Genomic variation in baboons from central Mozambique unveils complex evolutionary relationships with other Papio species.

BACKGROUND: Gorongosa National Park in Mozambique hosts a large population of baboons, numbering over 200 troops. Gorongosa baboons have been tentatively identified as part of Papio ursinus on the basis of previous limited morphological analysis and a handful of mitochondrial DNA sequences. However, a recent morphological and morphometric analysis of Gorongosa baboons pinpointed the occurrence of several traits intermediate between P. ursinus and P. cynocephalus, leaving open the possibility of past and/or ongoing gene flow in the baboon population of Gorongosa National Park. In order to investigate the evolutionary history of baboons in Gorongosa, we generated high and low coverage whole genome sequence data of Gorongosa baboons and compared it to available Papio genomes. RESULTS: We confirmed that P. ursinus is the species closest to Gorongosa baboons. However, the Gorongosa baboon genomes share more derived alleles with P. cynocephalus than P. ursinus does, but no recent gene flow between P. ursinus and P. cynocephalus was detected when available Papio genomes were analyzed. Our results, based on the analysis of autosomal, mitochondrial and Y chromosome data, suggest complex, possibly male-biased, gene flow between Gorongosa baboons and P. cynocephalus, hinting to direct or indirect contributions from baboons belonging to the "northern" Papio clade, and signal the presence of population structure within P. ursinus. CONCLUSIONS: The analysis of genome data generated from baboon samples collected in central Mozambique highlighted a complex set of evolutionary relationships with other baboons. Our results provided new insights in the population dynamics that have shaped baboon diversity.

A missing piece of the Papio puzzle: Gorongosa baboon phenostructure and intrageneric relationships.

Most authors recognize six baboon species: hamadryas (Papio hamadryas), Guinea (Papio papio), olive (Papio anubis), yellow (Papio cynocephalus), chacma (Papio ursinus), and Kinda (Papio kindae). However, there is still debate regarding the taxonomic status, phylogenetic relationships, and the amount of gene flow occurring between species. Here, we present ongoing research on baboon morphological diversity in Gorongosa National Park (GNP), located in central Mozambique, south of the Zambezi River, at the southern end of the East African Rift System. The park exhibits outstanding ecological diversity and hosts more than 200 baboon troops. Gorongosa National Park baboons have previously been classified as chacma baboons (P. ursinus). In accordance with this, two mtDNA samples from the park have been placed in the same mtDNA clade as the northern chacma baboons. However, GNP baboons exhibit morphological features common in yellow baboons (e.g., yellow fur color), suggesting that parapatric gene flow between chacma and yellow baboons might have occurred in the past or could be ongoing. We investigated the phenostructure of the Gorongosa baboons using two approaches: 1) description of external phenotypic features, such as coloration and body size, and 2) 3D geometric morphometric analysis of 43 craniofacial landmarks on 11 specimens from Gorongosa compared to a pan-African sample of 352 baboons. The results show that Gorongosa baboons exhibit a mosaic of features shared with southern P. cynocephalus and P. ursinus griseipes. The GNP baboon phenotype fits within a geographic clinal pattern of replacing allotaxa. We put forward the hypothesis of either past and/or ongoing hybridization between the gray-footed chacma and southern yellow baboons in Gorongosa or an isolation-by-distance scenario in which the GNP baboons are geographically and morphologically intermediate. These two scenarios are not mutually exclusive. We highlight the potential of baboons as a useful model to understand speciation and hybridization in early human evolution.

Ontogenetic scaling of fore- and hind limb posture in wild chacma baboons (Papio hamadryas ursinus).

Large-scale interspecific studies of mammals ranging between 0.04-280 kg have shown that larger animals walk with more extended limb joints. Within a taxon or clade, however, the relationship between body size and joint posture is less straightforward. Factors that may affect the lack of congruence between broad and narrow phylogenetic analyses of limb kinematics include limited sampling of (1) ranges of body size, and/or (2) numbers of individuals. Unfortunately, both issues are inherent in laboratory-based or zoo locomotion research. In this study, we examined the relationship between body mass and elbow and knee joint angles (our proxies of fore- and hind limb posture, respectively) in a cross-sectional ontogenetic sample of wild chacma baboons (Papio hamadryas ursinus) habituated in the De Hoop Nature Reserve, South Africa. Videos were obtained from 33 individuals of known age (12 to ≥ 108 months) and body mass (2-29.5 kg) during walking trials. Results show that older, heavier baboons walk with significantly more extended knee joints but not elbow joints. This pattern is consistent when examining only males, but not within the female sample. Heavier, older baboons also display significantly less variation in their hind limb posture compared to lighter, young animals. Thus, within this ontogenetic sample of a single primate species spanning an order of magnitude in body mass, hind limb posture exhibited a postural scaling phenomenon while the forelimbs did not. These findings may further help explain 1) why younger mammals (including baboons) tend to have relatively stronger bones than adults, and 2) why humeri appear relatively weaker than femora (in at least baboons). Finally, this study demonstrates how field-acquired kinematics can help answer fundamental biomechanical questions usually addressed only in animal gait laboratories.

