IN ANY CASE, even paired birds with their cuddle hormones up and running are not paragons of fidelity. According to Rhiannon West, a biologist at the University of New Mexico, this may be another reason why some bird species are smart. West proposes that it’s not just the challenges of maintaining pair-bonds in birds that have boosted their brainpower. Rather, she says, it’s “the complexity of achieving a successful pair bond and extra-pair copulations that is simultaneously driving the increase.” It’s what she calls an “intersexual arms race.”
