Nov 29

Do Fish Drink? (and other mini questions)

In September, I published a post entitled ‘Why are moths attracted to flames? (and other mini questions)’, which aimed to answer questions about the natural world that were neither large nor detailed enough to have their own entries. But one post certainly wasn’t enough to answer the myriad of mysteries concerning the animal kingdom, so here are four more mini questions to lay to rest.

Are there any animals found only in Britain and nowhere else?

Most islands are rich with species that are found nowhere else in the world (the New Zealand kiwi or the Madagascan lemurs, anyone?), but the British Isles was only isolated from mainland Europe a few thousand years ago and so evolution has not had much time to work its magic there. There are a few species found only here – a unique Celtic woodlouse, found only on the maritime cliffs of Glamorgan in South Wales, and a couple of fish restricted to certain deep lakes and lochs, for example – but the British only have one single endemic vertebrate: the Scottish crossbill.


Photo: Elaine R. Wilson

This bird, a finch with beak mandibles that cross over towards the tip to prise open pine cones, lives only in – would you believe it? – Scotland. It has had a rather rough time trying to earn its endemic distinction for it was long regarded as merely a subspecies of either the red crossbill or three parrot crossbill, all three kinds of which look extremely similar without close-up examination. Finally, in 2006, the Scottish crossbill was confirmed as a unique species, not on the basis of its physical appearance, but due to its distinct flight and its bird song, which some say has a Scottish accent to it.

What is the deadliest spider in the world?

This distinction almost certainly goes to the notorious Brazilian wandering spider, a terrifying monster that has venom up to five times more potent than the famous black widow. Bites from these spiders cause intense pain, irregular heart rhythm, vomiting and extreme internal hemorrhaging (not to mention acute and painful erections if you are a man).


Photo: Techuser

The Brazilian wandering spider is also extremely aggressive and often chases people with little to no provocation, so it’s no wonder that it causes more human deaths than any other spider. It doesn’t help that this spider likes to hide in banana bunches and so sometimes ends up in shipments of bananas destined for foreign countries, including, quite worryingly, the United Kingdom.

How do fish drink?

That all depends on what type of fish it is. Freshwater fish don’t really need to drink due to the process of osmosis – this is the diffusion of pure water molecules from an area of high concentration to an area of low concentration. Since these fish are constantly surrounded by very high concentrations of water molecules, the water diffuses directly through their skin. A freshwater fish’s kidneys need to work extra-hard to expel excess water so that the animal doesn’t absorb too much and dilute their bodily salts.

In the ocean, the opposite is true. These fish have no problem losing salt; now the issue becomes taking in too much salt. If sea-faring fish relied on osmosis in the ocean, water would actually move out of a fish’s body because the concentration of pure water molecules is lower in the salty sea than inside the fish. So the fish that live in the ocean must indeed drink from the water around them. The excess salt is excreted on their gills, where it returns to the ocean.


Photo: Laban712

From this, it is now abundantly obvious why saltwater fish cannot live in freshwater and vice versa. They have very different methods of dealing with water and salt in their bodies and those methods don’t really adapt to a different environment.

Do any animals other than mammals produce milk? (with thanks to Alex S. for providing this question)

By and large, milk is a white liquid produced by the mammary glands of mammals. If we use that definition, then no other animals can produce it. But there are a few species of bird in the world that produce a substance known as ‘crop milk’, which is semi-solid and a pale yellow colour. Pigeons are among them.


Photo: Discott
Speckled pigeon

Pigeons, immediately after hatching, do not eat solid food or even semi-digested food that has been regurgitated by their parents. Instead, for the first week or so of their lives, the chicks are fed on this specialised crop milk, which is secreted from the lining of the parent birds’ crops. Both male and female pigeons produce crop milk, which is rich in protein and fat, and they share in the feeding of the young until they are weaned onto more solid food. This is why you hardly ever see baby pigeons; they grow so quickly on this secretion that by the time they leave the nest, they are almost the size of an adult.

The only other birds to feed their young on crop milk are flamingos and a few species of penguin, even though the latter don’t possess crops.

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