What we don't know about great ape variation

Morphological variation of the Pan talus relative to that of Gorilla

OBJECTIVES

Differences in talar articular morphology relative to locomotion have recently been found within Pan and Gorilla. Whole-bone talar morphology within, and shared variation among, Pan and Gorilla (sub)species, however, has yet to be investigated. Here we separately analyze talar external shape within Pan (P. t. troglodytes, P. t. schweinfurthii, P. t. verus, P. paniscus) and Gorilla (G. g. gorilla, G. b. beringei, G. b. graueri) relative to degree of arboreality and body size. Pan and Gorilla are additionally analyzed together to determine if consistent shape differences exist within the genera.

MATERIALS AND METHODS

Talar external shape was quantified using a weighted spherical harmonic analysis. Shape variation both within and among Pan and Gorilla was described using principal component analyses. Root mean square distances were calculated between taxon averages, and resampling statistics conducted to test for pairwise differences.

RESULTS

P. t. verus (most arboreal Pan) talar shape significantly differs from other Pan taxa (p < 0.05 for pairwise comparisons) driven by more asymmetrical trochlear rims and a medially-set talar head. P. t. troglodytes, P. t. schweinfurthii, and P. paniscus do not significantly differ (p > 0.05 for pairwise comparisons). All gorilla taxa exhibit significantly different talar morphologies (p < 0.007 for pairwise comparisons). The more terrestrial subspecies of G. beringei and P. troglodytes exhibit a superoinferiorly taller talar head/neck complex.

DISCUSSION

P. t. verus exhibits talar morphologies that have been previously related to more frequent arboreality. The adaptations in the more terrestrial G. beringei and P. troglodytes subspecies may serve to facilitate load transmission.

Endocrinological correlates of male bimaturism in wild Bornean orangutans

Among primates, orangutans are unique in having pronounced male bimaturism leading to two fully adult morphs that differ in both physical appearance and behavior. While unflanged males have a female-like appearance, flanged males have the full suite of secondary sexual characteristics, including cheek flanges and a large throat sac. So far, hormonal correlates of arrested development in unflanged males and the expression of secondary sexual characteristics in flanged males have only been studied in zoo-housed individuals. In this study, we investigated fecal androgen and glucocorticoid metabolites as hormonal correlates of male bimaturism in 17 wild adult Bornean orangutans (Pongo pygmaeus) in Central Kalimantan, Indonesia. We predicted and found higher androgen levels in flanged males compared to unflanged males, probably due to ongoing strong competition among flanged males who meet too infrequently to establish a clear linear dominance hierarchy. Furthermore, we found no difference in fecal glucocorticoid metabolite concentrations between flanged and unflanged males, indicating that social stress is unlikely to explain arrested development in unflanged wild orangutans. The only actively developing male in our study showed significantly higher androgen levels during the period of development than later as a fully flanged male. This supports earlier findings from zoo studies that elevated androgen levels are associated with the development of secondary sexual characteristics.

Distinct and diverse: range-wide phylogeography reveals ancient lineages and high genetic variation in the endangered okapi (Okapia johnstoni)

The okapi is an endangered, evolutionarily distinctive even-toed ungulate classified within the giraffidae family that is endemic to the Democratic Republic of Congo. The okapi is currently under major anthropogenic threat, yet to date nothing is known about its genetic structure and evolutionary history, information important for conservation management given the species' current plight. The distribution of the okapi, being confined to the Congo Basin and yet spanning the Congo River, also makes it an important species for testing general biogeographic hypotheses for Congo Basin fauna, a currently understudied area of research. Here we describe the evolutionary history and genetic structure of okapi, in the context of other African ungulates including the giraffe, and use this information to shed light on the biogeographic history of Congo Basin fauna in general. Using nuclear and mitochondrial DNA sequence analysis of mainly non-invasively collected samples, we show that the okapi is both highly genetically distinct and highly genetically diverse, an unusual combination of genetic traits for an endangered species, and feature a complex evolutionary history. Genetic data are consistent with repeated climatic cycles leading to multiple Plio-Pleistocene refugia in isolated forests in the Congo catchment but also imply historic gene flow across the Congo River.

Variation in developmental arrest among male orangutans: a comparison between a Sumatran and a Bornean population

INTRODUCTION

The presence of two sexually active male morphs with different reproductive tactics in a single species is rare among mammals. The most striking case of bimaturism among primates is exhibited by the orangutan (Pongo spp), in which one adult morph, the unflanged male, irreversibly develops into another one, the flanged form, but may remain arrested in the unflanged state for many years. However, it has been suggested that such arrest is less common among Bornean orangutans (Pongo pygmaeus) compared to Sumatrans (Pongo abelii). To investigate this possible inter-specific difference we compared both the number of developing males and the ratios of the two male morphs at two long-term study sites, Suaq Balimbing on Sumatra and Tuanan on Borneo.

