From Here on Earth by Tim Flannery: Chapter 10 - Superorganismic Glue.
p126: This tendency towards civilised imbecility has left its physical mark on us. It's a fact that every member of the mini-ecosystems we have created has lost much brain matter. For goats and pigs it's around a third when compared to their wild ancestors. For horses, dogs and cats it may be a little less. But, most surprising of all, humans have also lost brain mass. One study estimates that men have lost around 10 per cent, and· women 14 per cent of their brain mass when compared to ice-age ancestors. It's easy to see why. The dog's sharp nose protects the sheep from danger, while the herder's knowledge of pasture means that the sheep don't even have to think about where they'll forage for the day. And of course the wether's bones and other scraps relieve the dog from having to hunt, while its meat and skin feeds and shelters the man. Overall, life for all members of our domesticated mixed feeding flock is made so much more accommodating that its members can invest less of their energy in brains and more in reproduction and fighting disease.