Why Are Some Animals Venomous?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Some animals are venomous because injecting chemicals is an extremely efficient way to subdue prey or deter a predator without a physical fight. Venom has evolved independently dozens of times across the animal kingdom, which is a strong sign of how well the strategy works.
Venom rather than strength
A physical struggle with prey is risky. A bite that can kill without a fight lets a small animal take prey it could never overpower, and lets a slow animal avoid being injured.
The same applies defensively. A creature with no speed or armour can still make itself extremely unprofitable to attack. Both uses explain why venom appears in animals as different as snakes, spiders, jellyfish, cone snails, shrews and the male platypus.
Venom is not poison
The distinction is about delivery. Venom is injected through a wound using fangs, stingers, spines or barbs. Poison is absorbed or ingested, so it harms whatever eats or touches the animal.
A rattlesnake is venomous. A poison dart frog is poisonous. Some species are both, and a few snakes are technically neither despite being widely believed dangerous. The useful rule is that if it bites you and you get ill it is venomous, and if you bite it and you get ill it is poisonous.
What venom actually does
Venoms are complex mixtures, often containing dozens of proteins and peptides, and they generally target one of a few systems:
- •Neurotoxins, which block nerve signals and cause paralysis
- •Haemotoxins, which affect blood clotting and damage vessels
- •Cytotoxins, which destroy tissue around the bite
- •Enzymes that break tissue down, sometimes beginning digestion
Why it evolved so many times
Venom systems have arisen independently in many lineages, a pattern called convergent evolution. Frequently the raw material was a gene for an ordinary protein, often a digestive enzyme, duplicated and then modified into something more damaging.
This is a common route in evolution. A copy of an existing gene is free to change without breaking the original function, which is why venom components so often look like modified versions of everyday body proteins.
Venom as medicine
Because venom molecules target specific receptors with high precision, they are a valuable source of drug leads. Several approved medicines derive from them.
Examples include a blood pressure drug developed from a Brazilian pit viper peptide, a diabetes treatment based on a compound from Gila monster saliva, and a powerful painkiller derived from cone snail venom. Researchers continue to screen venoms for compounds that act on pain, clotting and cancer pathways.
What to do about a venomous bite
Most traditional first aid for snakebite is actively harmful, and the list of things not to do is longer than the list of things to do.
Current medical guidance is straightforward:
- •Get to medical care as quickly as possible, since antivenom is the treatment
- •Keep the person calm and the bitten limb still and below heart level
- •Remove rings and watches before swelling starts
- •Do not cut the wound, suck out venom, apply a tourniquet or use ice
- •Note the appearance of the animal if it can be done safely, but do not chase it
The takeaway
Venom evolved repeatedly because injecting a chemical is a low risk way to hunt or defend, it is distinguished from poison by being delivered through a wound, and its precision has made it a genuine source of modern medicines.