Gene loss, thermogenesis, and the origin of birds.

Compared to related taxa, birds have exceptionally enlarged and diversified skeletal muscles, features that are closely associated with skeletal diversification and are commonly explained by a diversity of avian ecological niches and locomotion types. The thermogenic muscle hypothesis (TMH) for the origin of birds proposes that such muscle hyperplasia and the associated skeletal innovations are instead the consequence of the avian clade originating from an ancestral population that underwent several successive episodes of loss of genes associated with thermogenesis, myogenesis, and skeletogenesis. Direct bird ancestors met this challenge with a combination of behavioral strategies (e.