a proto-mitochondrion and decided to keep it as a lodger for the next 1.45 billion years – the mitochondria get somewhere to live and our cells get a ready source of energy.
Indeed, bacteria do a roaring trade in symbiosis, acting like bio-modifications for larger organisms. The bioluminescent bacterium Aliivibrio fischeri, for example, is found free-floating in the sea, but also forms an integral part of the Hawaiian bobtail squid’s light organ. The squid feeds the bacteria sugar and amino acids – and, in return, the bacteria’s bioluminescence hides the squid’s silhouette when viewed from below. One of SpaceX’s resupply missions to the International Space Station last year included 128 hatchlings of this squid to see if they still incorporated their symbiotic bacteria while in space.
Let’s take a look at some other interesting mutualisms.
Lichen
Lichens – those yellowy and green growths you see on old stone walls and tree trunks – are not organisms, but communities of fungi and green algae or cyanobacteria (“green-blue algae”). This is the relationship for which the word symbiosis, back in 1876, was coined.
The fungi’s filaments provide the structure on which the algae or cyanobacteria can live, and help them gather moisture and nutrients, while the algal or bacterial partners produce sugars via photosynthesis for the fungi. Lichens are, in the words of lichenologist Trevor Goward, ‘fungi that have discovered agriculture.’
About 20% of all fungal species have evolved this mode of life, and the fungus is more reliant on the relationship. While most of the algal and bacterial partners can survive on their own, most of the fungi that have developed this strategy depend on them to survive. Some fungi have even evolved to parasitise lichen – a sort of fungal third wheel to this fungus-alga relationship.
But the relationship gets even more complicated. It used to be thought that it was a simple one-to-one relationship between an alga and a fungus, but we now know that one fungus will happily associate with various species of bacteria or algae (sometimes at the same time in a sort of ménage à trois), meaning that a lichen is really an entire ecosystem, sustaining several different strains of algae, bacteria, viruses and fungi – a micro-organic city.
Ants and fungi
You may have heard of, or seen, ants’ mutualistic relationship with aphids, whom they farm for the sweet “honeydew” they secrete, protecting the aphids from predators in return. But the earliest, and most complex, association ants have is with fungus.
Long before humans started farming, long before we even existed, ants were farming fungus. It’s a relationship that started 50-60 million years ago, only in the Americas, and has been going strong ever since.
These fungus-growing ants are known as attine ants, or the attini tribe, and there are at least 200 species of them. Most of them are “leafcutters” – they bite off bits of grass and leaves, carry them back to the nest, and then use these as a substrate on which the fungus can grow. The fungus breaks down the indigestible cellulose of plants, converting it into more edible proteins and sugars the ants can harvest.
It’s thought that the attini’s ancestors were originally hunter-gatherers, feeding on a mix of insects, nectar – and leaf sections, which happened to grow a fungus. And the rest is history. The ants increasingly switched their diets to the cultivated fungus until eventually they became wholly subsistent on it.
While some of the attini will readily adopt a different strain of fungus if necessary, many have stuck with the same strain for millions of years. And each colony grows its own unique, ancient fungus – it occurs in that one colony and nowhere else. When the queen of such a colony leaves to form a new one, she’ll take a bit of the fungus with her to seed the next. So the colony and its fungus have co-evolved to the point where they entirely depend on each other to survive – the ants can only get nutrients from their particular fungal garden, and the fungus can only get its food from that colony of ants. It’s a precarious existence, vulnerable to bacterial infection, foreign fungi and even other ant colonies who have been known to steal another’s fungus – like ancient Mesopotamian city states abducting each other’s gods.
For that reason, the relationship between ant and fungus is a fiercely protective one. Ant and fungus contra mundum. The fungus chemically communicates with the ants to direct them to the food it wants. When plants get preyed upon by something like an ant, they start to release toxins into their leaves as a defence mechanism. If an ant brings back a toxic leaf, the fungus will detect it and tell the ants to switch to another food source.
