Impact of intrasexual selection on sexual dimorphism and testes size in the Mexican howler monkeysAlouatta palliataandA. pigra

Abstract

One of the goals of physical anthropology and primatology is to understand how primate social systems influence the evolution of sexually selected traits. Howler monkeys provide a good model for studying sexual selection due to differences in social systems between related species. Here, we examine data from the sister howler monkey species Alouatta palliata and A. pigra inhabiting southeastern Mexico and northern Guatemala. We use a resampling approach to analyze differences in sexual dimorphism of body and canine size. In addition, we compare testes size as a way of gauging the intensity of sperm competition in both species. Morphometric data were collected from wild-caught individuals, including body mass and length, and dental data were obtained from casts from wild individuals and from museum specimens. Although A. pigra individuals are larger than their A. palliata counterparts, we find that both species exhibit similar levels of sexual dimorphism for all of the variables considered. Testicular volume results indicate that A. palliata male testes are on average twice as large as those of A. pigra males, suggesting more intense sperm competition in the former species. Our study shows that A. pigra is not highly sexually dimorphic as was once thought, and testes size differences suggest the need for a clearer understanding of howler monkey social systems.

Habitat degradation impacts black howler monkey (Alouatta pigra) gastrointestinal microbiomes

“…As quantitative dietary data were expressed in time spent consuming resources, comparisons were standardized by body weight depending on individual age and sex (Kelaita et al, 2011). The time each individual spent feeding on food types and plant species was analyzed as proportion data after arcsine transformation.…”

The Howler Monkey as a Model for Exploring Host-Gut Microbiota Interactions in Primates

“…Fecal ammonia concentration estimates microbial protein metabolism, while VFA content can be used to estimate microbial carbohydrate fermentation. Because these values vary according to body size, all data were standardized by body weight for each age/sex class before analysis (Kelaita et al 2011 ).…”

“…Average wet masses of food items were used to estimate the average daily amount of grams of food ingested by each individual for each food type and plant species (Amato 2013 ), and the average kcal and grams of protein ingested by each individual was calculated using general estimates for Neotropical food types (Norconk et al 2009 ). Feeding data were standardized by metabolic body weight for each age/sex class before analysis (Kleiber 1975 ;Kelaita et al 2011 ).…”

Endocrinology of Howler Monkeys: Review and Directions for Future Research

“…Furthermore, a study that investigated male reproductive physiology and sperm production in three adult male and three juvenile male black-and-gold howler monkeys in captivity revealed that even low levels of fecal androgens were suffi cient for normal sperm count, quality, and motility (Moreland et al 2001 ). Additionally, small testes size relative to body size characteristic for black howler monkeys suggests an overall weak level of sperm competition in this howler monkey species (Kelaita et al 2011 ). In Rangel-Negrín et al's ( 2011 ) study of A. pigra , mean fecal androgen levels in males of two focal groups for which copulations were observed were not signifi cantly higher in weeks when copulations were observed compared to weeks without observed copulations.…”