The race is to the smart...and the fleet and the well camouflaged

Predators Prefer To Hunt Small-brained Prey

Predators such as leopards and chimpanzees consistently target smaller-brained prey less capable of escape, research at the University of Liverpool has shown.They avoid more intelligent prey such as monkeys which have exceptionally large brains and are more capable of escaping attacks.

------------------------snip--------------------

Animals with small brains lack behavioural flexibility and are probably less capable of developing new strategies to escape predators, compared with larger-brained species.

------------------------snip--------------------

"Some animals' ability to avoid being eaten by predators may be a contributing factor to the evolution of large brains across some species, adding to conventional theories which argue this is important for developing social relationships and using tools."

When we talk about co-evolution, we usually think of pairs of species: a flower and a bee, a lion and a zebra, a ground squirrel and a rattlesnake. But in reality, each species is involved in multiple co-evolutionary arms-races with a number of other species simultaneously. You evolve either general adapatations to survival that give you an average success rate against all other species, or you evolve highly specialized adaptations to beat one particular species (which is most abundant, most tasty, most dangerous, etc, where you live) and ignore the others.

So, some of the prey species got too smart, others too fast, others too stealthy for these predators to bother with any more - they lost those co-evolutionary races and are now focusing on other species that are still sufficiently dumb/slow/obvious in their environment even if they do not taste as well or cannot be found in such great numbers or ar in other ways an inconveneince to the predator - but they are the only ones around who are still an easy catch.

So, just like we stopped thinking about food chains and started thinking about food webs, we should stop thinking only about co-evolutionary pairs and start thinking about co-evolutionary webs.