From the team at BBC Wildlife Magazine

The origin of pieces: 13 amazing ways animals have evolved

Discover the origin and purpose of the structures of living things, from why baboons have swollen bottoms to the comically-large paws of moles.

Diamondback rattlesnake coiled on a dirt road.
Published: August 15, 2022 at 2:00 pm
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Animals and plants have specialised parts that carry out different functions. They can be used as a form of defence, to enable a species to move through its natural environment, attract a mate or process food. Discover more about fascinating wildlife anatomy.


How does a rattlesnake's rattle work?

No one wants to accidentally step on a rattlesnake. The snake doesn’t like it much either. Happily for all concerned, as it grows, a rattlesnake accumulates small hollow segments of each shed skin at the tip of its tail, which clank together menacingly when shaken. The result is a warning signal as archetypal as a wasp’s black-and-yellow stripes. Increasing the frequency of the rattle adds to the sense of urgency as danger approaches.

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How does a fish's swim bladder function?

Anatomy of a fish © Tigatelu/iStock/Getty
An anatomical drawing of a fish shows where its swim bladder is located. © Tigatelu/iStock/Getty

Inflatable bags of gas are useful organs whether you live on land (lungs) or in water (buoyancy aids). By inflating or deflating its swim bladder, a fish is able to ascend or dive as required. While lungs are aerated via the mouth, the swim bladder is fed by gases dissolved in the blood. The organs have the same evolutionary origins, though, both developing from simple air sacs used by ancestral fish to gulp air in oxygen-poor waters.

How does a horsefly bite?

Common horsefly
Not all adult horseflies bite - only the females have mouthparts able to break the skin and feed on blood. © Getty

The exquisite beauty of horseflies (those iridescent eyes!) belie their utter brutality. No mosquito-like precision here. They simply slice into flesh with a cluster of serrated blades and lap the blood that inevitably flows. They even lack the decency to anaesthetise the incision site - little point when their preferred victims (horses and cattle) are so ill-equipped to swat them. The first horseflies sipped nectar, as the males still do. Females, though, need a blood meal to provision their eggs.

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Why do baboons have swollen bottoms?

Hamadryas baboon's bottom.
Hamadryas baboon's bottom – a red derrière is a sign of sexual readiness. © Gypsy Picture Show/iStock/Getty

Scientists haven’t quite got to the bottom of the florid sexual swellings on the hind-quarters of female primates including baboons and chimpanzees. They are at their most spectacular during the fertile phase of the reproductive cycle, when males show the greatest interest in them. However, it’s not clear that females with the fruitiest booties enjoy greater reproductive success. In mandrills, it’s the males that have the rum bums – the more dominant the male, the more louche the tush.

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What is the purpose of a duck's speculum?

Mallard in flight.
Watch a mallard in flight for the best view of its purple speculum. © Tomas Rak/Alamy

An iridescent jewel nestles amongst the plumage of many ducks. Formed from secondary wing feathers, the speculum is most obvious during flight. Otherwise, it appears as a glinting rhomboid of saturated colour on each flank (purple in mallards, green in teal, white in gadwall, for example). It may play a role in species recognition, but it’s probably no coincidence that it is positioned on the very spot that ducks ritually preen themselves during courtship and competitive displays.

Why do moles have large paws?

European mole emerging from molehill.
European mole emerging from molehill © Arterra/Contributor/Getty

If you’re going to spend your life digging tunnels, you’d better have a good shovel. Moles have them in spades - well, two, anyway. Their massive, clawed front paws are built for excavation. Evolution has even transformed one of their wrist bones into an extra “thumb” to help them shift soil in bulk. Moles aren’t the only mammals to have a false thumb fashioned from a wrist bone. A similar structure helps giant pandas grip bamboo.

How does a jellyfish sting?

