Predation is known to have a significant effect on life history, eliciting predictable responses. Physical constraints of body shape and size may also limit life history divergence. There may be a trade-off between adapting to predation, and limits placed by constraints that decrease life history divergence. We test this idea in the Costa Rican livebearing fish Alfaro cultratus. This species has a keeled ventral surface and does not develop a distended abdomen when pregnant like other livebearers. We describe the life history of A. cultratus in 20 different populations across predator and non-predator environments. We found significantly lower reproductive allotment in predator environments relative to non-predator environments, but no significant difference in female or male size at maturity, number of offspring, or size of offspring. We found that A. cultratus exhibit isometric patterns of allocation for clutch dry mass in relation to female dry mass in predator and non-predator environments. We suggest that body shape constraints in this species limit the life history divergence we typically see in predator and non-predator environments in other species.