RESULTS

First, we observed a significantly greater number of flanged than unflanged males per month in the Tuanan study area, whereas the opposite pattern held at Suaq. Second, the same contrast held for the total number of identified individuals over the study, with more flanged than unflanged males at Tuanan and the opposite at Suaq. These differences were mainly due to transient males. For Tuanan, the identification results were confirmed by detailed genetic analyses. Finally, we recorded a higher proportion of unflanged males that became flanged during any given year at Tuanan than at Suaq.

CONCLUSION

These results show that developmental arrest is far more common at Suaq than at Tuanan. Preliminary comparisons suggest that this is a general contrast between the island taxa of orangutans and should help efforts to identify the function and proximate control of developmental arrest in orangutan males.

Bonobos fall within the genomic variation of chimpanzees

To gain insight into the patterns of genetic variation and evolutionary relationships within and between bonobos and chimpanzees, we sequenced 150,000 base pairs of nuclear DNA divided among 15 autosomal regions as well as the complete mitochondrial genomes from 20 bonobos and 58 chimpanzees. Except for western chimpanzees, we found poor genetic separation of chimpanzees based on sample locality. In contrast, bonobos consistently cluster together but fall as a group within the variation of chimpanzees for many of the regions. Thus, while chimpanzees retain genomic variation that predates bonobo-chimpanzee speciation, extensive lineage sorting has occurred within bonobos such that much of their genome traces its ancestry back to a single common ancestor that postdates their origin as a group separate from chimpanzees.

Demographic history and genetic differentiation in apes

Comparisons of genetic variation between humans and great apes are hampered by the fact that we still know little about the demographics and evolutionary history of the latter species. In addition, characterizing ape genetic variation is important because they are threatened with extinction, and knowledge about genetic differentiation among groups may guide conservation efforts. We sequenced multiple intergenic autosomal regions totaling 22,400 base pairs (bp) in ten individuals each from western, central, and eastern chimpanzee groups and in nine bonobos, and 16,000 bp in ten Bornean and six Sumatran orangutans. These regions are analyzed together with homologous information from three human populations and gorillas. We find that whereas orangutans have the highest diversity, western chimpanzees have the lowest, and that the demographic histories of most groups differ drastically. Special attention should therefore be paid to sampling strategies and the statistics chosen when comparing levels of variation within and among groups. Finally, we find that the extent of genetic differentiation among "subspecies" of chimpanzees and orangutans is comparable to that seen among human populations, calling the validity of the "subspecies" concept in apes into question.

Developmental changes in the facial morphology of the Borneo orangutan (Pongo pygmaeus): possible signals in visual communication

Orangutans display remarkable developmental changes and sexual differences in facial morphology, such as the flanges or cheek-pads that develop only on the face of dominant adult males. These changes suggest that facial morphology is an important factor in visual communication. However, developmental changes in facial morphology have not been examined in detail. We studied developmental changes in the facial morphology of the Borneo orangutan (Pongo pygmaeus) by observing 79 individuals of various ages living in the Sepilok Orangutan Rehabilitation Centre (SORC) in Malaysia and in Japanese zoos. We also analyzed photographs of one captive male that were taken over a period of more than 16 years. There were clear morphological changes that occurred with growth, and we identified previously unreported sexual and developmental differences in facial morphology. Light-colored skin around the eyes and mouth is most prominent in animals younger than 3 years, and rapidly decreases in area through the age of approximately 7 years. At the same time, the scattered, erect hairs on the head (infant hair) become thick, dense hairs lying on the head (adult hair) in both sexes. The results suggest that these features are infant signals, and that adult signals may include darkened face color, adult hair, whiskers, and a beard, which begin to develop after the age of approximately 7 years in both sexes. In females, the eyelids remain white even after 10 years, and turn black at around the age of 20; in males, the eyelids turn black before the age of 10. The whiskers and beards of adults are thicker in males than in females, and are fully developed before the age of 10 in males, while they begin to develop in females only after approximately 20 years. White eyelids and undeveloped whiskers and beards may be visual signals that are indicative of young adult females. Our results also show that the facial morphology of the unflanged male is similar to that of the adult female, although it has also been pointed out that unflanged males resemble younger individuals.

Western lowland gorillas (Gorilla gorilla gorilla) as seasonal frugivores: use of variable resources

The gorillas studied at Bai Hokou, Central African Republic, between August 1990 and October 1992 consumed 239 kinds of foods from 138 species of plants and invertebrates, including the fruits of 77 species. Seeds were present in 99% of all fecal samples (n = 859). Although gorillas ate fleshy fruit whenever it was available, herbaceous plants and fibrous fruits were consumed year-round and were important during times of fleshy fruit scarcity. At Bai Hokou and across their range, resources are temporally discontinuous, and western gorilla diet exhibits marked seasonal and interannual variation. Although their large body size lends them dietary flexibility relative to chimpanzees, seasonal fruit-eating shapes the foraging and ranging patterns of western lowland gorillas.

