3.2.2 Alouatta pigra Lawrence, 1933

Type: Adult male, collected by A. Murie, 4 May 1931, Museum of Zoology, University of Michigan.

Type locality: Uaxactun, Petén, Guatemala.

Common name: Central American black howler.

Alouatta pigra is distributed across the Peninsula of Yucatán in Mexico, Belize, and north and eastern Guatemala (Fig. 3.1). Most authors followed Lawrence’s taxonomic arrangement of the Mesoamerican howlers, until 1970, when James D. Smith analyzed two sympatric populations of howler monkeys from the state of Tabasco, Mexico. He examined the skulls, teeth, and/or pelage of 238 specimens from across the range of A. palliata (sensu Lawrence 1933) and maintained that the two (partially sympatric) forms from his sample constituted two well-differentiated species, A. palliata and A. pigra. Alouatta palliata luctuosa Lawrence, 1933, he considered a junior synonym of A. pigra, given that the only extant specimen (from Mountain Cow, Cayo District, Belize) fell within the range of variation of A. pigra. Horwich (1983) supported Smith’s recognition of two distinct species and commented on differences in group size between A. palliata and A. pigra (the latter having consistently smaller group sizes than the former), as well as differences in male genitalia; A. pigra males have fully descended testes from infancy, the testes of A. palliata males do not descend until they are subadults. Cortés-Ortiz et al. (2003) using mitochondrial DNA sequence data corroborated that A. palliata and A. pigra fall into two reciprocally monophyletic groups that separated approximately 3 MA, supporting their status as distinct species. Steinberg et al. (2008) also supported this distinction based on chromosome number (A. palliata 2n = 54 and A. pigra 2n = 58) and on the male sex determination system (X 1 X 2 Y in A. palliata and X 1 X 2 Y 1 Y 2 in A. pigra). All samples of A. pigra analyzed by Cortés-Ortiz et al. (2003) and Steinberg et al. (2008) came from Mexico, and the validity of A. pigra luctuosa from the Cayo District in Belize has yet to be evaluated with genetic data.

Regarding the proper name for the Central American black howler monkey, as already mentioned, Brandon-Jones (2006) provided evidence that the A. villosa type (skull of a young adult female in the British Museum [Natural History]) is recognizably distinct from A. palliata and that the type locality can be identified as “Mexico.” However, further studies exploring the similarities of the A. villosa -type specimen to a large sample of crania from A. pigra from Mexico, Guatemala, and Belize, as well as the differences with other black howlers (i.e., A. nigerrima and A. caraya) of known geographical provenance would provide compelling evidence for the suggested status of A. villosa as senior synonym of A. pigra. Given the widespread use of A. pigra and the benefits of taxonomic stability, we continue to call this taxon A. pigra until further studies are completed.