They have dog-like muzzles and males have large canine teeth. Depending on the species, body mass varies between 17 and 30 kg for adult males, and between 10 and 15 kg for females, resulting in a sexual dimorphism in mass ranging between 1. The species differ notably in their fur color, body size and sexually selected characteristics, such as the distinct capes which are most pronounced in Guinea and hamadryas baboons but also present in olive baboons.
Females of all species develop sexual swellings of the anogenital region when they are fertile. These swelling change throughout the menstrual cycle, such that maximum swelling typically coincides with ovulation Higham et al.
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The size and shape of the swellings varies considerably among species Figure 2 ; Petersdorf et al. A Phenotypic variation between species. Pictures show adult males and females. B Crania of male baboons. C Sexual swellings of female baboon during peak estrus. Species are grouped by social organization uni- and multi-level and dispersal behavior male- or female-biased dispersal.
All baboon species are largely terrestrial during the day but retreat to sleeping trees or cliffs during the night.
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They exhibit great ecological flexibility, allowing them to occupy habitats including semi-deserts grasslands, woodland savannas, humid forests, and Afroalpine grasslands over 3, meters above sea level Chala et al. Baboons eat a broad range of foods, although their diet mainly consists of plants, including fruit, seeds, leaves, and roots.
They also eat insects and other arthropods and, occasionally, kill small antelopes, hares, rodents, birds and smaller monkeys Goffe and Fischer, ; Swedell, Phenotypic differences between species are well recognized Jolly, , and based on their molecular phylogeny, baboons are generally split in two major groups: north and south Dunn et al.
However, genetic evidence reveals a complex evolutionary history of the genus Papio. Analysis of mitochondrial DNA yields a phylogeny that includes several major haplogroups or clades — groups of individuals who belong to a specific mitochondrial lineage. These haplogroups reflect the geographic origin of the respective specimens better than their external phenotypes or taxonomic classification, making species appear to be paraphyletic and polyphyletic when mapped onto the mitochondrial phylogeny Zinner et al.
Comparisons of whole genome sequences confirm the six baboon species taxonomy and suggest that the initial evolutionary divergence separated a southern lineage that ultimately produced Kinda, chacma and yellow baboons, from a northern lineage that produced olive, hamadryas and Guinea baboons Rogers et al.
Ancient hybridization events appear to have affected the genetic makeup of all species. For instance, Guinea baboons most likely experienced genetic admixture with a "ghost lineage" that is probably extinct, or that is at least not represented in the sample of genomes analyzed to date Rogers et al. Multiple episodes of admixture and introgression have been linked to climate change and range expansion Rogers et al. Similar evolutionary mechanisms, including gene transfer by introgressive hybridization, are now recognized to have influenced the evolution of Neanderthals, Denisovans and modern humans Green et al.
However, the absence of genomic data from non-sapiens African hominins presently hinders our ability to ask questions about ancestral African hominin hybridization Scerri et al. Baboons allow us to study the impact of gene flow in an extant model. Of particular interest for understanding baboon evolution is how changes in population density and spatial structure, such as the opening and closing of forest and other barriers, gave rise to different social systems Jolly, The range expansion of the genus appears to be of particular relevance.
Given a southern African origin, modern baboons experienced a tremendous expansion of their range, possibly linked to changes to the habitats, animal communities and climate that occurred during the Pleistocene and that gave baboons the chance to disperse into the savanna belt north of the tropical forest zone Dolotovskaya et al.
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Like humans and other savanna species, baboons have thus been subject of recurrent range shifts, fragmentation, and isolation and reconnection of populations Zinner et al. In summary, baboons can serve as a valuable model for evolutionary divergence and hybridization, triggered by climatic changes and the expansion and fragmentation of populations in the African savanna. Such analyses are also highly relevant for a better understanding of early hominins.
The six baboon species vary substantially in their social characteristics, including social organization, social style and mating patterns. Group size varies within and among species. In chacma, olive and yellow baboons, group size ranges from about a dozen up to roughly one hundred animals Markham et al.
Sex ratios in baboons vary too; some groups are fairly balanced, while adult females in other groups can outnumber adult males by about 10 to 1 Swedell, Chacma, olive, Kinda and yellow baboons — recently dubbed "COKY" baboons Jolly, — live in multi-male-multi-female groups, in which related females constitute the stable core, while males leave the group they were born into and join another. Clear rank hierarchies among males and females can be discerned based on aggressive interactions, including threats, chases and physical aggression, as well as signals of submission.
In females, related individuals known as matrilines typically occupy adjacent ranks. For female chacma, olive and yellow baboons, female kin constitute the most important social partners Silk et al. In Kinda baboons, however, males are the most significant grooming partners for females Petersdorf et al.
Females of all COKY species interact and mate with several males in the group. Consorts may last from several hours up to several days. Consort success and thus mating success is often related to male dominance status Gesquiere et al. Male competition and aggressiveness vary considerably between species. Infanticide is frequent in some populations of chacma baboons Palombit et al.
Lactating females often form close ties to specific males, which are often the sires of their infants Huchard et al. These relationships appear to be an adaptation against infanticide by recent immigrant males Palombit et al. These differences in male competitive regimes are reflected in their dispersal behavior: male chacma baboons in the Okavango delta, for instance, do not emigrate from their natal group until after they are fully grown Beehner et al.
