Lateral Enamel Formation and Life History in New World Monkeys

The field of anthropology has seen a continuous interest in the human life history evolution. Since the modern human life history pattern is unique when compared to the rest of the primates, the question of when and why the modern human life history arose has become a frequent topic of research. The relationship between dental development and life history profiles within extant and fossil primates has not been systematically tested, especially in the anterior dentition. In order to explore this relationship, this study tests the hypotheses that there is a relationship between life history and estimated lateral enamel formation time as well as percent of perikymata in the cervical half of the crown. This study examined these relationships in incisor replicas of twelve extant species of platyrrhines. The measures of life history and related variables include: encephalization quotient, age at weaning, age at reproductive maturation, and average lifespan in addition to brain size and body size. Platyrrhines were chosen for their wide life history variation and their use in previous studies examining dental development and life history. Results show that there are relationships of differing strengths between all variables for each incisor type. Estimated lateral enamel formation time is strongly related to brain size, body size, age at weaning and age at reproductive maturation. The percent of perikymata in the cervical region of the crown is significantly related to brain size, age at reproductive maturation, and weaning age, although these relationships are much weaker and are not found in all tooth types. These results suggest that the relationship between dental development, specifically lateral enamel crown formation time, and life history is robust and should be explored further. Chapter 1: Introduction The life history of hominins and non-human primates has become a topic of increasing interest in the field of paleoanthropology (e.g., Beynon and Dean, 1986; Dean et al., 2001; Guatelli-Steinberg et al., 2007; Smith, 2008). Modern human growth and development has been a focal point of anthropological research since its inception as a discipline (Boas, 1932). Patterns, variation, and causes of human growth continue to be of interest to researchers (Bromage et al., 2009; Crews and Bogin, 2010; Guatelli-Steinberg et al., 2012). Life history theory defines stages of growth in an evolutionary framework. Life history profiles involve trade-offs between energy allocated to various stages of growth and reproductive output, and can therefore be seen as evolutionary strategies. The modern human life history strategy is a result of millions of years of evolutionary forces (Bogin, 1999). As a result of our evolutionary history, modern humans have a unique pattern of growth unlike that of other living primates (Bogin, 1999). Specifically, modern humans have prolonged periods of growth and development before reaching adulthood (Bogin,1999). Our unique life history pattern has raised three main research questions for anthropologists. First, when did the modern human pattern evolve? Secondly, why did this pattern evolve? And finally, what evidence can be used to answer the former questions? (Dean, 2006). Aspects of dental development, such as first molar eruption (Smith, 1989), molar crown formation time (Macho, 2001), and occlusal enamel formation rates (Dean, 2006) have been shown to be associated with brain size, body size, and pace of primate life histories across the order. These aspects of dental development can be instrumental in answering questions about the pace of life history, specifically, when the prolonged juvenile period arose in the hominin lineage. There are additional aspects of dental development that have not been fully researched that may also further knowledge about the evolution of life history. Aspects of enamel microstructures such as perikymata (incremental growth lines on the enamel surface) may also be related to the pace of life history (e.g. Dean et al., 2001). Much previous research on dental development and life history focuses on molars (e.g. Smith, 1989), and therefore these relationships need to be tested in the anterior dentition. Studies involving fossil hominins often utilize anterior dentition (Dean et al., 2001; Ramirez-Rozzi and Bermudez de Castro, 2004, Guatelli-Steinberg et al., 2005), but a robust relationship between dental development and life history in these teeth has yet to be established (Guatelli-Steinberg, 2009). In fossil studies where teeth cannot be sectioned, anterior teeth (incisors and canines) provide a benefit over posterior teeth (molars and premolars) in that a larger portion of their enamel formation time is represented by perikymata on the enamel surface. The present investigation focuses on the potential relationships between estimated lateral enamel formation time in anterior teeth and several life history variables as well as associated aspects of primate biology (e.g., brain size). This investigation also examines potential relationships between perikymata distribution on anterior teeth and these same life history and associated variables. Tests of these relationships are performed within platyrrhines, chosen for their wide variation in life history patterns. Platyrrhines were also chosen for this investigation because few studies have focused on the connections between dental development and life history in this infraorder (Henderson, 2007; Hogg and Walker, 2011). Section 1: Life History The life history pattern of a particular species involves, “a series of growth and maturational phases ultimately related to the scheduling of reproduction and lifetime reproductive output” (Kelley and Smith, 2003). Phases of growth differ in timing and length between species. In other terms, life history is a “...strategy an organism uses to allocate its energy toward growth, maintenance, reproduction, raising offspring to independence, and avoiding death” (Bogin and Smith, 2012). Similarly, Stearns, (1992) defines life history as, “...demographic traits-birth, age and size at maturity, number and size of offspring, growth and reproductive investment, length of life, and death-connected by constraining relationships tradeoffs...” (Stearns, 1992). Across the primate order life history variables tend to be correlated, and these variables are connected to brain size, body size, and dental development, specifically eruption timing (Smith, 1989). Research on dental development, brain size, body size, and life history suggests in a general relationship; primates with larger body size tend to have slower dental development (Harvey and Clutton-Brock, 1985; Schwartz et al., 2005). There are some exceptions to these relationships, specifically in lemurs (Schwartz et al., 2005). Brain and body mass have been discovered to be correlated with other life history variables such as age at weaning (Smith,