Red-handed Howler - Taxonomy

Taxonomy

Considerable taxonomic confusion has surrounded this species. Until 2001, most authorities included the Amazon black howler as a subspecies (or simply a taxonomically insignificant variation) of the red-handed howler, though its distinction had already been pointed out much earlier. The red-handed howler remained variable in ecology, colour and pattern of the fur, shape of the cranium, and shape of the hyoid bone (of great importance in the voice of the howler monkeys; a likely isolating mechanism between the species), but a geographical pattern was not clear, resulting in it being treated as a monotypic species. In 2006, a major review of the Brazilian members of the genus Alouatta was able to match some of the variations to geography (though further study was recommended), resulting in the recognition of the Spix's red-handed howler and Maranhão red-handed howler as species separate from the red-handed howler. Even with these as separate species, the colour and pattern of the fur of the red-handed howler remains variable. Most adults are black with reddish-brown hands, feet and distal part of their tails, but some are entirely black (resembling the Amazon black howler), reddish (resembling the red howlers) or somewhere in between.