Why are there no mammals smaller than a shrew, and why shouldn’t you give dogs human chocolate?

We explore the curious questions that science can answer

Shrews need to eat a prodigious amount of food to stay alive (Getty/iStock)

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. 

Do any animals have eyeballs that retract when they close their eyes?

As they lack well-developed eyelids, frogs do withdraw their eyes into their sockets while they are in resting mode. They are also able to use their eyeballs during swallowing, in order to help push food down their throat. They retract one eye at a time, which presumably creates some kind of peristaltic motion.

Does a spider have a heart?

Spiders have hearts, but they aren’t as complicated as ours. The spider heart is really just a tube that hangs at the back of the abdomen cavity. Two or three pairs of openings called ostia are found on either side of the heart. Through these blood enters, and is pumped forwards and backwards from the heart. Back flow is prevented by valves. This means that the heart keeps the blood pumping whatever position the spider is in – even if it is upside down. The spider has an open circulation system: although blood travels through blood vessels to a specific area, it then flows into the open spaces and bathes the cells with oxygen and nutrients.

What makes the muscle of a bird ‘white’ or ‘dark’ meat?

White and dark poultry meat results from the amount of a pigment in the muscle called myoglobin. This pigment is closely related to haemoglobin, an oxygen-carrying component of blood. The oxygen-carrying ability of myoglobin allows the muscle to continue to work for long periods of time using a process called oxidative metabolism. Once all the oxygen “stored” in the muscle’s myoglobin is used, the muscle then uses a process called anaerobic glycolysis to produce the contractions needed for work. If the muscle is used repeatedly for extended periods of time, the amount of myoglobin in the tissue increases. That is why migrating birds such as ducks and geese have darker meat in the breast muscles (the muscles used for flight). Chickens’ legs are also muscled with darker meat because the birds tend to use them more than those in the breast.

Why shouldn’t you give dogs human chocolate?

Chocolate contains theobromine, which is poisonous to dogs. A dose of 50mg per pound can be fatal to a dog. Milk chocolate contains 45mg of theobromine per ounce and unsweetened baking chocolate contains 400mg per ounce. Just one ounce of unsweetened baking chocolate can kill a small breed dog. Theobromine, when ingested by dogs releases epinephrine causing the heart to race and serious cardiac arrhythmias.

Signs include vomiting, diarrhoea, excessive urination, and hyperactivity followed by depression, coma, seizures and death.