Comparison of Intersexual Relationships between Western-Lowland Gorillas and Siamangs.

Types of mating systems are important selective factors in the evolution of social behavior/dynamics in social animals. We examined how mating systems influenced the male-female (intersexual) relations of two species of ape at the Cincinnati Zoo, western-lowland gorillas (Gorilla gorilla) and siamangs (Symphalangus syndactylus). The two species share many similarities and differences, but most relevant to this study were their different mating systems. Siamangs are largely monogamous, where one male mates with one female, whereas western-lowland gorillas are polygynous, where one male mates with multiple females. We used focal scan sampling where we studied every individual from each study group for a specified period. At each minute, we recorded the current focal animals\u27 behavior as well as their distance (proximity) from all opposite sex individuals. We found that the siamangs spent 80.3% of their time in close proximity (\u3c 2m) while the gorillas spent 25.4% of their time at the same distance. No instances of affiliative or aggressive interactions were observed between the gorillas. The siamangs, however, spent 11.6% of their time performing affiliative interactions and \u3e1% performing aggressive interactions. This research supported previous studies that found that within monogamous mating systems intersexual relations are characterized by more time spent in proximity and more affiliative interactions than in polygynous systems