What on Earth? Your questions answered

The tiniest mammals; How do cells grow? A burning issue
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Why are there no mammals smaller than a shrew?

Why are there no mammals smaller than a shrew?

The answer to this question comes down to heat loss. Shrews are mammals, so in common with other mammals, they maintain a constant body temperature that's nearly always above that of their surroundings. As a result, they lose heat from their bodies to their surroundings. The source of the heat is the energy that the mammal gets from food. The more heat is lost, the more food is needed to replace this energy loss.

Heat loss is a bigger problem for smaller animals because they have a high surface-area-to-volume ratio: they have a lot of surface area to lose heat through. And, as the smallest mammals, shrews suffer from heat-loss like no others. As a result, they need to eat a prodigious amount of food to stay alive.

This principle explains why mammals that live in cold climates tend to be large (for example, polar bears). These have quite a large volume for a relatively small amount of surface area, so they do not lose heat so readily.

Any mammal smaller than a shrew would not be viable on a heat-loss basis. If you think about animals smaller than shrews, they are always cold-blooded, for example insects.

In an embryo, how do the cells begin to specialise? How do they decide, for example, to become brain cells?

This answer applies to mammals only. Other animals can have very different developmental strategies.

In the initial stages, the fertilised egg divides to form a ball of cells. Cell position in the ball gives the cue for the first differentiation step. Cells on the outer surface of the ball become one kind of tissue (which will contribute to the placenta only) and cells on the inside become another cell type, which will give rise to the baby that is born. It is believed that the number of cell contacts is the key feature: outside equals a few cell contacts, inside equals many.

Once you have two cell types, there is further scope for interaction. Some of the "outside" cells are in contact with "inside" cells, and some are not. This leads to another two different types of cell arising. And so it goes on in a cascade of increasing complexity. During that cascade, genes come into the picture. While every cell has the same set of genes, its position (outside/inside) determines which genes are read and used to make the proteins that the genes encode. Depending on which genes the cell uses, even more varying sets of genes are activated, and so on, leading to this astonishing number of different tissues such as brain cells, bones and others. Just from one fertilised egg.

What happens to the candle wax when a candle burns? Where does it go?

Candle wax is the fuel which is burnt to keep the candle going. When you light a candle, the flame melts the wax at the top of the candle into liquid form. This liquid travels up the wick via capillary action and is transformed into a gas by the heat of the flame. This gas combines with oxygen in the air, giving out the heat and light that you see as a flame. Candle wax is a mixture of chemicals, which contain carbon, nitrogen, oxygen and hydrogen. When combined with the oxygen in the air, new chemicals are created. These are gases – carbon dioxide, nitrogen, and water vapour – which float into the atmosphere, so you can't see where the wax has gone.

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