<p>In species under sexual selection, morphological and behavioral traits strongly influence reproduction, which ultimately affects copulation rates and reproductive success. One of the most conspicuous traits are the sexual size dimorphism, such as the permanent or transient enlargement of males, as in the case of squirrel monkeys. In this study, we unveil some key aspects of the “fatted male phenomenon” in four captive groups of Amazonian Collins’ squirrel monkey. We examined their reproductive behavior, dominance, and network power to investigate whether male fattening enhances their copulatory success and to find the presence of pre-copulatory sexual selection factors. Our results show that males in fatted condition increased their copulation rate and displayed higher levels of aggression towards females and other males. We did not find any evidence of greater female tolerance, receptivity, or attraction to fattened males, nor did we find any influence of the fattening condition on the social dominance of males. The evidence suggests that pre-copulatory sexual selection operates in two simultaneous ways in this species. For fatted males, it operates through the imposition on females. In contrast, the affiliative network positions allow the non-fatted males to stay socially closer to females, thus allowing them to achieve successful copulation and some degree of reproductive success. Our study provides strong evidence for two alternative male reproductive strategies that may explain the persistence of fatted and non-fatted phenotypes across generations, and suggests that a variety of male strategies may be used by males of different vertebrate species.</p>