Eli5: Why do birds and fish come in such a spectacular variety of colors and shapes compared to other animals?
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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.
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.
Also, why are domestic cats all pretty much the same size?
> 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](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2781854/)
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.
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.
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.
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?
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