Well, two things:

1. Their colors only stand out compared to other reptiles and mammals. Amphibians and nearly every kind of invertebrate come in just as many spectacular colors. The better question is "why don't mammals have such a spectacular variety of colors compared to other animals?"

2. Their variety in shape doesn't really stand out amongst anyone; obviously invertebrates all have enormous variation, but even boring old mammals have everything from tiny/round mice to lanky/springy deer to weird long ferrets to giant stocky rhinos.

It's also worth noting that fish in particular may be so widely varied because they're miscategorized; there's a push among some biologists to split up "fish" into several differently groups because there's so much more variation among "fish" than among equivalent groups.

Mammals don't have as much color variety because our color comes almost exclusively from melanin, which can only do shades of tan/brown/black. Most other animals can synthesize more pigments than that.

But birds in particular do have some other tricks up their sleeves:

Some birds, famously the flamingo but also plenty others, can absorb pigments from their food that their body can't make by itself.

Most green and blue birds actually don't use specc pigments, but their feathers structurally create that color (which is why they're so iridescent).

I don't know if either of those apply to fish.

This is not a complete answer, but in some species, bright colors help the males to attract females. This is true for various bird and fish species, however there are also plenty who don't exhibit sexual dimorphism. Like I said, not a complete answer.

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Corvids (for example) have a variety of bright and vibrant splashes all over their body; those colors just aren't in the human visual spectrum.

"There's no such thing as a fish"

It's also worth noting that colourful mating displays aren't useful to mammals because most mammals are colourblind. And since bright colours are expensive, and tan/brown/black/white does a perfectly good job of camouflage, there's no advantage for mammals to have the bright colours.

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Well, be careful with that one, because I usually hear that one in the context of denigrating paraphyly, and paraphyly is a useful and valid method of taxonomic classification as long as there are also equivalent monophyletic words.

Also that birds aren't pressured as highly into camouflage because of their ability to fly. But given that they're not unique in their colors, I'm not sure how big of a contributor that actually is.

I was hoping people would search and find my favorite podcast. Obvs there is such a thing as a fish; for the sake of accuracy it's probably better to say for our taxonomy there is no branch that contains all the creatures we commonly call fish while also omitting every creatures we don't call a fish.

No monophyletic branch. But there's a paraphyletic branch, and a definition based on that won't be any less objective or consistent.

I have heard good things about the podcast, though

I assume paraphyletic is a short way of saying basically the thing I just did: "there is a single fish branch if you're okay with a bunch of non-fish in it"

I'll piggy back on this to supplement the evolutionary reasons for this.

Mammals, birds, and fish all use sexual selection as a primary basis for their reproductive process. Demonstrating you are an exceptional member of your species is vital to attracting a mate and reproducing successfully. Typically, a Male of a species competes with others for the attentions of the choosy female. To do so they must prove they are stronger, faster, smarter, better fed, and generally more capable of supporting and/or protecting the dependent female than their competitors, or at the very least that his genes are worth the significant female energy investment to bearing offspring.

In the mammalian world, these competitions are best done in a way that is synergistic with survival demands. What better way to prove you can survive better than an opponent than to overcome them in some physical contest: Fight, be bigger, thump louder, etc. often while demonstrating a handicap that accentuates your ability to overcome (think Ginormous antlers that take a lot of food and energy to make and whose weight alone is a hinderance)? Even if you lose this contest you'll survive, learn from the experience, and the innate toughness it required in the first place makes you better to survive in the wild generally speaking.

Birds cannot tolerate the physical stresses of violent competition because the physics they rely on do not allow their skeletons to support it. It would be way to easy for hollow bird bones to break turning a fight lethal 90% of the time. How to compete then? For them, physical indicators are better: bright colourful pigmentation is energy intensive to produce. It will fade if the male is malnourished. It also reveals the presence of disease or parasites. The disadvantage of being so visible proves you are cunning as you're still alive despite it. Coupled with other attributes and performances this allows males to compete with Displays instead of fighting, for a much more survivable experience.