Initial studies on the contributions of body size and gastrointestinal passage rates to dietary flexibility among gorillas

Large body size has been traditionally seen as the primary dietary adaptation of gorillas, facilitating their consumption of fibrous foods (Schaller ¿1963 The Mountain Gorilla; Watts ¿1990 Int. J. Primatol. 11:21-45). Nevertheless, recent research has emphasized frugivory among western lowland gorillas, as well as the influence of habitat and seasonality on gorilla diet and behavior across subspecies (Watts ¿1990 Int. J. Primatol. 11:21-45; Tutin et al. ¿1991 Philos. R. Soc. Trans. Lond. Biol. 334:179-186; Remis ¿1994 Ph.D. Thesis, ¿1997a Am. J. Primatol. 43:87-109, ¿1997b Am. J. Primatol. 43:111-133, ¿1998 Primate Locomotion: Recent Advances, p 95-(1)108, ¿1999 Primates 40:383-396; Nishihara ¿1995 Primates 36:151-168; Goldsmith ¿1999a Int. J. Primatol. 20:1-23, Goldsmith [1999b] Nonhuman Primates, p 58-63). This study provides preliminary data to address the physiological underpinnings of dietary flexibility among gorillas, and their consumption of a broad range of fibrous and tannin-rich foods. To date, little is known about the digestive physiology of the African apes (but see Milton ¿1984 Adaptations for Foraging in Nonhuman Primates, p 249-279, Milton [1984] ¿1999Evol. Anthropol. 8:11-20; Milton and Demment ¿1988 J. Nutr. 118:1082-1088; Lambert ¿1997 Ph.D. Dissertation), although gastrointestinal morphology and proportions are roughly similar among species ( Chivers and Hladik ¿1980 J. Morphol. 166:337-386). This study provides additional experimental data on the gastrointestinal passage times of gorillas (Gorilla gorilla gorilla) fed a captive diet in a zoological park setting and discusses results in relation to field research on gorilla feeding ecology. In this study, 480 small plastic markers were fed to six captive gorillas. The mean gut retention time (MRT) of the adult gorillas in this study was 50 hr, longer than the 31 hr reported for chimpanzees fed a similar diet (Lambert ¿1997 Ph.D. Dissertation). These data suggest that gorillas may retain foods in their gastrointestinal tracts longer than smaller hominoids, and that the large body size likely forms the primary basis for consumption of fiber. This research provides additional data to contribute to our understanding of the relationships of body size and morphology to ecology, and the evolution of body size, foraging strategy and social organization among the African apes.

Genetic Diversity in Hominoid Primates

Humans are only one of the species produced by the hominoid evolutionary radiation. Common and pygmy chimpanzees (our closest relatives), gorillas, orangutans, and the lesser apes also belong to this group. In humans, patterns of genetic variation are becoming increasingly better characterized by modern molecular methods. Understanding human variation in an evolutionary context, however, requires comparison of human patterns with those of other hominoids, to reveal features shared among hominoids and those unique to humans. Genetic variation among chimpanzees, gorillas, and orangutans is beginning to be characterized, so that comparisons are now possible.

From genetic data, several different kinds of information can be reconstructed, including the evolutionary relatedness of subspecies and populations, time estimates for evolutionary divergences, past population dynamics, extent of gene flow over geographical landscapes, and group social structure. Knowledge of hominoid genetic variation is also relevant to applied fields such as primate conservation and medicine.

Genomic differentiation among natural populations of orang-utan (Pongo pygmaeus)

BACKGROUND

Orang-utans exist today in small isolated populations on the islands of Borneo (subspecies Pongo pygmaeus pygmaeus) and Sumatra (subspecies P. p. abelii). Although, on the basis of their morphological, behavioral and cytogenetical characteristics, the Bornean and Sumatran orang-utan populations are generally considered as two separate subspecies, there is no universal agreement as to whether their genetic differentiation is sufficient to consider and manage them as species, subspecies or population level taxonomic units. A more precise phylogenetic description would affect many conservation management decisions about captive and free-ranging orang-utans.

RESULTS

We analyzed the amount and patterns of molecular genetic variation in orang-utan populations using cellular DNA from orang-utans from two locations in Sumatra and nine locations-representing four isolated populations-in Borneo. Genetic and phylogenetic analyses of mitochondrial DNA restriction fragment length polymorphisms, nuclear minisatellite (or variable number tandem repeat) loci and mitochondrial 16S ribosomal RNA sequences led to three major findings. First, the genetic distance and phylogenetic differentiation between Sumatran and Bornean orang-utans is large, greater than that between the common chimpanzee, Pan troglodytes, and the pygmy chimpanzee or bonobo, Pan paniscus. The genetic distance suggests that the two island subspecies diverged approximately 1.5-1.7 million years ago, well before the two islands separated and long enough for species-level differentiation. Second, there is considerable endemic genetic diversity within the Bornean and Sumatran orang-utan populations, suggesting that they have not experienced recent bottlenecks or founder effects. And third, there is little genetic differentiation among four geographically isolated populations of Bornean orang-utans, consistent with gene flow having occurred between them until recently.

CONCLUSIONS

Our results are consistent with the view that the genetic differentiation between Sumatran and Bornean orang-utans has reached the level of distinct species. Furthermore, our findings indicate that there is not a genetic imperative for the separate management of geographically isolated Bornean populations.