Do you have hair? Bones inside your ear? A wrinkled wrapper around your brain? You may be a mammal.

The real test, and the root of the word, is whether your mother had mutated sweat glands which produced a butterfat emulsion which you were encouraged to suck out of her body as a newborn. (Note: more disgusting to come.)

Entering the world via a food factory is handy, and no doubt helps explain the success of mammals at adapting to a wide range of environments. Milk is a wonder food, providing all the nutritional needs of a baby. Lactose, or "milk sugar," is an energy-providing sugar composed of two monosaccharide molecules (what we called "simple sugars" in school), galactose and glucose. To absorb lactose, the villi in the small intestine of the baby produce the enzyme lactase, which splits each big lactose molecule into two digestible pieces.

Once the baby matures, it doesn't need to bust up lactose molecules anymore, so the villi stop making lactase and the kid subsists on kola nuts or Oreos or whatever.

This was working fine until humans with their big brains with extra-wrinkled wrappers started getting ideas. Instead of working all day in the field, they asked, why not keep some goats and sheep around and squeeze milk out of their mammary glands for breakfast?

This probably caused some indigestion at first (see below), but over time people appeared who accidently kept making lactase as they grew older, so they could tolerate milk as adults. Over lots of time, a pattern appeared in which human mutants with lactase persistence became the norm in Northern European populations, where dairy foods were common, while in African and Asian regions only a small minority of people enjoyed the ability to eat dairy with impunity.

Despite my Northern European ancestry, I did not get the new feature. My case is mild, which is why it took me so long to recognize that my symptoms have a name. While ice cream and cheese cause no issues, I have long known that a tall glass of milk, without any food, can lead to trouble. The lactose goes straight into my gut without being broken down. This is a bonanza for the bacteria which I host (or, as some biologists seem to be saying, which host me). The gut flora feast on the sugar, and my alimentary canal becomes a kind of septic brewery.

Everyone knows what the byproducts of brewing are, and an early sign that I am having a milk reaction is what the British, with their knack for handling indelicate matters tactfully, refer to as "wind." Before it is released, the gas takes up space, so bloating and cramping are additional symptoms of the misfortune.

Additional sugar in the bowel also attracts fluid due to osmotic pressure. This inevitably leads to what the British, in their refinement, refer to as "the shits." A certain British subject went so far as to classify bowel products by shape and texture, resulting in the Bristol Stool Chart, available on coffee mugs, mousepads and t-shirts. Thus I might tastefully warn housemates not to enter the W.C. after delivering a "Type 7."

It's good to be a mammal, and I am especially pleased with the wrinkled wrapper (the neocortex, Latin for "new rind") around my brain that enables me to understand what is going on elsewhere in the body and take steps to avoid discomfort.

Additional facts learned:

• "Dysentery" is not an infection, but a symptom of infection. More info, for those interested.

• Borborygmi is the most entertaining term for stomach rumblings.

• While trying to find out if any other creatures consume the milk of other species (seems pretty rare) I found some adorable stories of interspecies adoption, which might be a good point to end on.