The reasons behind Fish colouration are less clear afaik. Best guesses are that the particular colour pattern of a fish allows them to camouflage in their colourful environment better.

To put it in perspective: there are species of fish that are more closely related to humans than they are to some other species of fish

Also, why are domestic cats all pretty much the same size?

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> As a result of events that occurred during the early history of mammals, eutherian mammals retain only two of the four cone opsin gene families found in many other vertebrates. Very likely during this same time frame, the elaborate system of coloured oil droplets characteristic of photoreceptors in many vertebrates were also abandoned, as was a portion of the specific retinal circuitry dedicated to processing colour information (Jacobs & Rowe 2004). These changes left most eutherian mammals with a single dimension of colour vision. Primates subsequently escaped this restriction by evolving a series of visual system alterations that provided opportunities for expanded colour vision.

From Evolution of colour vision in mammals

This means that mammals generally have poor color vision, primates like us being an exception. No need for flashy displays, they couldn't see them.

As for birds (aka avian dinosaurs), a lot of them have four color receptors, one more than our three, and their plumage is in fact more complex and dazzling than what we humans can even see. (Similar to flowers who sport designs only visible in the ultraviolet, meant for insect eyes.)

Fishes generally have good color vision, even into the ultraviolet, but as light decreases with depth those living deep down can be left only with rods in their retinas, i. e. be color blind.

In both cases, fishes and birds, the display of colors is used to attract mates, sometimes to scare rivals away.

Fantastic shapes (which can be found in mammals too, in the form of weird, cumbersome horns, for example) are thought to be the result of sexual selection, the females (generally) "preferring" these as a sign of health and thus good genes and lack of parasites. (A bird with a ridiculously oversized tail has to be good at survival to make it to the mating season.)

A last point about fishes: water drag increases with temperature. So while the big ones like sharks, tuna, &c., are always streamlined, it makes no sense for small tropical ones, and they can indulge in attractive body forms instead. But you don't see that in cold waters.

Please elaborate

We use the big sloppy category "fish" to describe all kinds of creatures under the sea. It's like if we bundled together all the chimps and certain kinds of birds and maybe one mushroom and decided those were all called arbs because they like to live in trees. It's a useful word in the real world, but the category doesn't map cleanly to any evolutionary branch

>It's also worth noting that fish in particular may be so widely varied because they're miscategorized; there's a push among some biologists to split up "fish" into several differently groups because there's so much more variation among "fish" than among equivalent groups.

While this is true, the splits would be jawless fish, sharks and rays, lungfish and kin, and everything else. So a huge chunk of the diversity of fish, and especially the colorful fish, is actually in one group of fish.

It's really a case of birds, fish, reptiles, and various invertebrates vs mammals. Mammals are the odd ones out. The main reason is that bright colors are usually used for sexual selection or species identification. Most mammals lack full color vision and rely more heavily on scent and sound for sexual selection and species identification. So mammals lack a lot of the really bright coloration. Primates are the oddballs of the group that rely more on sight and less on smell.

Mammals do come in some pretty varied shapes though, not many equivalent groups contain species as diverse in body shape as bumblebee bats and blue whales. But in terms of decorative frilly bits, again, those are often about sight based signalling.

All fishes are one group if you include tetrapodomorpha (amphibian, reptiles including bird, mammals).

https://biologue.plos.org/wp-content/uploads/sites/7/2020/05/Fig1_FToL2-scaled.jpg

If you don't, then fishes are not one group. Not even bony fishes.

> Not even bony fishes.

Sure, but there are 8 living species of bony fish that would not be in the group, so the vast majority of all color and shape diversity in fish is in that one group.

And it's far more accurate to say those 8 species aren't "true" fish than to say that fish doesn't exist as a category.

Which species?

Apparently this has two answers.