On the ant’s side, the attini have evolved to produce antibiotic and antifungal cultures on their thorax and legs, which kill off any potential bacterial and fungal parasites that the ant might inadvertently pick up and bring back to the colony. In other words, the ants have developed antibacterial and antifungal armour to protect their colony’s fungal garden.
Frogs and spiders
Frogs are usually a natural prey item for tarantulas, but for the microhylids – narrow-mouthed frogs – the spiders are an unlikely ally.
By associating with these large spiders, the frog and its offspring are protected from predators – other spiders, snakes and lizards – and also get to feed on the insects attracted to prey remains left by the spider. In return, the frog protects the tarantula’s eggs by eating the ants that prey on them.
Young spiders, who perhaps don’t recognise the partnership with the frog, have been observed to examine the frogs with their mouthparts before letting them go unharmed. Presumably, the frogs’ skin toxins are unpalatable to the tarantulas so they just let them hang around. So began the association.
The first description of this relationship, in 1989, was specifically between the dotted humming frog (Chiasmocleis ventrimaculata) and the Colombian lesserblack tarantula (Xenesthis immanis) in southeastern Peru. But it’s now been found to occur across other countries – India and Sri Lanka, so far – and between different species of both tarantula and frog.
This suggests that this mutual relationship has evolved more than once, and probably occurs in more places and between more species than we currently know of. Yet despite this being a common association, it’s not a necessary pairing. Both spider and frog can live quite happily without ever meeting. Very much a marriage of convenience.
Humans and honeyguides
But it’s not all fungi and creepy-crawlies. Humans have developed mutual relationships with animals too.
While we’ve domesticated and trained plenty of species to hunt for us – dogs, raptors and cormorants – such relationships with wild animals are rare. Yet they do exist. In many parts of sub-Saharan Africa, people and a species of wax-eating bird called the greater honeyguide work together to find wild bees’ nests.
The honeyguide (amusingly named Indicator indicator as a nod to its behaviour) first attracts a person’s attention with a wavering, chattering call, and then it flits from tree to tree, directing the human to an active bee nest in its territory. It then waits nearby while the human subdues the bees with smoke, hacks open the nest and harvests the honey. Afterwards, the bird swoops in to feed on the bees’ eggs, larvae and pupae, waxworms, and beeswax (it’s one of the few birds that can digest wax).
This foraging partnership was first recorded in print by Portuguese missionary João dos Santos in 1588 when, while on mission in what is now Mozambique, he noticed a dun, starling-sized bird slipping into his church to nibble his wax candles. But it’s thought that this honeyguide-human partnership goes back some 1.9 million years to our ancestor Homo erectus.
Several peoples engage in this partnership, including the Yao in Mozambique, the Awer in Kenya, the Hadza in northern Tanzania, and the Boorana in Ethiopia, each with their own way of calling the bird, telling them they’re ready to hunt for honey. The Boorana opt for a loud whistle known as the fuulido, the Hadza for a melodious one, the Awer whistle on the shell of the giant African land snail, and the Yao use a loud trill followed by a short grunt: ‘brrr-hm’.
And it works. The fuulido of the Boorana doubles their encounter rate with honeyguides, and the Hadza’s rate of finding bee nests increased by 560% when partnered with honeyguides, leading them to significantly higher yielding nests than those found without the birds.
But relationships like this hang in the balance. Deforestation and urban development are reducing the habitat for wild bees and honeyguides, and the Awer youth in Kenya apparently show little interest in continuing the practice. But this is also a sign of increased wealth in these areas. The peoples who practice honey hunting have always done so out of necessity. Now they have easier access to alternative sources of sugar, including through beekeeping, and more opportunities for schooling and employment. The honeyguide’s call increasingly goes unanswered, and yet elders continue to pass down the secrets of the partnership to willing ears.
The call of the modern world needn’t drown out that of the wild.