Humans are unique among animals in maintaining large, stable coalitions of unrelated individuals, strongly bonded by mutual trust.and:
Unlike other social animals, humans are very good at establishing and maintaining relations with agents beyond their physical presence; social hierarchies and coalitions, for instance, include temporarily absent members.
I guess some primatologists will say Boyer underestimates the ability of some apes to maintain complex webs of relationship, and I'd add work by Scott Atran to his reading list.
P.S. Other comments on Boyer include this, this and this.