Perch a lizard on a desert rock beside a copper replica that cannot move, then run the day and see which one holds its body temperature steady.

Data format

A trial is one day, dawn to dusk. Every ten simulated minutes each subject records one row, so a full day gives 73 rows for the live animal and 73 for the replica. Each row carries Trial, Time, Day Type, Subject, Body Size, Air Temperature, Ground Temperature, Sunlight, Body Temperature, the two ends of the preferred range, and what the subject was doing: Position, Posture, Skin Darkness and Gaping. Time is recorded as a clock reading, so it plots on a real axis. Temperatures are in whichever unit the gauge is showing, Celsius or Fahrenheit, and the column names say which. Switching units changes the whole table, so it never mixes the two. A finished day sends itself to the table, so trials stack up side by side.

Model details

Body temperature is the only thing being tracked. It comes from adding up four flows of heat every moment: sunlight absorbed by the skin, heat conducted to or from whatever the body is lying on, heat carried away by the air, and water evaporating from an open mouth. Nothing is scripted. Where the animal goes and how it sits are its own responses, and every one of them changes those four flows.

Field herpetologists measure exactly this way: a hollow copper replica set out beside a live animal shows what the animal's temperature would be if it did nothing (Hertz, Huey and Stevenson 1993). The copper replica here runs the identical heat balance. The single difference is that it has no behaviour, so its posture and its skin colour are fixed properties of the cast and it stays where it is put. It is always made the same size as the live animal, because a replica of a different size would not be measuring the same thing.

How quickly a body can change temperature depends on its mass. Surface area grows more slowly than volume does, so a large animal has less skin per gram and takes longer to heat or cool. That is not a setting in the simulation, it falls out of the geometry: the three sizes work out at roughly ten, nineteen and twenty-nine minutes to make most of a temperature change.

Simulated time runs six hundred times faster than real time, so a day takes about a minute and a half. While the animal is turning or walking the clock stops: the sun holds still and nothing heats or cools until it arrives. A move takes no simulated time, and the temperature shown can never be ahead of the body. The Celsius and Fahrenheit buttons beside the gauge change only what is shown and what the table records: the model always works in Celsius.

Heat radiated away to the sky is folded in with the heat carried off by the air, as one combined term, which is also what the rising red marks above a warm animal are drawn from. Body orientation to the sun is a real cooling behaviour and is deliberately left out, so posture here changes how much of the body touches the ground and how much air moves over it, not how much sun it catches. The burrow is held at one temperature, because soil at that depth barely feels the daily swing. The numbers are calibrated for the central bearded dragon, Pogona vitticeps, which lives in the semi-arid interior of eastern Australia; the plants in the scene are the mulga and spinifex of that country.

Standards

HS-LS1-3 Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis.

Design intent

• The control is a real instrument, not a switch. Field herpetologists (Hertz, Huey and Stevenson 1993) put hollow copper or agar replicas out beside live animals. The replica records what body temperature a lizard would have at that spot if it did nothing, and the gap between the animal and the replica is the standard measure of thermoregulation. A student can defend that comparison, and nothing has been switched off to get it.
• The preferred range is drawn across the replica's column too, and a student may reasonably object that a lump of copper has no preferences. It does not. The band is not a property of either object: it is the range this species defends, and both readings are held up against it. The difference between the two deviations is the standard field measure of thermoregulation.
• Body size is the axis for generalizing. All three animals are the same species with the same preferred range, so mass is the only thing that changes. A larger body has less skin per gram, so it lags further behind the environment and shuttles less often. Same mechanism, different pace.
• The three days are chosen for what the animal has to do on each. On a Cool day it never leaves the rock, because there is no thermal challenge to answer. On a Typical day it shuttles between rock and shade. On a Heatwave shade stops being enough: it gapes, then goes underground, and its temperature climbs above its preferred range anyway. The burrow is only needed that day, which is how a student works out that a refuge is needed only when the cheaper option has run out.
• The day itself is the changing environment. Air temperature, sunlight and the burrow temperature all follow how high the sun climbs, and all three are recorded in every row. Sunlight is recorded so that a student can plot it against air temperature. Sunlight peaks at noon, but the air keeps warming into the afternoon. Plotted against skin darkness and body temperature, sunlight also shows why the animal darkens its skin or flattens its body. Nothing on screen states the finding: it is in the data.

Discussion prompts

• Both subjects sat in the same sun all day. Why did only one of them stay near 3595 degrees?
• What does the replica measure that a thermometer held in the air does not?
• Compare two runs that differ only in body size. Which animal held its range more tightly, and which one had to work harder to do it?
• Run the same animal on a Typical day and then on a Heatwave. What does it start doing that it did not need to do before, and what does that cost it?
• On a Cool day the live animal ends up warmer than the replica. How can moving and changing colour make an animal hotter than a lump of copper in the same place?
• How many different animals would you want to run before you would be willing to say this is how the species works?

How do I explore?

1. Drag a lizard out of the crate and onto the rock. Pick the small, the medium or the large one. (On a keyboard: Tab to a lizard, press Enter or Space to pick it up, then use the arrow keys.)
2. Drag the copper replica out of its tray and put it down somewhere. Beside the animal on the rock is the usual place.
3. Pick a day in the top left: Cool, Typical or Heatwave. Then press Play in the top bar to run it from dawn to dusk.
4. If you do not want to watch the whole day, press Skip to dusk at the end of the sun's path. It finishes the day at once and still records every reading.
5. When the day ends it sends its readings to the table on its own. Change one thing, run another day, and the two trials sit side by side.

What should I do/notice?

• Watch the two tubes on the right. The green stripe across them is the range this lizard tries to stay inside.
• Watch what the animal actually does: it turns to face where it is going before it sets off, then walks. Notice whether it lies flat or stands up tall, and whether it goes dark or pale. Every one of those changes its temperature.
• The replica cannot do any of it. Where does it end up by the middle of the day?
• Each trial has one live animal in it. To compare sizes, run the small animal for a day, then the large one on the same kind of day. Which one is on the move more?

What about the data?

Put Time along the bottom and Body Temperature up the side, then colour the points by Subject. You get two lines from the same day: one that stays in a narrow band and one that does not. Preferred Min and Preferred Max give you the band to draw across it.

Run the same animal on all three days and colour by Day Type. Look for where it was: Position only says Burrow on one of them.

One day only shows you one animal, so it cannot tell you how the species works. Run the small, the medium and the large animal on the same kind of day and colour by Body Size to see what stays the same and what changes. Trial keeps your days apart even when the settings match, so you can repeat a day and check you get the same answer twice. Position, Posture, Skin Darkness and Gaping record what the subject was doing at that moment, which is where you look to explain a turn in the line.