Ask a field biologist the age of the lizard in their hand and you will usually get a hedge: adult, subadult, large male. Body size is the obvious proxy and the worst one. Lizards grow fast early and then slowly, almost asymptotically, so two animals of identical length can be three years apart in age. Worse, growth rate varies with food, rainfall and altitude — the same species will run on a different clock on two sides of the same valley.
The honest answer has to come from somewhere other than a ruler. In temperate reptiles, it comes from bone.
Why bone keeps time
Reptiles in a seasonal climate do not grow at a constant rate. Through the warm months they feed, bask and lay down bone quickly; through winter brumation, growth all but stops. When it resumes in spring, the boundary between the two phases is preserved as a thin, dense, sharply defined layer in the cortical bone — a line of arrested growth, or LAG.
Count the LAGs and you have counted winters.
One line, one winter. It is the closest thing herpetology has to a tree ring, and it is doing the same job for the same reason.
The technique built on this is called skeletochronology, and it has been used on amphibians and reptiles since the 1970s. Its great virtue is that it is minimally invasive: you do not need the animal's skull, or its femur, or the animal at all after the first minute. A single phalanx — one toe bone — carries the whole record.
What the bench work actually looks like
The glamour ends at the field notebook. What follows is a fortnight of careful, repetitive laboratory work:
- Fixation. The sample is fixed, usually in formalin or ethanol, and the soft tissue cleared away.
- Decalcification. The mineral is removed with a weak acid or a chelating agent, leaving a bone soft enough to cut.
- Sectioning. The phalanx is sectioned transversely at the diaphysis — the mid-shaft — at a thickness measured in micrometres. Too thick and the lines blur together; too thin and the section tears.
- Staining. Haematoxylin binds preferentially to the dense LAG tissue and throws the lines into contrast.
- Counting. Under a light microscope, at magnification, by eye. Twice, on different days, without looking at the first count.
That last point is not fussiness. LAG counting is an interpretive act. Some lines are double, laid down during an unusual cold snap in the growing season. Some are faint. And in older animals, the innermost lines may be gone altogether.
The problem of the missing first years
Bone is not a passive archive. As a lizard grows, the marrow cavity expands outward and destroys the oldest, innermost bone — the record of the animal's first year or two. The technical name is endosteal resorption, and it means that in a long-lived individual, a naive LAG count is a minimum age, not an age.
The standard correction is back-calculation. You measure the diameters of the surviving LAGs, plot them against the diameters of known first-year LAGs from young animals of the same population, and estimate how many lines the cavity has eaten. It is a reconstruction, and it should be reported as one.
This is the part of the method I find most worth defending to people outside the field. The technique is not weakened by admitting where it is uncertain — it is made usable. A number with an honest error attached is science. A number without one is decoration.
What you get for the trouble
From a set of LAG counts across a population, you can read things no amount of measuring tape will give you:
- Age structure — how many year-classes the population holds, and whether the young ones are missing.
- Age at sexual maturity — the LAG after which growth visibly slows, cross-checked against reproductive condition.
- Longevity — the oldest individual anyone has actually documented, rather than guessed.
- Growth rate — the spacing between lines, which is a record of how good each particular year was.
That fourth one is the quiet gift. The distance between two LAGs is a measurement of a single summer: how much rain fell, how many insects there were, how well one small animal did. A lizard caught in 2021 is carrying a legible account of 2017.
I have never got over that.