The phrase “survival of the fittest” is often used to describe the way species evolve over
time. It highlights that only the most successful have offspring, passing on their genes to the next generation. However, by using language like this, it’s easy assumed that all species in an ecosystem are constantly working against each other, and that only the most aggressive or strongest species can survive. While it is true that predators must catch and kill their prey and parasites must take resources away from their hosts, these are only examples of a wide diversity of relationships involving multiple species collectively known as symbiosis.
Humans often separate themselves from discussions of the ‘natural world’, but it shapes us
as much as we shape it (if not more)! Inspired by ZooLab’s own ‘Symbiosis’ workshop, let’s take a look at some of the ways humans have formed relationships with the various species in our environments throughout history!
Mutualism
‘Symbiosis’ is a term used to describe any type of long-term, repeated interaction between
two different species. Mutualisms are special kinds of evolved relationships in which both
partners benefit from the interaction, evolving specially to provide for their partner. Great
examples are everywhere; think about how bees pollinate flowers in return for sugary nectar,
or how clownfish eats an anemone’s parasites in return for a safe place to live. Often, these
cooperative relationships shape ecosystems just as much as negative relationships like
parasitism and predation. Let’s explore some of the most dazzling examples for human/ nature relationships below.
Pets
No discussion of human-animal relationships would be complete without first talking about
our furry, feathery and scaly friends. Are our relationships with our pets mutualisms? It’s
difficult to say for sure.
Historically, dogs and cats could absolutely be described as mutualists. By encouraging ancestral dogs around our campfires, we gained both protection and a hunting companion, and the dogs gained reliable campfire scraps and warmth. The same can be said of mice-catching ancestral wildcats. In modern times, whilst we still love spoiling cats and dogs with food and warmth, the benefits we gain are less clear. Outside of specialist cases like guide and livestock-herding dogs, it seems that much of our benefit from modern relationships with pets can be chalked up to the fact that we simply enjoy their friendship, companionship, and cuteness!
Honeyguide
The honeyguides are a family of starling-sized birds from Africa and Asia. They are notable
for their rather precarious fondness for beeswax and bee grubs, a taste that has led to their
unique relationship with the native peoples in their range, such as the Yao, Hadza and Boran. Using a special call, some honeyguides will guide human trackers directly to honeybee nest. The humans use smoke to subdue and excavate the colony to collect its honey, leaving the wax and grubs for the birds. Studies have found that the birds reduce the hunting time of the Boran people by over 60%, and that up to 10% of the Hadza diet can be attributed to honeyguide collaboration. The relationship is thought to date back thousands if not millions of years. As both species benefit, it is a classic example of a mutualism.
Gut Microbiota
Almost every animal that has a gut has it's own gut microbiota, the scientific name for the
population of bacteria, archaea and fungus that live inside our digestive system! In most
cases, they actively help us by breaking down our food, producing essential vitamins and
preventing harmful bacteria from growing by secreting chemicals and competing for space.
There is even a rising body of evidence suggesting that a diverse and healthy gut community
is important to protect us against conditions such as asthma and obesity.
We get our first community of microbiota from our mothers during birth and go on to develop it mostly through our diet. Our microbiome benefits from transmission to the next generation as well as a safe and stable environment with lots of energy supplies, whilst we benefit for the breakdown of hard-to-digest foods and protection from disease. Each human genome contains about 23,000 genes, but a human body can contain up to 600,000 microbial genes. This incredibly diverse communities of microbes are key to our survival and adaptability, representing as close a relationship as one can find in nature.
Parasitism
Unlike the mutualisms discussed above, parasitisms are a more nefarious kind of association. Here, the parasite benefits by stealing resources from its host without providing anything in return. Humans are no stranger to parasites; as a result of our wide range, varied diet, proximity to animals and tendency to live close together, we are easy targets.
Tapeworms
The pork tapeworm follows a typical tapeworm life cycle. In its adult form, it grows to around
5 meters in length by feeding on the partially digested nutrients within our small intestines.
When mature, it releases egg-containing segments of its body to be passed by the host
through their poop. If an unfortunate pig encounters the eggs, it has a chance to be infected, and if people end up eating the infected pork, the cycle starts again! In the majority of cases, tapeworm infections show no obvious symptoms as the worms carefully avoid the immune system. However, large infections can cause serious health problems. These mostly occur in poorer countries, stemming from lower levels of regulation over agricultural hygiene standards, so future investment and education remains crucial to protect local communities against this threat.
‘Nits’, Head and Body Lice
Most schoolchildren will come to know the horror of a new set of tiny, six-legged neighbours moving in upstairs. Head lice (Pediculus humanus capitis), sometimes known as ‘nits’ (although scientifically this just refers to their highly-visible white eggs) are a species of blood-sucking insect that carry out their entire lifecycle - from hatching and growing through breeding and laying eggs – on our noggins. They spread from person-to-person when we touch our heads together, so it makes sense that we most commonly encounter them when we’re young and crowded together in schools every day.
Head lice are thought to be harmless. Despite their liking for human blood, they aren’t known to carry any diseases. However, the story is complicated by their evil twin: the body louse (Pediculus humanus humanus). While they might look extremely similar, they differ in behaviour. Unlike the head louse, the body louse prefers to lay its eggs on clothing rather than human hair. Far from being harmless, body lice are thought to transmit many diseases including typhus and trench fever. It has even been suggested that the harmless head louse may have historically acted as a human mutualist by priming the human immune system to combat the harmful body lice. Whatever the reality, these lice highlight how complicated natural relationships can be, and how they might have changed over our long history.
Commensals
There is a final, perhaps less appreciated type of symbiotic relationship to consider, one to
which humans are no strangers. So-called commensalisms are relationships in which one
partner benefits while the other suffers no costs or benefits: it is completely unaffected.
Eyebrow Mites
One of the best examples of human commensalism, has us turning to a very small many-legged skin-dwelling beasties, one thought to be found living on more than 99% of people across the world. We are of course talking about eyebrow mites (of the genus Demodex). Compared to these tiny 0.3-0.4 mm long arachnids, our previously mentioned head lice are gigantic, at only 1/10th of the size. This minuscule stature comes in handy, given they live almost entirely within our hair follicles and the sebaceous glands below them, eating the oily sebum produced to moisturise our hair. In fact, they only leave the follicles at night, when they slowly prowl across our skin between their feeding sites on our eyebrows, noses and cheeks.
Despite this skin-crawling nature, eyebrow mites are actually thought to be completely
harmless, having inhabited our bodies for thousands of years. There have even been
suggestions that they provide a housekeeping role by consuming harmful bacteria on our
skin’s surface. And without articles like this, you probably wouldn’t even notice them. They
represent just one more example of our interconnectedness with the natural world to a
degree that we don’t fully appreciate, in that our own bodies represent a symbiotic
ecosystem of hundreds or thousands of species.