Horwich RH, Brockett RC, James RA, Jones CB (November 2001) Population structure and group productivity of the Belizean black howling monkey (Alouatta pigra): implications for female socioecology. Primate Report 61: 47-65.
Abstract: The assembly and architecture of populations are functions of decisions made by individuals for the optimization of lifetime survival and reproductive success. We analyzed the results of 12 longitudinal surveys (209 group counts) of Belizean black howling monkeys (Alouatta pigra) at the Community Baboon Sanctuary (CBS) in an attempt to describe population structure and group productivity over time. Similar to previous reports of black howlers at several sites, modal group size was found to be one adult male and 2 adult females. Group size ranged from 2 to 16 and maximum female group size* was 4 as reported for other species of polygynous Alouatta. Population density ranged from 8.14 - 178.19 individuals per km^2, one of the highest densities ever recorded for A. pigra. Group size was significantly positively correlated with population density, and 52% of the variance in group size was explained by population density. Female group size and number of immatures [J+I+ sub-adults] per group were positively correlated. An analysis of the least squares regression line for female group size and the number of immatures per group found 9 of 12 surveys experiencing density-dependent conditions. Relative reproductive success [RRS], the mean number of immatures : females per female group size, decreased with increasing female group size suggesting that females in larger groups are at a disadvantage due to decreases in survivorship and/or fecundity [i.e., no Allee Effect]. Again, density-dependent conditions appeared to be operating. Our analysis of gains and losses to 19 groups from 1995-1997 suggests that the black howler population at the CBS is at equilibrium or slightly increasing, primarily as a result of recruitment of immatures (infants, juveniles, and/or sub-adults). If female black howlers at the CBS experience density-dependent conditions, they may undergo significant food competition contrary to predictions of the "ecological model" for folivorous primates. The different conditions predicted by the least squares regression analysis (density-dependence, density-independence, or an advantage to large groups [see method used on page 54: "An indirect test of female survivorship and/or fecundity]) may define the domains of selective pressures generating variations in group size as a function of decisions made by individuals optimizing inclusive fitness. These and other findings have important implications for female social relations Alouatta. At present we cannot distinguish between competition for limiting food resources and infanticide as the proximate mechanism [or, both?] limiting female group size in Belizean black howlers and other polygynous howlers.
*Related Reference
Jones CB, Milanov V, Hager R (2008) Predictors of male residence patterns in groups of black howler monkeys. J Zool 275: 72-78.
Abstract: The assembly and architecture of populations are functions of decisions made by individuals for the optimization of lifetime survival and reproductive success. We analyzed the results of 12 longitudinal surveys (209 group counts) of Belizean black howling monkeys (Alouatta pigra) at the Community Baboon Sanctuary (CBS) in an attempt to describe population structure and group productivity over time. Similar to previous reports of black howlers at several sites, modal group size was found to be one adult male and 2 adult females. Group size ranged from 2 to 16 and maximum female group size* was 4 as reported for other species of polygynous Alouatta. Population density ranged from 8.14 - 178.19 individuals per km^2, one of the highest densities ever recorded for A. pigra. Group size was significantly positively correlated with population density, and 52% of the variance in group size was explained by population density. Female group size and number of immatures [J+I+ sub-adults] per group were positively correlated. An analysis of the least squares regression line for female group size and the number of immatures per group found 9 of 12 surveys experiencing density-dependent conditions. Relative reproductive success [RRS], the mean number of immatures : females per female group size, decreased with increasing female group size suggesting that females in larger groups are at a disadvantage due to decreases in survivorship and/or fecundity [i.e., no Allee Effect]. Again, density-dependent conditions appeared to be operating. Our analysis of gains and losses to 19 groups from 1995-1997 suggests that the black howler population at the CBS is at equilibrium or slightly increasing, primarily as a result of recruitment of immatures (infants, juveniles, and/or sub-adults). If female black howlers at the CBS experience density-dependent conditions, they may undergo significant food competition contrary to predictions of the "ecological model" for folivorous primates. The different conditions predicted by the least squares regression analysis (density-dependence, density-independence, or an advantage to large groups [see method used on page 54: "An indirect test of female survivorship and/or fecundity]) may define the domains of selective pressures generating variations in group size as a function of decisions made by individuals optimizing inclusive fitness. These and other findings have important implications for female social relations Alouatta. At present we cannot distinguish between competition for limiting food resources and infanticide as the proximate mechanism [or, both?] limiting female group size in Belizean black howlers and other polygynous howlers.
*Related Reference
Jones CB, Milanov V, Hager R (2008) Predictors of male residence patterns in groups of black howler monkeys. J Zool 275: 72-78.