Dunbar’s Number

Robin Dunbar is an anthropologist/primatologist who discovered a correlation between the size of a primate’s brain (or really its neocortex) and the size of that animal’s social network. For example, clever chimpanzees tend to live in groups of 60 or so individuals, who maintain complex intimate social relations (yet chimpanzees don’t really care about outsiders without elaborate introductions). Howler monkeys tend to live in groups of 6 or 7. Dunbar studied primate brains until he believed he found the correlation index… then he applied it to human beings based on our own neocortices (is that the right word?). The number he arrived at was around 150. He posited that this is the average number of stable meaningful social relationships we can have at once. Here is a humorous (yet oddly serious) article which explains the concept elegantly (albeit with some fairly salty language and preachy talk).

When one starts looking for the number 150, it crops up all over the place. Hunter gatherer tribes were (and still are) limited to about that number. Military companies of all sorts of different armies throughout history have been that size. Business consultants say that this is an ideal size for companies (come to think of it there are 150 people at the company where I work) or for departments of companies.

But of course the 150 people I work with are not the entirety of my social interactions. I have 500 or so Facebook friends and not a one of them is from work….and the people I am closest to are not always on Facebook. And there are people I know about but have never (and will never) meet (like Susanna Hoffs, the emir of Qatar, and…Robin Dunbar). High functioning individuals like Presidents, CEOs, and world famous artists probably know many thousands of people—or at least know the one or two key pieces of information which makes each contact useful.

So there are lots of troubles and quibbles with Dunbar’s number…yet if you really write out everyone you have a true worthwhile meaningful relationship with you will probably come up with about 150 (if you are a gregarious adult with a full life in a big city—you can have many fewer close relations and there is nothing wrong with that…it doesn’t mean you are a capuchin monkey or something).

(Not that there is anything wrong with that either)

There is a line we draw around our tribe. Within this line are people we care about and need, outside it are… others—people we may care about in the abstract, or because they share a language, or a characteristic, or a nationality with us…but who are not dear to our heart in the same way as our intimate associates. The writer I linked to in the first paragraph up there asks us to imagine having a beloved pet…or two beloved pets…or six, or 23. How long would it be before our love and our attention were so diluted that we only cared about them in the most general abstract terms (or just outright despised them as a furry horde)? Whether you accept the premise of Dunbar’s number or not, it is a worthwhile question. If our brains are built by evolution in such a way as to make an “us” and a “them” what does it mean for all of us?

Advertisement

The Echidna

The Short-Beaked Echidna (Source: M McKelvey/P Rismiller/)

Last year featured an in-depth examination of Echidna, the terrifying “mother of monsters” from Greek mythology.  To start this year on a glorious high note, here is an essay concerning the actual echidnas (Tachyglossidae), a family of mammals from Australia and New Guinea.  The echidnas were much wronged when explorers named them after a hellish demigoddess.  Although I have never met—or even seen—a living echidna, they are one of my favorite creatures for many reasons.  Combining a gentle temperament with fascinatingly alien intelligence, the echidna is a delightful animal whose taxonomical oddity reveals the strange paths of fate which life takes over great expanses of time.

Along with the charismatic platypus, the echidna is the last of the egg-laying monotremes.  Monotremes are a very different sort of mammal than the other two major divisions of mammals, the eutheria and the metatheria.  The teeming eutheria (familiar mammals like shrews, manatees, picas, goats, and humans) nourish their fetal young by means of a placenta.  The ancient metatheria (marsupials) sustain their developing young in a special pouch.  The monotremes predate both groups and give evidence of mammals’ origins.  Genetic studies suggest that the monotremes originated from some reptile-like ancestor about 220 million years ago.  The long and tangled family history of the mammals and their antecedents will have to wait for another post–suffice to say that monotremes have been here for an extraordinarily long time.  The surviving monotremes, however, are not primitive atavists, but extraordinarily advanced descendants of those ancient progenitor mammals.  They have evolved and survived in varying fashions over the long eons.  Over those millions and millions of years, the echidna developed a very interesting brain.

Echidnas have the largest neocortex relative to bodymass of any creature.  The neocortex (which Hercule Poirot always creepily referred to as “the little grey cells”) is involved in higher brain functions such as spacial cognition, logic, and problem solving.  This special tool has taken the echidna far: like humans, and unlike almost all other creatures, echidnas live in very diverse habitats.  Actually it is the short-beaked echidna (Tachyglossus aculeatus) which lives in different habitats—the other two extant species live in tropical New Guinea and are little known to science.  Although all three species seem to share most traits, I am really writing solely about Tachyglossus aculeatus which ranges from the hot dry desert scrub, to the tropical rainforest, to the coast, to the cold snows of the Australian Alps (where they can lower their body temperature a few degrees above freezing and hibernate).  Echidnas live on termites and ants, omnipresent social insects which are evolutionary winners in their own right.  Echidnas dig up these insects with powerful razor claws and gobble them down using a long sticky tongue which zips in and out of a toothless tube-like mouth.  Echidnas are not known to fight each other or other animals.  In the great evolutionary battle they are pacifists (provided you are not an ant or termite) and if approached aggressively they will curl into a ball and trust their sharp spine-like hairs to keep them safe.  They are also phenomenal burrowers and can quickly tunnel down through anything other than solid stone.

An orphaned puggle being handraised.

Because of their cleverness, relatively little is known about echidnas.  They are difficult to capture since they disdain baits and can figure out most traps.  Similarly, in zoos, echidnas have proven extremely gifted at escape.  Their mating habits are largely mysterious to us but seem to involve non-confrontational competition.  The female echidna is followed for an extended period of time by a train of interested males.  In response to an unknown signal, the male echidnas begin frantically digging, trying to nudge one another out of the way.  Just how the victor emerges from this competition is unknown, but one the female has chosen, the other males walk away with no obvious rancor.  After laying her egg, the female immediately rolls it into a pouch-like fold on her abdomen.  Once the puggle has hatched, the mother echidna solicitously tends it for seven months, after which it roams off free and solitary.

Echidnas have an extra sense, electoreceptivity, but for them it is much weaker than it is in their close cousins the platypuses.  It has also been noted that echidnas vibrate. Water placed near captive echidnas shows distinct ripples in the surface. Perhaps they vocalize on frequencies beneath the range of human hearing (as do elephants).  Speaking of captivity, Echidnas have survived for up to 30 years in zoos even though it is a difficult environment for the blithesome free-roaming animals.  It is believed they live twice that long in the wild—but, again, nobody really knows.

Agh! Get away from there! I though you had a large neocortex!

Likewise nobody knows the echidnas’ total population numbers or how healthy the species is.  What is known is that, sadly, even the intelligent and peaceful echidnas are running into problems in the modern world.  Like other good-hearted pedestrians, echidnas are often killed by careless drivers.  Echidnas face increasing habitat destruction from human houses, farms, and roads.  Likewise they must deal with new predators, the dingos, which have discovered that urinating on balled-up echidnas will cause the latter to uncurl for a moment in stunned disgust (giving the ruthless dogs a chance to rip into their guts).

I wonder what echidnas think of us.  They know of our traps, our radio-tracking devices, and they know how to avoid aborigine hunters.  They are becoming wise to our deadly cars and to the dirty tricks of dingos.  Still they remain curious about people and will sometimes come out of the wilderness in groups to examine our suburbs and cities before melting back into the wild.  Humankind took a long while to understand that echidnas are not dim-witted reptilian pincushions but rather clever and highly developed generalists.  Do they generously think the same of us, or do they put humankind from their mind as something foul when they head back to the ancient, open outback?