We Are Each A Community

In our world of antibacterial soaps, we have learned that bacteria are evil, dirty, sickness-causing agents to be eliminated at all costs. Although some bacteria can cause sickness, bacteria in general are actually a critical component of animal bodies. A human body has ten times as many bacterial cells as human cells and a hundred times as many bacterial genes as human genes, and this pattern is likely true for most animals. We animals have bacterial communities living on our skin, fur, feathers, scales and exoskeletons. We have bacteria in our guts, respiratory systems and reproductive tracts. And bacteria live in glands that are specialized for grooming or scent communication. These bacteria play critical roles not just in how our bodies work, but also in how we behave.

We have known for a long time that bacteria live in communities of multiple bacterial species on and throughout animal bodies and they help our bodies in many critical ways. These bacterial communities are essential for effective digestion, helping to extract energy and nutrients from food and to produce vitamins. They also aid the immune system by fighting pathogens and training our immune systems. Some bacteria can even produce neurotransmitters, chemicals in the brain that influence how we perceive, think, and behave.

Animal behavior can play important roles in how these bacteria are shared and thus the health of animal communities. For example, social relationships can affect what bacterial species each individual is exposed to. Animals that spend more time together and that eat and drink from the same sources are more likely to infect one another with their bacteria. Furthermore, gut bacteria generally do not survive well outside of their animal hosts, so they are generally transmitted during birth (although some species transmit bacteria from mother to offspring by having the offspring eat parts of the mom's poop). For this reason, twins and mother-offspring pairs are more likely to have similar gut bacteria than animals that have other family relationships or no family relationship at all.

However, the idea that bacteria play important roles in animal behavior is a relatively new and exciting area of research. For example, mice that do not have gut bacteria get more stressed out than mice that have normal gut bacteria. These stressed out bacteria-free mice can be made to have normal stress reactions by putting normal mouse gut bacteria in their digestive tracts. How do gut bacteria affect how the brain responds to stress? It turns out, gut bacteria can produce neurotransmitters that interact with nerves in the gut to affect the brain!

Another interesting way in which bacteria influences behavior is through communication using odor. Bacteria that live in scent glands (which are common among mammals) and even in human armpits may produce odors that contribute to the scents that the host animals are already producing. Bacteria are influenced by their host animal's genes, health, diet, hormones, family relationships and social interactions. In this way, changes in bacterial communities can influence how the host animal or its secretions smell, communicating this information to other animals. This information is important for all kinds of social interactions: knowing who your relatives are, finding a mate, avoiding aggressive competitors, and even recognizing individuals in your group. For example, even humans, which are not a species thought to rely heavily on odors for communication, can match armpit scents that belong to twins. These abilities could be seriously messed up by affecting these bacterial communities. Interestingly, when red foxes and Indian mongooses were given antibiotics, their scent glands produced different odorous chemicals. Although data like these show the potential for how our overzealous elimination of bacteria could muck up some really important social processes, more research is needed on exactly how social behavior is affected.

Although researchers are just scratching (and sniffing) the surface of this complex interaction between bacterial communities and their animal hosts, this research could provide a whole new way of understanding our environments and ourselves. We may find that everything from antibiotics to antibacterial soaps, and from deodorants to perfumes, may be affecting how we perceive and interact with our world. These products could also have strong effects on how animals interact with one another and on how our environment interacts with us all. We are all interconnected in more ways than we have ever imagined.

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Further reading:

Archie, E.A. and Theis, K.R. Animal behaviour meets microbial ecology, Animal Behaviour, 82, 425-436 (2011). DOI: 10.1016/j.anbehav.2011.05.029.

Image Credits:

Lactobacillus (the purple rod-shaped things) is a common bacterial species in reproductive tracts. Lactobacillus_sp_01.png by Janice Carr from the CDC at Wikimedia Commons.

Armpit by Ambro at freedigitalphotos.net.