The mechanism behind the female-biased sexual size dimorphism in snakes (SSD) is usually linked to a delay in females' maturation and/or higher growth rates during their immature phase. Studies on growth rates of immature snakes have been focused on sex differences related to feeding rates instead of food conversion rates. We tested the hypothesis that growth rates of immature female lanceheads are higher than that of immature males and determined if this difference is related to differential food conversion rates between genders. During 18 months, we collected data on growth rates in juveniles from a litter of a species (Bothrops fonsecai) with a female-biased sexual size dimorphism under a controlled feeding regime. Our results showed that females, even receiving the same amount of food than males, presented higher growth rates in all parameters measured (snout-vent length, mass, stoutness, snout-vent length-specific growth rate and mass-specific growth rate). Our data support a mechanism of establishment of sexual size dimorphism in a female-biased species apart from feeding rates but linked to a different feeding conversion between sexes. Based on our results, we also suggest that these differences in feeding conversion must be linked to sexual differences in energy assimilation efficiency.