Jellyfish, Port Eilzabeth, South Africa.
The stinging cells, called cnidocytes, are on the tentacles. © wildestanimal/Getty

Jellyfish, corals, anemones – collectively, cnidarians (with a silence c) – aren’t the liveliest of creatures. Yet they perform one of the fastest movements in nature, one that cannot be captured even by high-speed video techniques. Their potent stings are delivered by tiny barbed harpoons, each packed into a single cell full of venom. When triggered, they discharge explosively, like an inverted finger of a rubber glove popping back out under pressure, accelerating faster than a bullet in a gun barrel.

How does a ruminant's stomach work?

Reindeer grazing in the tundra, Spitzbergern, Svalbard, Norway.
Reindeer grazing in the tundra, Spitzbergern, Svalbard, Norway. © Sylvain CORDIER/Contributor/Getty

The multi-chambered stomachs of cattle, deer and other ruminants are dedicated to releasing the considerable energy contained in the complex sugars that form the structural bulk of plants (indigestible roughage to you and me). The magic happens in the rumen, a fermentation chamber containing an ecosystem of microbes with biochemistry skills lacking in mammals. The resulting cud is regurgitated and chewed again (with a contented far-away look in the eyes) ready for a more conventional digestive process.

Why do roses have thorns?

Rose and thorns.
A rose's thorns are prickles. © Saskia Rischka/EyeEm/Getty

One might assume that spines, thorns and prickles are different names for the same things. Botanists would disagree, though. They are different structures used for a common purpose: defence against hungry herbivores. Spines - most spectacularly deployed by cacti - are highly modified leaves. Thorns are pointed branches or stems. While a hawthorn’s thorns are true thorns, a rose’s famous thorns are actually prickles – simple outgrowths from the bark, more akin to thick, sturdy hairs.

Why does an amblypygid have jaws?

Tailless whip scorpions belong to the Amblypygi order. © Graham Wise/Flickr
Tailless whip scorpions belong to the Amblypygi order. © Graham Wise/Flickr Creative Commons licence

Are there any jaws more fearsome than the enormous spiked, hinged contraptions wielded by an amblypygid? Strictly speaking, they are not jaws at all, but palps, used by many other arachnid groups as sensory organs. In amblypygids, that job is done by the ludicrously long first pair of walking legs, which silently sweep the surroundings for prey. Woe betide anything that stumbles within touching distance.

Why does a swallowtail caterpillar have an osmeterium?

A swallowtail caterpillar.
A swallowtail's osmeterium smells like rancid pineapple to human noses. © Buddy Mays/Getty

Unique to members of the swallowtail butterfly family, this other-worldly organ is usually stowed away in a pouch behind the caterpillar’s head, but when danger strikes, it is inflated spectacularly to deter predators with a combination of visual and chemical shock and awe. Predators are repelled by its repugnant smell and its resemblance to a snake’s forked tongue.

What is ram’s horn squid’s shell?

Ram's horn squid's (Spirula spirula) shell on a beach. © Nandani Bridglal/Getty
Ram's horn squid's ( Spirula spirula) shell on a beach. © Nandani Bridglal/Getty

Though it looks like a loosely-coiled snail shell, this is more akin to the cuttlefish 'bones' enjoyed by pet budgies. It’s the internal buoyancy organ of a ram’s horn squid, a deep-sea cephalopod that was observed alive in the wild for the first time only in 2020. The species is largely tropical, but the horns’ buoyancy means they do occasionally turn up on UK beaches.

How do remora fish attach to other animals?

Giant manta with remora attached and being cleaned by Clarion angelfish. © Michele Westmorland/Getty
Giant manta with remora attached and being cleaned by Clarion angelfish. © Michele Westmorland/Getty

Dorsal fins are used conventionally to prevent a fish rolling to the side while swimming and to help them negotiate tight turns. Remoras, though, have transformed theirs into a structure designed to get other animals to do their swimming for them. The bizarre suction cup on the top of a remora’s head allows them to hitch rides from whales, sharks and other big fish.


Main image: Diamondback rattlesnake coiled on a dirt road. © twildlife/iStock/Getty


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