Venom and animals can be pretty weird.
For instance, the Funnel web spiders. Funnel webs, for the most part, aren't actually all that big. Sydney Funnel webs, for instance are only about 10-50mm in size (1/2 to 2 inches).
That puts very few vertebrate animals within their size range, basically small lizards and no mammals that I could name off the top of my head.
From there, it starts getting a little more... fucky.
If you are bitten by a Funnel web, you can expect some of these fun symptoms:
- Sweating, drooling and crying
- Tingling, muscle spasms
- Your heart will begin to pump into overdrive
- Shortness of breath, nausea, vomiting
Essentially the neurotoxins start pushing your nervous system into overdrive, switching them on and off rapidly. Salivary glands, tear ducts, sweat glands, your heart and other muscles. Death is usually caused by brain swelling, or your heart starting to beat so irregularly that your blood is no longer moved around properly and you die.
Fortunately, that hasn't happened to anyone since the antivenom was released in the 1980's. Result.
Venom, generally, is designed to incapacitate a threat as quickly as possible. It's usually tailored to incapacitate prey items as soon as possible, so in the Funnel web's case this involves overloading the nervous system as soon as possible. A big dose of Funnel web venom into an insect almost instantly incapacitates it as its nervous system starts fighting it and secures the kill for the Funnel web.
So, makes sense for insects, right? But what about vertebrates, why is it so toxic to vertebrates? Well, surprisingly... it isn't. In fact, cats, dogs and small rodents are effectively immune to Funnel web venom. If your dog gets bitten, it might get a little sick for a few hours and have quite a painful bite, but it'll be fine.
In fact, in some cases they can take over 100 times the lethal dose of a human and survive. Primates are one of the only vertebrates that respond so poorly to Funnel web venom. Primates, excluding us, being something that is conspicuously absent from the Australian mainland.
Funnily enough, Australia has quite a few native tarantulas; they're fairly poorly studied and relatively drab things to look at, but surprisingly, have venom that is almost the reverse of Funnel webs.
If you get bitten, you're looking at some pain, sweat and vomiting -- but you'll live. If your cat or dog gets bitten? They'll be dead within 30 minutes.
So, uh.... What the fuck, Spiders, why?
Honestly, just bad luck. Funnel webs, by chance, have a compound in their venom that is just especially lethal to primates.
Add to that the fact that males wander during breeding season and are, possibly for defensive reasons, much more venomous than females (who don't wander ordinarily), like to hide in cool dark places during the day and some species happen to live in suburbia... and you can see where trouble arises.
In fact, this is a trend that you can find with a lot of venomous animals. Almost none have any reason to envenomate and kill primates.
The blue ring octopus is another example of this. Their venom works in the reverse to Funnel webs. It suppresses the nervous system until you are paralyzed. There's not an antivenom available and the treatment is actually just to keep you breathing until your body breaks down the venom on its own.
This is handy for their prey species like shellfish and crabs, they drill the venom into the shells and inject venom either incapacitating the prey or in the case of shellfish, crippling the muscles holding the shell shut.
What a lot of people don't realize, however, is that all cephalopods (squid, octopus, cuttlefish) are venomous, for the same reasons. We just happen to respond very poorly to blue rings.
And the list goes on.
tldr; Venom is strange and sometimes we get caught in the crossfire.