Since 1976 this study has been conducted at Hato Masaguaral, a cattle ranch with abundant wildlife in the llanos of Venezuela. It has involved regular censuses of more than 40 red howler groups comprising a population of about 350 animals. While these censuses provided detailed and long-term demographic data, they also enabled the discovery of infanticide following male invasions of one-male and multi-male bisexual groups.
These invasions invariably resulted in the establishment of multi male groups, which included an invader and the resident adult male or several invading males. Additionally, females whose infants were killed copulated thereafter with several males. Female choice of several sexual partners led to questions of paternity of offspring born after infanticide, and challenged the most popular explanation for the evolution of this behavior.
Hence, a detailed study of social and sexual interactions within invaded groups was initiated in 1988. This study provided more startling facts about red howler infanticide that supported an alternate explanation for the function of infanticide. This information was published in Ethology in 1996. However, the predictions based on the different explanations for infanticide have yet to be tested fully in red howlers. Future research on red howlers will focus on:
The red howler study is one of the few long-term investigations being conducted on South American primates today. The demographic information is one of most detailed data sets available for any new or old world primate. It has illustrated the demographic features of a growing population, and in the last few years, the characteristics of a declining population as well.
In 1978, this study was the first to document infanticide in a new-world monkey species, although primate investigations have been conducted in South America since the 1930s. Subsequently, other new-world primates were also found to be infanticidal. Furthermore, investigations of social interactions within invaded red howler groups has provided the most comprehensive observational data found anywhere, to explain the evolution of infanticide.
Hence we will continue the red howler investigation to evaluate the function of a behavior that seems to be as common in new-world primates as in old-world primates.