Over the past decade, studies of Kinda baboons have broadened our knowledge of morphological and behavioral variation within the genus. Kinda females exhibit small sexual swellings Figure 2 and give inconspicuous calls Petersdorf et al. Chacma females, in contrast, give loud copulation calls, which function to incite male-male competition O'Connell and Cowlishaw, Hamadryas baboons — in contrast to the COKY baboon species described above — live in a multi-level society with reproductive units, called "one male units" comprising one sexually active leader male, a variable number of females, and sometimes a follower male Kummer, Associations between several one-male units constitute a clan Abegglen, ; several clans and unaffiliated bachelor males form a band, the main ecological unit, and multiple bands coalesce at resources, especially sleeping sites, to form troops Schreier and Swedell, Recent behavioral and genetic studies of hamadryas baboons show that leader and follower males tend to be maternally related, in line with the fact that they disperse only rarely.
Guinea baboons also live in a multi-level society. Several units comprising a primary male, 1—6 females, young, and occasional secondary males make up parties, and 2 to 3 parties constitute a gang within a larger community Fischer et al. Male Guinea baboons maintain strong bonds and a high degree of spatial tolerance Fischer et al. Some, but not all males with strong bonds are highly related, suggesting that the existence of kin in the group promotes male tolerance Patzelt et al. In striking contrast to most other baboon species, aggression between males is so rare that it is not possible to discern a dominance hierarchy with certainty Dal Pesco and Fischer, Males engage in extended ritualized greetings that apparently function to reinforce delineations between parties and to test bonds between males Dal Pesco and Fischer, Females freely transfer between units, parties and gangs.
Female tenure with a given male may vary between weeks and years Goffe et al. Both Guinea and hamadryas baboons exhibit female-biased dispersal Kopp et al. Note that many of the most significant differences in social behavior between species have been observed across different populations in multiple African sites, as well as in captivity.
Thus, there is good evidence that the variation we describe here reflects true species differences and not just variation between populations. Yet, characterizing the variation within species in greater detail would be extremely valuable.
Despite the variation in social organization and aggressiveness between the different baboon species, there is very little variation in the vocal repertoires and call types within the genus Hammerschmidt and Fischer, This suggests that the structure of vocal patterns is highly conserved.
Because species vary in their aggressiveness and their propensity to affiliate, they also differ in the frequency with which they use signals that either relate to fighting ability or "benign intent", respectively Faraut et al. Variation in social organization and in the nature and extent of competition over resources between baboon species is thought to result in differential selective pressure on social cognition Amici et al.
To date, most of the work on baboon social knowledge has been done on chacma baboons that exhibit steep dominance hierarchies known as despotism. A suite of studies by the American primatologists Dorothy Cheney and Robert Seyfarth and colleagues revealed that chacma baboons have sophisticated social knowledge reviewed in Cheney and Seyfarth, For instance, the animals represent the nested hierarchical rank relationships of their group members Bergman, , track the consortship status of pairs in their group Crockford et al.
Field playback experiments revealed that baboon species respond differently to social information. While the territorial chacma baboons respond strongly to apparent intruders Kitchen et al. Similarly, chacma baboons respond strongly to simulated rank reversals Bergman, or break-ups of existing consortships Crockford et al. The somewhat surprising responses of the Guinea baboons may be a result of the high gregariousness of the species, where deviant interaction patterns may initially be classified as "social noise" Faraut and Fischer, In summary, these findings suggest that the content of what is represented, namely the associations between different individuals or their group memberships, appears to be relatively similar across the two species, while the value of different types of social information may vary substantially in relation to the type of society.
Over the past decade, baboon research has provided ground-breaking insights into the relationships between social status, social relationships, health and fitness measures such as offspring survival and longevity. Data from two long-term studies of baboon behavior and life history suggest that sociality enhances the fitness of females.
For example, infants born to yellow baboon females who are more socially integrated have higher survival than infants of less social mothers Archie et al. As in many other primates, higher-ranking male baboons sire more offspring than other males Altmann et al. Higher-ranking females have shorter periods before they resume menstrual cycling following birth Gesquiere et al. Consistent with this, feeding on crops in olive baboons Higham et al. A number of studies have investigated the proximate mediators of the relationship between behavior and fitness.
In particular, many researchers have taken advantage of non-invasive ways to measure glucocorticoid hormones, a class of hormones known to mediate the energetic demands that accompany social and ecological challenges. For example, lactating chacma females that were at risk for infanticide because a new male immigrated into the group exhibited elevated glucocorticoid hormones compared to female counterparts that were not at risk Beehner et al.
Additionally, loss of a close female relative increases glucocorticoid concentrations, and this increase may be responsible for initiating a compensatory broadening and strengthening of female grooming networks Engh et al. Several studies have investigated the relationship between glucocorticoid concentrations, rank and social stability in male baboons. In a long term-study of yellow baboons, high-ranking males had lower glucocorticoid concentrations, regardless of hierarchy stability, while alpha males may experience higher concentrations than expected for their rank Gesquiere et al.
Nonetheless, high-ranking yellow baboon males get sick less often and heal from wounds faster, suggesting that these high-ranking males are in better health and do not suffer trade-offs from these extra demands Archie et al. Higher-ranking chacma baboon males also had higher glucocorticoid concentrations Bergman et al.
More recently, baboons were also established as a promising model for studying the impact of sexually transmitted diseases on mating behavior. These findings highlight how pathogens may impose important selective pressures in mate choice and ultimately social evolution.