Teacher sheet · Ages 15–16 · about 45–50 minutes
Endothermy, size and heat loss
Lesson page: echoesof.science/school/mammals/ages-15-16/ · Pupil worksheet: echoesof.science/school/mammals/ages-15-16/print/
Learning aims
- Recall the defining features and origin of mammals.
- Explain endothermy and its energy cost using BMR data.
- Calculate and apply surface-area-to-volume ratios.
- Collect, plot and evaluate cooling data as a model of body size.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why are there no mammals smaller than a few grams? |
| 10 min | Read the text; SA : V worked example. |
| 25 min | Cooling investigation and graph. |
| 5 min | Heat in, heat out game. |
| 5 min | Evaluate the model: how is a beaker unlike a mouse? |
Materials for the experiment
- Three beakers of different sizes (e.g. 100 ml, 250 ml and 600 ml), same glass type
- Hot water at about 50–55 °C (prepared by the teacher)
- Three thermometers or temperature probes with data logger
- Measuring cylinder, ruler, stopwatch
- Heat-proof mat
Safety: Only the teacher pours the hot water; keep it below 55 °C. Stand beakers on a heat-proof mat away from the bench edge. Report breakages; do not pick up broken glass by hand.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Bigger animals lose more heat, so they need more food per kilogram. | They lose more heat in total, but less per kilogram, because SA : V is smaller. |
| Warm-blooded means the body is always warmer than the surroundings. | Endotherms keep a relatively constant internal temperature; it can be lower than a hot environment. |
| All mammals are placental. | Monotremes lay eggs and marsupials have very premature young. |
Worksheet answers
- 3, 1.5 and 0.75 cm⁻¹. Smaller bodies have more surface per unit volume, so they lose heat faster for their mass and need a higher metabolic rate per kilogram to stay warm.
- Any four: hair (keratin); mammary glands; sweat glands; muscular diaphragm; four-chambered heart; heterodont teeth; malleus and incus from jaw bones; endothermy.
- Higher SA : V gives a faster cooling rate. Uncertainties: different starting temperatures, evaporation from the surface, stirring, thermometer reading, heat stored in the glass.
- Fur traps a layer of air close to the body; blubber is a thick layer of fat that insulates against cold water; both slow heat loss. Blubber also stores energy for when food is short.
Quiz answers
- What is the SA : V ratio of a sphere of radius 2 cm? 1.5 cm⁻¹. SA : V = 3/r = 3/2 = 1.5 cm⁻¹.[7]
- If an animal doubles in size (all lengths ×2), its mass increases by about… 8 times. Volume scales with length cubed: 2³ = 8.[7]
- Why does a mouse have a higher BMR per gram than an elephant? It has more surface area for its mass, so it loses heat faster. Smaller endotherms lose heat faster.[7]
- Roughly how many times greater is a human male's BMR (about 1700 kcal/day) than an alligator's SMR (60 kcal/day)? About 28 times. 1700 ÷ 60 ≈ 28.[7]
- When do the earliest known mammal fossils date from? The early Jurassic, about 205 million years ago. Mammals evolved from cynodont synapsids.[1]
- What fraction of a 600 kg Weddell seal could be blubber at most? About 40%. 240 ÷ 600 = 0.40.[6]
Going further
Use published data on body mass and metabolic rate for several mammals to plot log(BMR) against log(mass) and find the gradient.
Sources used in this lesson
- [1] Biology 2e, 29.6 Mammals (OpenStax (Rice University)): reliability High
- [2] What is a mammal? (Carnegie Museum of Natural History): reliability High
- [6] How marine mammals stay warm (hands-on activity) (NOAA Office of Education): reliability High
- [7] Biology 2e, 33.1 Animal form and function (OpenStax (Rice University)): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/mammals/sources/
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