Embryonic and fetal morphology of the black agouti

Understanding embryonic and fetal development is fundamental to interpreting life-history strategies associated with neonatal survival in mammals. This study provides a detailed characterization of the intrauterine development of the hystricognath rodent, the black agouti (Dasyprocta fuliginosa), one of the most hunted species in the Amazon. The developmental patterns are compared with those of other precocial and altricial mammals, offering insights into species-specific reproductive strategies. A total of 90 conceptuses (2 embryos and 88 fetuses) from 49 pregnant females collected from subsistence hunting in the Peruvian and Brazilian Amazon were analyzed for external and internal morphological features. The mean prolificacy was 1.84 ± 0.59 offspring per pregnant female, with a sex ratio of 1.26:1.00 (female:male). Fetal dorsal length (TDL) ranged from 0.57 to 23.3 cm. External development followed a consistent and sequential pattern, beginning with the appearance of outer ears, eyelid and limb buds, followed by ossification, fused eyelids, tactile pelage, differentiated genitalia, skin development, covering pelage, nail formation, tooth eruption, and ultimately eyelid opening. Fetal age estimation, based on the formula ∛W = 0.078(t - 20.8), showed a strong correlation between TDL and gestational age. All external biometric measures and absolute visceral organ masses were strongly correlated with TDL. The relative mass of the spleen, the tubular digestive organs, and the thymus increased across pregnancy; while liver, lungs, and kidneys decreased. The developmental pattern observed supports the classification of the black agouti as a precocial species, with neonates exhibiting early functional independence.