Kinda baboons (Papio kindae) and grayfoot chacma baboons (P. ursinus griseipes) hybridize in the Kafue river valley, Zambia.

The ranges of small kinda (Papio kindae) and much larger grayfooted chacma (P. ursinus griseipes) baboons adjoin in the Kafue National Park, Zambia. In a visual survey of baboons at 48 sites in the Kafue River drainage we found that, contrary to previous reports, groups at the species interface near the town of Ngoma are phenotypically diverse and presumably formed by multigenerational hybridization. Mitochondrial and/or Y-chromosome genetic markers from fecal samples (N=164) collected at 29 sites support this conclusion. Groups with phenotypic signs of a history of hybridization also had taxon-specific mitochondria and Y-haplotypes from both parental species. Although the distribution of mitochondrial haplotypes largely mirrored that of external phenotypes, a significant proportion of male specimens from grayfoot as well as hybrid groups carried kinda Y-chromosomes, and kinda Y-chromosomes were involved in all observed cases of mitochondrial/Y-chromosome discordance. These observations are consistent with, though they do not prove, a population history in which the range of chacmas and the hybrid zone have advanced at the expense of the kinda range. They also suggest that, unexpectedly, kinda male×chacma female matings are much more common than the reciprocal cross in the ancestry of hybrids. We suggest that distinctive male kinda behavior and the "juvenile" appearance of kinda baboons of both sexes, perhaps combined with obstetric difficulties of a small kinda female carrying the large offspring of a chacma male, may account for this bias.

Can fertility signals lead to quality signals? Insights from the evolution of primate sexual swellings.

The sexual swellings of female primates have generated a great deal of interest in evolutionary biology. Two hypotheses recently proposed to elucidate their functional significance argue that maximal swelling size advertises either female fertility within a cycle or female quality across cycles. Published evidence favours the first hypothesis, and further indicates that larger swellings advertise higher fertility between cycles. If so, a male preference for large swellings might evolve, driving females to use swellings as quality indicators, as proposed by the second hypothesis. In this paper, we explore this possibility using a combination of empirical field data and mathematical modelling. We first test and find support for three key predictions of the female-quality hypothesis in wild chacma baboons (Papio ursinus): (i) inter-individual differences in swelling size are maintained across consecutive cycles, (ii) females in better condition have larger swellings and higher reproductive success, and (iii) males preferentially choose females with large swellings. We then develop an individual-based simulation model that indicates that females producing larger swellings can achieve higher mating success even when female-female competition is low and within-female variance in the trait is high. Taken together, our findings show that once sexual swellings have evolved as fertility signals, they might, in certain socio-sexual systems, be further selected to act as quality signals. These results, by reconciling two hypotheses, help to clarify the processes underlying sexual swelling evolution. More generally, our findings suggest that mate choice for direct benefits (fertility) can lead to indirect benefits (good genes).

Specializations of the granular prefrontal cortex of primates: implications for cognitive processing.

The biological underpinnings of human intelligence remain enigmatic. There remains the greatest confusion and controversy regarding mechanisms that enable humans to conceptualize, plan, and prioritize, and why they are set apart from other animals in their cognitive abilities. Here we demonstrate that the basic neuronal building block of the cerebral cortex, the pyramidal cell, is characterized by marked differences in structure among primate species. Moreover, comparison of the complexity of neuron structure with the size of the cortical area/region in which the cells are located revealed that trends in the granular prefrontal cortex (gPFC) were dramatically different to those in visual cortex. More specifically, pyramidal cells in the gPFC of humans had a disproportionately high number of spines. As neuron structure determines both its biophysical properties and connectivity, differences in the complexity in dendritic structure observed here endow neurons with different computational abilities. Furthermore, cortical circuits composed of neurons with distinguishable morphologies will likely be characterized by different functional capabilities. We propose that 1. circuitry in V1, V2, and gPFC within any given species differs in its functional capabilities and 2. there are dramatic differences in the functional capabilities of gPFC circuitry in different species, which are central to the different cognitive styles of primates. In particular, the highly branched, spinous neurons in the human gPFC may be a key component of human intelligence.

