Primatology at the 2023 annual meeting of the Society for Integrative & Comparative Biology

Abstract

Alexandra Kralich (U. Pennsylvania) investigated whether body size in orangutans is binary or a spectrum by comparing the body size of unflanged males to flanged males and females. Orangutans are considered highly sexually dimorphic; adult males are twice as large as females and exhibit bimaturism (or a plastic polymorphism) in which some but not all adult males develop cheek pads known as face flanges, along with a laryngeal throat pouch used for mate calling. Historically, the unflanged condition in males was considered a temporary phase of subadult development but study of wild individuals revealed that males can remain unflanged for life. The proximal hormonal mechanism for flanging is unknown, and we have only recently started to learn about body size and behavioral differences between adolescent, unflanged, and flanged males. Using stature and mass to assess for size, Kralich examined 96 skeletons from wild individuals at 12 museums (looking at associated skins when available) and used the skin, dentition, and long bone fusion to determine age. Kralich found that the size of unflanged males falls between flanged males and females (with younger ages being closer to the female range) and that femur lengths of unflanged males overlapped with or exceeded those of flanged males. Kralick proposes that the larger femur sizes in unflanged males is due to a longer developmental growth period. These data show that while flanged males are generally larger, biological sex is a range or a spectrum, even in a species considered highly sexually dimorphic. Because some males go through both states, we may need to rethink sexual selection, which suggests that unflanged males are less desirable to females. Rather, the unflanged males are successful with previously unmated females and in times of rank instability, and one study found that more than half of offspring at the field site were fathered by unflanged males. Thus, there may be a disconnect between social status and mating success, and developing flanges may even have the disadvantage of the flanges eventually becoming shriveled, which marks the individual as a “past‐prime” male. Kralick argues that “sexual dimorphism” is not an appropriate term to describe the variation observed in orangutans and so new terminology is needed. Further, we should avoid projecting gendered thinking onto nonhuman primates because our expectations may limit our understanding of natural variation. Brett Frye (Emory and Henry College) studies the effects of environmental factors on health outcomes and investigated the impacts of litter size on the health of captive and free‐range callitrichine (i.e., marmosets, tamarins) primates. Gestation and infancy are critical periods of development and environmental influences can shape physiology and behavior over the entire life course. Most primates have only one offspring at a time, but callitrichines routinely have litter sizes up to six, with twins being the most common. Siblings are a source of stress, so Frye examined how litter characteristics (i.e., size and sex composition) influence outcomes such as survival and neonatal development as well as sex differences. Using longitudinal data from an impressive 27,000 birth records, Frye found no overrepresentation in any specific litter size or sex composition, but in all seven of the callitrichine species, singletons have a much higher survival rate. Using common marmosets (n = 30), Frye measured size and cognitive development and found that same sex litters grew to be larger and that there were differences in behavior between same‐sex and mixed litters: females performed better on cognitive tests and males performed better if they had (a) female sibling(s). Katie Hinde (Arizona State U.) studies development and maternal investment and investigated the impact of glucocorticoid (GC) levels in milk on behavior and cognition in rhesus macaques. In an individual's life, energetic resources go towards bodily maintenance, growth and development, storage, and reproduction, all of which except the latter are supported by mother's milk. The first year of life is a critical period for neuronal organization and neurobiological development. During this period, the level of exposure to hormonal components of milk can influence behavioral development (i.e., exploring, playing). One such component is GC, which is involved in endocrine signaling (e.g., for acute and chronic stressors). Hinde looked at milk GC levels at both early and late lactation time points in macaques and found that offspring from mothers with naturally high cortisol levels have higher cognitive performance while offspring from mothers with lower cortisol levels are more impulsive. Maternal GC levels were the same between sexes, but more GC resulted in daughters with more social behavior, suggesting that there may be an adaptive sex‐based difference in ability to receive GC signal (e.g., intestinal GC receptors). Studying the neuroenergetics of GC in milk, Hinde found that male and female offspring have different periods of