The apolipoprotein E (APOE) gene appears functionally monomorphic in chimpanzees (Pan troglodytes).

BACKGROUND: The human apolipoprotein E (APOE) gene is polymorphic, with three primary alleles (E2, E3, E4) that differ at two key non-synonymous sites. These alleles are functionally different in how they bind to lipoproteins, and this genetic variation is associated with phenotypic variation for several medical traits, including cholesterol levels, cardiovascular health, Alzheimer's disease risk, and longevity. The relative frequencies of these alleles vary across human populations, and the evolution and maintenance of this diversity is much debated. Previous studies comparing human and chimpanzee APOE sequences found that the chimpanzee sequence is most similar to the human E4 allele, although the resulting chimpanzee protein might function like the protein coded for by the human E3 allele. However, these studies have used sequence data from a single chimpanzee and do not consider whether chimpanzees, like humans, show intra-specific and subspecific variation at this locus. METHODOLOGY AND PRINCIPAL FINDINGS: To examine potential intraspecific variation, we sequenced the APOE gene of 32 chimpanzees. This sample included 20 captive individuals representing the western subspecies (P. troglodytes verus) and 12 wild individuals representing the eastern subspecies (P. t. schweinfurthii). Variation in our resulting sequences was limited to one non-coding, intronic SNP, which showed fixed differences between the two subspecies. We also compared APOE sequences for all available ape genera and fossil hominins. The bonobo APOE protein is identical to that of the chimpanzee, and the Denisovan APOE exhibits all four human-specific, non-synonymous changes and appears functionally similar to the human E4 allele. CONCLUSIONS: We found no coding variation within and between chimpanzee populations, suggesting that the maintenance of functionally diverse APOE polymorphisms is a unique feature of human evolution.

Leptin associations with age, weight, and sex among chimpanzees (Pan troglodytes).

BACKGROUND: Leptin is a hormone secreted primarily by adipocytes, a lipostatic signal to the hypothalamus, and is often correlated with adiposity. Associations between leptin, age, and development are unknown in human's closest evolutionary relative, the common chimpanzee (Pan troglodytes). METHODS: Serum leptin was assessed cross sectionally in association with age, weight, and sex in healthy captive chimpanzee males (n = 47) and females (n = 49) to test hypotheses related to predicted differences in leptin levels with body mass, development, and sexual dimorphism. RESULTS: Leptin increased with age and weight among females, but not in males. Leptin was overall higher in females compared to males. CONCLUSIONS: Sex differences in leptin were most evident during adolescence and adulthood, despite similar increases in weight in both sexes indicating that sexual maturation is a key divergence point for differential somatic investment in adiposity and leptin levels between male and female chimpanzees.

Specific gravity as an alternative to creatinine for estimating urine concentration in captive and wild chimpanzee (Pan troglodytes) Samples.

The measurement of hormones in urine has become a widely used technique in primatology. Because urine concentration varies according to fluid intake, concentration must be measured in each sample collected, and hormone values are always expressed per unit of concentration. Traditionally, creatinine has been used as a concentration index, but some studies in humans have shown that creatinine varies among populations and even within and between individuals within a population, and that it begins to degrade after just one freeze-thaw cycle. In addition, creatinine measurement is relatively time-consuming and expensive and creates hazardous waste. In this study, we tested the hypothesis that specific gravity, or the ratio of the density of a sample to that of water, is highly correlated with creatinine measurement in urine samples collected from captive chimpanzees at the New Iberia Research Center in Louisiana and wild chimpanzees at the Ngogo study site in the Kibale National Park, Uganda. We found that specific gravity and creatinine were highly correlated in both captive (N=124) and wild (N=13) chimpanzee samples, and that specific gravity measurement was robust to actual and simulated transport conditions and repeated freeze-thaw cycles. We recommend that researchers consider specific gravity measurement as a preferable alternative to creatinine measurement in their studies of primate endocrinology.

Age, rank, and personality effects on the cortisol sedation stress response in young chimpanzees.

Primates and other mammals exhibit a glucocorticoid response to somatic and psychosocial stressors. The pattern and degree of response varies both within and between species, but the factors affecting within-species variability have rarely been considered. Here we describe the 90-min serum cortisol response of 14 juvenile and adolescent chimpanzees to the stress of sedation with ketamine hydrochloride. We show age differences in both baseline cortisol and time of peak cortisol, with younger individuals (ages 4-5 years) showing higher baseline levels and an earlier peak response than older individuals (ages 8-9 years). However, we found no sex or rank-related differences in any of the sedation measures: baseline cortisol, cortisol peak time, peak cortisol level, or cortisol change from baseline. We also examined the relationship between these sedation stress measures and behavioral style score, and found that individuals scoring high in the "mellow" behavioral style component showed a greater stress response than individuals scoring lower in this component. Future studies should consider the role of individual differences in age and personality in the cortisol response to stress.

Testosterone in juvenile and adolescent male chimpanzees (Pan troglodytes): effects of dominance rank, aggression, and behavioral style.

Testosterone is a steroid hormone with diverse effects on male reproductive function and behavior. The relationship between testosterone and social behavior such as mating and aggression has been investigated in a variety of primate species, but few such studies have been conducted on chimpanzees, and even fewer on primates during the juvenile and adolescent periods. This study explores the relationship between baseline urinary testosterone and behavioral variables including dominance rank, rates of aggression toward peers, and behavioral style in 16 juvenile and adolescent male chimpanzees (Pan troglodytes) living at the New Iberia Research Center in Louisiana. Behavioral observations and urine collection occurred during four research periods, each a year apart. After correcting for the positive association between testosterone and age, testosterone was positively associated with both dominance rank and rates of aggression directed at others. It was negatively associated with rates of aggression received. Individuals scoring highest in the "mellow" behavioral style component showed higher levels of testosterone than individuals scoring lowest in this component, an effect that may be partially due to the confounding effect of rank. The results of this study suggest that hormonal changes during the period preceding adulthood are not simply programmed physiological processes tied primarily to age-related change, but that important age-independent relationships with behavior also exist.