For one, dogs have been bred to be tools and assistants for a wide variety of situations. Cats were pretty much just "eat the pests," which is what they were gonna do anyways. So dogs got whatever morphology and size was needed for their job, but cats were just bred to be whatever they already were.

Secondly, there's actually a coincidence of genetics that gives dogs more natural variation than most animals, that both made more dramatic artificial selection possible for them and kept them the same species throughout all of it. Cats don't have that, and it would be harder to breed them in so many different directions.

A melanin variant can produce an orange shade. Tigers look very bright to us, but not to their prey, which can't make out the redness.

The ancestors of mammals were nocturnal and it's difficult to see colours in dim light. This makes it less useful to produce colourful pigments since they won't be noticed.

Lungfish and coelocanths are more closely related to us than to other fish

Sort of?

Paraphyletic is saying "everything in this evolutionary branch except these things".

Take reptiles for example. In common usage, it excludes birds, even though birds are a subset of dinosaurs, which are a subset of reptiles.

The exclusion of birds from the reptile "group" makes it paraphyletic.

Also fun fact! The most closely related animal group to birds is actually crocodilians (crocodiles, alligators, etc)

All life started in the water. Imagine at some point two species of fish diverged from a common ancestor, species X and species Y. Both became ancestors to many subsequent species. Some descendant species of X (irl: lobe finned fish) eventually came to land, and that's where we descent from. Now, thousands of modern fish species have evolved from species X, and thousands have from species Y. A far descendant from species X might look morphologically similar to a species that evolved from Y (they are both "fish") but it shares a closer common ancestor with humans.

Life is created, presumably starting from one species. Then there's an explosion of life. Now all kinds of life. Some have fins. Some have feathers. Some have feet.

Some develop into something else, as different to the ones with fins as humans are. But that design fails. Better to have fins. Now they have fins.

Humans come along and see it and go hey, that's a fish.

But this 'fish' maybe used to be a lizard and then became a fish. So it went:

Step 1: ancestor

Step 2: something else

Step 3: something else

Step 4: lizard?

Step 5: fish?

Meanwhile, 'fish' #2 went:

Step 1: ancestor

Step 2: fish?

So you have one fish that came from a lizard and one fish that came from something else entirely. As a result, you have one fish that has a shared common ancestor with humans say maybe one billion years ago and another fish that has a shared common ancestor with humans three billion years ago. That means fish #1 and humans are related by one billion years while fish #1 and fish #2 are related by three billion years.

Conclusion: some fish are more closely related to humans than to other fish, and the category of fish is meaningless.

This is also true of crabs and trees. Mother nature proves to us over and over again that crabs, fish, and trees are the most superior earthly life forms.

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Great answers, and I don't (didn't until now) have the biology behind it, but isn't the environment on which each species habitats a reason for it also? If we see it as a painting, birds "background" is sky (blue) andr trees (green) and colorful flowers so to camouflage they need to be colorful. Same goes for fish: sea (blue) , seaweed (greenish-yeallow), corals colorful. Also fish have this silver reflective effect to mimic sunlight in the sea. And packs of birds white belly black top, so when they Move in hundreds they look like one big bird. Also there are the octopus and other sea creatures but also chameleons that can change color. Insects that look like flowers or sticks. On the contrary, lions and tigers and pumas and other mammals live in Savannah, where the background is mainly ochre yellow from the sun and grass so that is their best camouflage option. The spots on some help with the camouflage also. Mice and castors and the rest have the ground as a "background" so dark grey or dark brown would be the best because they live under the surface. I mean, if I think about zebras, they didn't make the best choice, but at least they move in packs and it's quantity over quality. So isn't firstly the survival the evolution reason, and then the mating?

>because most mammals are colourblind

this one has always been weird to me, apes aren't the only mammal that spent a lot of its time finding red berries in a green background, why were we the only ones lucky enough to get trichromatic vision?

Fun note with structural colour: our eyes don't have any blue pigment, but we do have blue structural colour.

it's a sloppy shorthand, fine as a joke but not an explanation