Vertebrates may be born highly dependent (altricial) or may rapidly gain independence (precocial). Primates are generally considered somatically precocial. However, all are at least initially helpless, and many primates have a prolonged phase of juvenility. In this chapter, we discuss how selection may influence the relative timing of appearance of morphological features (heterochrony). Newborn primate morphology offers unique insights into the roles of prenatal and postnatal growth processes, primarily because metabolic costs for growth commence a transition from the mother to the infant at this point in time. With this in mind, primates vary remarkably at birth in dental eruption and mineralization status as well as limb skeleton ossification (e.g., wrists and ankles). We also discuss evidence, still relatively scant, that at birth primates vary greatly in the degree to which neural organs (e.g., brains, eyes) have achieved adult size and proportions. In preparation for morphological descriptions to follow, the reader is introduced to the concept of modularity of growth: different parts of the skeleton or even parts of regions have different rates of growth and development.

Skeletal Anatomy of the Newborn Primate was written to broaden our knowledge of non-human primates from a comparative and developmental perspective. This chapter explains that the main focus of our book is on the inherently risky neonatal period. The “neonate,” or newborn, is considered here to be a perinatal primate of up to seven days postnatal age. However, there is no simple way to physically identify primate newborns, not in the same many have defined “infants,” based on dental maturity. This is precisely what makes the neonatal stage so interesting: primates, like most other groups of mammals, vary in how rapidly they attain physical maturity. This introductory chapter discusses terminology and methodological challenges in studying newborns.

Feeding ontogeny in primates has three stages. In utero, nutrition is gained maternally. After birth, primates suckle. We know little about functional variation in these stages. The transition to adult feeding – highlighted by weaning – varies across species. Variation is tied to many socioecological and morphological influences across primates. Primate feeding apparatus ontogeny is affected by many factors. Diet exhibits a complex relationship with the clearest signal marked by rapid dental mineralization and eruption in folivorous strepsirrhines. Mineralization varies across primates. Emergence and eruption of postcanine teeth tends to follow size in both suborders with smaller taxa showing earlier emergence, the exception being rapid eruption in some folivores. Compared to teeth, less is known about the musculoskeletal ontogeny of the feeding apparatus. Most studies compare closely related species and link musculoskeletal robustness to challenging diets. Looking forward, better understanding of primate feeding apparatus growth will require improved samples (a challenge for long-lived species) and emphasis on the evolutionary significance of feeding throughout ontogeny.

In this chapter we introduce concepts in dental development, microanatomy of the tooth germ and mineralizing crowns, and terminology relating to dental morphology. Subsequently, tooth morphology in newborn hominoids (apes and humans) is discussed based on the literature, followed by accounts of the extent of crown mineralization at birth in a newly described sample of tarsiers, Old World monkeys, New World monkeys, and strepsirrhines (lemurs and lorises). Morphology of crowns is described in all species in which the crown is completely formed at birth. The chapter ends with a brief discussion of the “perinatal” trajectory of dental development in selected primate species based on a comparison of species at different stages (fetal, neonatal, and older infant), including some at similar known ages.

Life-history theory pertains to the entirety of prenatal and postnatal ontogeny, and therefore morphology of the newborn offers an important perspective on how primates invest in their young. Generally, longer gestations yield larger neonates that are weaned later, become sexually mature later, and have larger brain masses. But can the variations in skeletal maturity are birth explained by life-history traits? Here, we examine new somatic data on 47 species of primates in light of life-history traits and modularity of growth among body regions. “Snout” length is uniformly diminutive in newborns compared to adults, although some scaling differences are already apparent at birth (relatively longer palates in strepsirrhines and tarsiers). Correlations of life-history characteristics indicate gestational length has a significant influence on facial functional matrices, with a positive correlation with permanent tooth and eye size, and a negative correlation with deciduous tooth germ size. We may as yet lack a broad enough perspective on brain size at birth, but some existing observations suggest primates preferentially prioritize prenatal brain growth over general somatic growth.

Primate locomotor development is a protracted process. We summarize the time course of locomotor development in approximately 50 primates distributed across the extant radiation. Despite substantial variance, we identify several broad trends. Primates are somewhat precocial at birth – born with their eyes open and able to strongly grasp. Locomotor onset age generally increases with body mass, although certain taxonomic groups (e.g., lemurids and cercopithecids) develop early for their size whereas others (e.g., indriids and hominids) develop relatively late. Initial locomotor movements are similar across primates and dominated by quadrupedal crawling. Only later do more specialized forms of locomotion emerge (e.g., leaping and brachiation), often in concert with functional changes in musculoskeletal anatomy (e.g., maturation of intermembral indices, center of mass position, and bony muscle leverage). We advocate viewing locomotor development as a fundamental life-history parameter, responding to the same evolutionary pressures shown to be fundamental to other aspects of primate life history (e.g., predation, resource access, body size, encephalization).

In this chapter we discuss the vertebral column, ribs, and sternum from a developmental perspective. The axial skeleton of newborn hominoids (apes and humans) is discussed based on the literature, followed by accounts of osteology in a newly described sample of newborn tarsiers, Old World monkeys, New World monkeys, and strepsirrhines (lemurs and lorises). The neonatal vertebral column is fragmented in skeletonized specimens, because in most vertebrae, actively growing synchondroses connect the right and left neural arches and connect the centrum to the arches. Transverse processes, portions of the articular facets, and the ventral arch of C1 are also cartilaginous in most primates. However, tarsiers and most monkeys have an ossified C1 ventral arch. The chapter ends with a brief discussion of the early postnatal trajectory of axial skeleton ossification in selected primate species based on a comparison of species at different stages (neonatal and older infant), including some at similar known ages.

Birth represents a transitional point in time – a static moment that borders a time of complete dependency and the march toward independence. This chapter reviews some basic concepts of vertebrate development and histology. Of particular relevance are tissue changes that reflect differentiation of mineralized tissues: skeletogenesis and odontogenesis, beginning with changes to the primitive connective tissue (mesenchyme). Viewed by microscopy, mesenchyme condenses prior to differentiation into cartilage, bone or tooth germs. Most mesenchymal condensations for the skeleton form during the embryonic period, but they continue to appear during the fetal period or postnatally for some structures (e.g., successional teeth). Most growth of the skeleton occurs during the fetal and postnatal periods. Mineralized connective tissues have different available growth processes. The contributions of appositional growth (new matrix is added to existing matrix), interstitial growth (cell replication and matrix production within existing tissue), and bone modeling (selective osteoclastic and osteoblastic activity) are discussed according to types of bones and skeletal regions.