(still joking, and paying the karma price of it. But Devil Shoes are Devil Shoes) If what matters is the number of animals suffering, regardless of brain size for instance, then clearly what should be done is to devastate environments enough that concentrated quantities of insects are not able to live there. In particular, Ants seem to take up a substantial proportion of number of living animals. So destroying anything that makes Ants prosper is a good idea.
Raising cattle means destroying forests. Fields can take fewer ants than forests (Wilson, Holldober. Superorganism 2008). So raising cattle diminishes animal suffering, by indirectly avoiding ant suffering.
I suppose I should put a MEMETIC HAZARD warning here, although this sort of thing, unlike the original basilisk that Eliezer had a fit over, is closer than your own heartbeat the moment you start taking reason seriously.
Clearly, the moral thing to do is to destroy all life. Even the microbes must go, lest the Earth re-evolve life again. But wait! Suppose there is life elsewhere in the universe? No, what we must do is destroy everything on Earth that has a nervous system except us, then get to the stars as quickly as possible and seed them with von Neuman...
(This post is completely jocose. If you can't take it, don't read it. I'm making fun of Rationalists, of Me, of homo economicus, of Vegans and of things I really praise, like Consequentialism and Outsourcing. It is not serious. The Sheldon Sarcasm sign has been lifted, your asperger side has been warned)
One of the features of rationality is that it allows you to mix different units.
By rationally behaving economically, you learn, for you, how many apples costs an orange.
Vegetarians and Vegans sell diminishing suffering. They claim to have the best price in the market, only Singularitarians and Existential Risk avoiding competes with their numbers. Utilitarians are a good target market.
Then a Lesswronger came and noticed that, and said: Well, why not buy someone to be a vegetarian for you, here.
Awesome price actually. You shock a few humans (notice that humans are animals, who clearly would rather be shocked than eaten), one of them enough to make him vegan.
So why not take this to the next level?
Figure out the reproductive cycle and eating habits of this beetle that makes people vegetarian. Make sure the evidence is solid.
Get a basement lab full of them.
Ship them alive to cities where more people consume meat. Wait for population growth.
Save a lot of animals!
Seems straightforward, but is it?
Also, are there similar strategies for other groups? Are there easy, but strange, shortcuts for selfish hedonists, immortalists, rational altruists? Utilitarian hedonists? The ancient school of negative utilitarianists? Cryonicists?