Altricial patterns in prenatal skeletal development of two large neotropical primates: The Poeppig's Woolly Monkey and the Juruá Red Howler Monkey

Understanding prenatal skeletal development is essential for pregnancy monitoring and veterinary care, as well as for understanding reproductive strategies, evolutionary differences, and ecological adaptations. This study provides an ultrasonographic description of prenatal bone development in two large neotropical primates, the Poeppig's woolly monkey (Lagothrix lagothricha poeppigii, n = 24) and the Juruá howler monkey (Alouatta juara, n = 7). Bone mineralization timing and skeletal biometric data were evaluated in relation to total dorsal length (TDL), which averaged 15.7 ± 5.3 cm in the woolly monkey and 13.9 ± 7.0 cm in the howler monkey. Gestational age correlated strongly with TDL (r2 = 0.99; p < 0.01), CRL (r2 ≥ 0.92; p < 0.01), and body mass (r2 ≥ 0.95; p < 0.01). The highest correlation coefficients between TDL and mineralization were observed with head circumference, scapula and humerus width, and long bone diaphysis, which are suitable indicators of gestational age. The relatively early mineralization of the appendicular skeleton, combined with the absence of secondary ossification centers in both species, suggests skeletal development typical of altricial species, with low neonatal resistance to muscular traction but the ability to cling to mothers, ensuring survival associated with prolonged maternal dependence. Allometric growth was observed during gestation, resulting in hindlimbs slightly longer than forelimbs, common in the locomotor category “arboreal quadruped,” with suspensory movements relying on forelimb strength and mobility. This study provides a morphological basis for fetal development, supporting a better understanding of neonatal survival in non-human primates.