Specialization in pyramidal cell structure in the sensory-motor cortex of the Chacma baboon (Papio ursinus) with comparative notes on macaque and vervet monkeys.

The systematic study of pyramidal cell structure has revealed new insights into specialization of the phenotype in the primate cerebral cortex. Regional specialization in the neuronal phenotype may influence patterns of connectivity and the computational abilities of the circuits they compose. The comparative study of pyramidal cells in homologous cortical areas is beginning to yield data on the evolution and development of such specialized circuitry in the primate cerebral cortex. Recently, we have focused our efforts on sensory-motor cortex. Based on our intracellular injection methodology, we have demonstrated a progressive increase in the size of, the branching structure in, and the spine density of the basal dendritic trees of pyramidal cells through somatosensory areas 3b, 1, 2, 5, and 7 in the macaque and vervet monkeys. In addition, we have shown that pyramidal cells in premotor area 6 are larger, more branched, and more spinous than those in the primary motor cortex (MI or area 4) in the macaque monkey, vervet monkey, and baboon. Here we expand the basis for comparison by studying the basal dendritic trees of layer III pyramidal cells in these same sensory-motor areas in the chacma baboon. The baboon was selected because it has a larger cerebral cortex than either the macaque or vervet monkeys; motor cortex has expanded disproportionately in these three species; and motor cortex in the baboon reportedly has differentiated to include a new cortical area not present in either the macaque or vervet monkeys. We found, as in monkeys, a progressive increase in the morphological complexity of pyramidal cells through areas 3b, 5, and 7, as well as from area 4 to area 6, suggesting that areal specialization in microcircuitry was likely to be present in a common ancestor of primates. In addition, we found subtle differences in the extent of the interareal differences in pyramidal cell structure between homologous cortical areas in the three species.

Molar enamel thickness in the chacma baboon, Papio ursinus (Kerr 1792).

Modern humans exhibit increasing relative enamel thickness from M1 to M3. Some biomechanical (basic lever) models predict that the more distal molars in humans encounter higher occlusal forces, and it has been postulated that this provides a functional explanation for the observed gradient in relative enamel thickness. However, constrained three-dimensional models and experimental observations suggest that there is a reduction in bite force potential from M1 to M3, which would be consistent with the tendency for humans to reduce the size of the distal molars. In this regard, it has been postulated that the distal increase in enamel thickness is a consequence of crown size reduction; thus, it is unnecessary to invoke functional scenarios to explain this phenomenon. We assess these competing proposals by examining relative enamel thickness in a catarrhine primate (Papio ursinus) that exhibits crown size increase from M1 to M3. The molar row of P. ursinus is positioned relatively far forward of the temporomandibular joint, which results in the baboon being able to exert relatively greater muscle forces during posterior biting in comparison to modern humans. Thus, a significant distalward gradient of increasing enamel thickness would be expected in P. ursinus according to the hypothesis that posits it to be functionally related to bite force. The present study reveals no significant difference in relative enamel thickness along the molar row in P. ursinus. This finding lends support to the notion that the relatively thicker enamel of human distal molars is related primarily to their reduction in size. This carries potential implications for the interpretation of enamel thickness in phylogenetic reconstructions: the relatively thick molar enamel shared by modern humans and some of our fossil relatives may not be strictly homologous, in that it may result from different underlying developmental mechanisms.

Specialization in pyramidal cell structure in the cingulate cortex of the Chacma baboon (Papio ursinus): an intracellular injection study of the posterior and anterior cingulate gyrus with comparative notes on the macaque and vervet monkeys.

This study forms part of an ongoing investigation of pyramidal cell structure in the cingulate cortex of primates. Recently we have demonstrated that layer III pyramidal cells in the anterior cingulate gyrus are considerably larger, more branched and more spinous than those in the posterior cingulate gyrus (areas 24 and 23, respectively) in the macaque and vervet monkeys. Moreover, the extent of the interareal difference in specialization in pyramidal cell structure differed between the two species. These data suggest that pyramidal cell circuitry may have evolved differently in these closely related species. Presently there are too few data to speculate on what is selecting for this specialization in structure. Here we extend the basis for comparison by studying pyramidal cell structure in cingulate gyrus of the Chacma baboon (Papio ursinus). Methodology used here is the same as that for our previous studies: intracellular injection of Lucifer Yellow in flat-mounted cortical slices. We found that pyramidal cells in anterior cingulate gyrus (area 24) were more branched and more spinous than those in posterior cingulate gyrus (area 23). Moreover, the complexity in pyramidal cell structure in both the anterior and posterior cingulate gyrus of the baboon differed to that in the corresponding regions in either the macaque or vervet monkeys.