Advances in diet, medicine and the cleanliness of water have massively boosted human life expectancy in the last century. It is now increasing by three months per decade in the affluent west. Is the life expectancy of our pets increasing at a similar rate, proportionally speaking?

Andrew Franklin, London, UK

As a human I do not need to go to the doctors every year and get my boosters so why do animals...is this just a  money making scheme by the drugs companies and /or vets?  Do the diseases these vaccines protect against mutate quickly more like  the human flu?

How long can you keep a tiger cub as a pet? I have read of people doing so, but surely, for very obvious reasons, there is a time limit to how long you can keep a carnivore in your living room.

Peter Higginson, Paris, France

I keep two PET bottles filled with tap water. As I don't intend to use the water unboiled anyway, I hardly ever refill them. Mostly I use them as weights for some fitness training (like dumbells). But once when the water was turned off I actually used water from one of the bottles and refilled it afterwards. Now in comparison with this newly-filled bottle I see that the old one has lost some water. No air has come in instead of it, but the bottle has become a bit floppy. I understand that water molecules are smaller than most molecules in the air, so it is understandable there might be pores through which it can evaporate and nothing comes in to replace it. But are these pores in the bottle or lid itself or does the vapour go through between bottle and lid?

I understand that a dog is not capable of producing the sounds required to converse in a human language. However, would it be possible to develop a language made up of sounds that both a human and a dog could produce so that they may "converse" on some level?

I am currently babysitting three tankfuls of assorted tropical fish while my daughter is away on holiday. They seem to be very active all the time. Do they ever sleep, even though they appear to have no means of closing their eyes?

On a related theme, do captive fish appreciate they are safe, and does that influence their behaviour?

Catherine Spalding, Bristol, UK