why would we need them? We've lost our fur and tail. Why not lose the pheromones as well?
We haven't lost our fur. We've
almost lost our fur. That's a big difference. Genes don't die off very quickly just because they're not needed. Our DNA is full of "junk genes" that are not even expressed. A junk gene that gives us a physical characteristic that's not harmful can survive for many generations, because it's not being selected against. Genes that work against survival disappear quickly.
On a personal note, although not conscious of any pheromone or "smell" I enjoy breathing with my nose very close to wife's skin when we go to bed to sleep.
You would not smell it. The emerging layman's definition of a human pheromone is an "unconscious odor."
Somehow they know they are brothers and sisters and this is "taboo" even among mice.
They don't "know" it is taboo; that's the whole point about pheromones. They trigger instinctive reactions. The scent of a mouse's own bloodline is simply not sexually attractive.
That's a reasonable hypothesis for humans too. We seem to have a strong sexual attraction to potential mates outside of our own gene pool from just physical appearance, so pheromones may have evolved to reinforce it. Miscegenation (cross-breeding of human "races") was a serious taboo in the Old South (of the USA) and even in the Reconstructed South (after the Civil War a failed attempt was made to rebuild it in the image of the North), but it was an artificial taboo. Planters routinely mated with their female slaves and had what were called "mulatto" children. Modern DNA analysis has shown that this practice was widespread.
One of the most sensational cases was South Carolina Senator Strom Thurmond, who died just four years ago, one of the most embarrassingly Unreconstructed Confederates in modern America. He was a rabid segregationist and once ran for President under the banner of the short-lived "Dixiecrat" Party, whose platform was to reverse the trend toward racial integration. Shortly before his death it was discovered that he had had a secret affair with a black woman and she had borne his child.
Even humans who claim to hate people of other ethnic groups can feel a very strong sexual attraction to them. For those of us who think ethnic diversity is just great, the attraction can be overwhelming.
Not all mammals are like this at all. Gorillas are just the opposite. The alpha male forms a pack with his daughters and granddaughters and continues to breed with them. Inbreeding is so extreme in gorillas that biologists say if you look at the skulls from two gorillas on opposite ends of their range, you'd think they were two different species. Dogs have no aversion to mating with their own siblings, parents or other close relatives.