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Mammals are synapsids; a group called the cynodonts are thought to be their ancestors, and the earliest known mammal fossils are from the early Jurassic, about 205 million years ago. Today there are about 5,200 species.

Defining features include hair made of keratin, mammary glands, sweat glands, a muscular diaphragm and, like birds, a four-chambered heart. Two of the three middle-ear bones, the malleus and incus, evolved from jaw bones.

Mammals are endotherms: they keep a relatively constant body temperature. Hair traps a boundary layer of air close to the body, and humans have sweat glands over most of the body surface.

Experiment: Modelling body size and cooling

1. Fill each beaker to the same fraction of its height with water at about 50 °C, measuring the volume of water in each.
2. Estimate the surface area of water and glass in contact with the air and bench for each beaker (treat it as a cylinder: 2πrh + πr² for sides and base, plus πr² for the top), and calculate SA : V.
3. Start all three readings together. Record the temperature of each every minute for 15 minutes, stirring gently before each reading.
4. Calculate the mean cooling rate (°C per minute) for each beaker over the first 10 minutes.
5. Plot cooling rate against SA : V. Describe the pattern.
6. Repeat with one beaker wrapped in cotton wool or fleece as a model of fur or blubber, and compare.

Beaker	Volume (cm³)	Surface area (cm²)	SA : V (cm⁻¹)	Cooling rate (°C/min)
Small
Medium
Large
Medium, insulated

Questions

1. Calculate SA : V for spheres of radius 1 cm, 2 cm and 4 cm, and use the results to explain why small mammals need a high metabolic rate per kilogram.

2. Give four features that define a mammal.

3. Use your cooling data to describe the relationship between SA : V and cooling rate. Give two sources of uncertainty.

4. Explain how fur and blubber reduce heat loss, and name one other use of blubber.

Sources for every fact on this sheet: echoesof.science/school/mammals/sources/

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