Gelada

Common name

The gelada has been referred to by other names, including the "gelada baboon", "bleeding-heart baboon", or simply "baboon", implying a monophyletic relationship with baboons, which historically included (apart from Theropithecus) the genera Papio (true baboons), and Mandrillus (mandrills and drills). Since the 1990s, however, molecular phylogenetic studies ^[9] clarified relationships among papionin monkeys, demonstrating that mangabeys of the genus Lophocebus are more closely related to Papio and Theropithecus, while mangabeys of the genus Cercocebus are more closely related to Mandrillus. These findings largely invalidated any scientifically based justification for referring to mandrills and drills as baboons, as doing so while excluding the unbaboon-like Lophocebus mangabeys would create a polyphyletic group. The status of geladas was less clear and the relationships among Papio, Lophocebus, and Theropithecus continue to reflect high levels of uncertainty, which are further complicated by the discovery of the kipunji. Nevertheless, the most recent and extensive phylogenetic study to date demonstrates that, while large fractions of the genome show an alternative history, the dominant relationship across the genome supports a closer relationship between Papio and Lophocebus, with Theropithecus as the outgroup.^[10] As a close sister relationship between Papio and Theropithecus is the least-supported scenario in recent studies, i "gelada baboon" and other names implying a close relationship with baboons, with increasing clarity, are not scientifically justified, leading researchers to advocate for the common name to be simply "gelada".^[11]

Social structure

Geladas live in a complex, multilevel society similar to that of the hamadryas baboon. The smallest and most basic groups are the reproductive units, which include up to 12 females, their young, and one to four males, and the all-male units, which are made up of 2–15 males. The next level of gelada societies are the bands, which are made up of two to 27 reproductive units and several all-male units. Herds consist of up to 60 reproductive units that are sometimes from different bands and last for short times. Communities are made of one to four bands whose home ranges overlap extensively. A gelada typically lives around 15 years.^[18][22][23]

Within the reproductive units, the females tend to be closely related and have strong social bonds.^[22] Reproductive units split if they become too large. While females have strong social bonds in the group, a female only interacts with at most three other members of her unit.^[22] Grooming and other social interactions among females usually occur between pairs.^[24] Females in a reproductive unit exist in a hierarchy, with higher-ranking females having more reproductive success and more offspring than lower-ranking females.^[25] Closely related females tend to have a similar hierarchical status.^[25] Females generally stay in their natal units for life; cases of females leaving are rare.^[26] Aggression within a reproduction unit, which is rare, is usually just between the females.^[24] Aggression is more frequent between members of different reproductive units and is usually started by females, but males and females from both sides can join and engage if the conflict escalates.^[24]

Males can remain in a reproductive unit for four to five years.^[22] While geladas have traditionally been considered to have a male-transfer society, many males appear to be likely to return and breed in their natal bands. Nevertheless, gelada males leave their natal units and try to take over a unit of their own. A male can take over a reproductive unit either through direct aggression and fighting or by joining one as a subordinate and taking some females with him to create a new unit.^[22] When more than one male is in a unit, only one of them can mate with the females.^[24][26] The females in the group together can have power over the dominant male. When a new male tries to take over a unit and overthrow the resident male, the females can choose to support or oppose him. The male maintains his relationship with the females by grooming them rather than forcing his dominance, in contrast to the society of the hamadryas baboon. Females accept a male into the unit by presenting themselves to him. Not all the females may interact with the male. Usually, one may be his main partner.^[27] The male may sometimes be monopolized by this female.^[27] The male may try to interact with the other females, but they are usually unresponsive.^[27]

Most all-male units consist of several subadults and one young adult, led by one male. A member of an all-male unit may spend two to four years in the group before attempting to join a reproductive unit. All-male groups are generally aggressive towards both reproductive units and other all-male units.^[24] As in reproductive units, aggression within all-male units is rare. As bands, reproductive units exist in a common home range.^[28] Within the band, members are closely related and between the units there is no social hierarchy. Bands usually break apart every eight to nine years as a new band forms in a new home range.

Researchers from the University of the Free State in South Africa, while observing gelada during field studies, discovered that the monkeys were capable of "cheating" on their partners and covering up their infidelity. A nondominant male mates surreptitiously with a female, with both suppressing their normal mating cries so as not to be overheard. If discovered, the dominant male attacks the miscreants in a clear form of punishment. It is the first time that evidence of the knowledge of cheating and fear of discovery have been recorded among animals in the wild. Dr. Aliza le Roux of the university's Department of Zoology and Entomology believes that dishonesty and punishment are not uniquely human traits, and that the observed evidence of this behaviour among gelada monkeys suggests that the roots of the human system of deceit, crime, and punishment lie very deep indeed.^[29]

Mixed-species association was observed between solitary Ethiopian wolves and geladas. According to the study's findings, gelada monkeys typically do not move on encountering Ethiopian wolves, even when they were in the middle of the herd; 68% of encounters resulted in no movement and only 11% resulted in a movement greater than 10 m (33 ft). In stark contrast, the geladas always fled great distances to the cliffs for safety whenever they encountered aggressive domestic dogs.^[30]

Reproduction and parenting

When in estrus, the female points her posterior towards a male and raises it, moving her tail to one side.^[31] The male then approaches the female and inspects her chest and genital areas.^[31][32][33] A female will copulate up to five times per day, usually around midday.^[32] Breeding and reproduction can occur at any time of the year, although some areas have birth peaks.^[14][34]

Most births occur at night. Newborn infants have red faces and closed eyes, and they are covered in black hair.^[32] On average, newborn infants weigh 464 g (16.4 oz).^[35]

If a new male assumes mastery of a harem, females impregnated by the previous leader have an 80% likelihood of aborting, in a phenomenon known as the Bruce effect.^[36] Females come into estrus quickly after giving birth, so males have little incentive for practising infanticide, although it does occur in some communities in the Arsi region of Ethiopia, which may be an incentive for females to abort and avoid investing caring for an infant that will most likely be killed.^[37]

Infanticide in geladas remains fairly uncommon, though, compared to many primates that live in one-male units such as gorillas or gray langurs. The females that cancel their pregnancy are thought to bond with the new leader faster.^[38] When a male loses his position as dominant harem master, the females and new leader may allow him to remain in the social unit as a nonbreeding resident to act as a babysitter. This way, the ex-leader can protect any infants he had fathered from being killed by the new leader, the females can protect the infants fathered by him, and when the new leader faces a potential rival, the ex-leader will be more inclined to help support him in keeping rivals at bay.

Mortality among infants occurs at its highest in the wet season, but on average, over 85% of infants survive to their fourth birthday, one of the great advantages of living in an environment with a food source few other animals can exploit, so is unable to sustain many large predators.^[39]

Females that have just given birth stay on the periphery of the reproductive unit. Other adult females may take an interest in the infants and even kidnap them.^[32] An infant is carried on its mother's belly for the first five weeks, and thereafter on her back.^[32][40] Infants can move independently at around five months old. A subordinate male in a reproductive unit may help care for an infant when it is six months old.^[32]

When herds form, juveniles and infants may gather into play groups of around 10 individuals. When males reach puberty, they gather into unstable groups independent of the reproductive units. Females sexually mature at around three years, but do not give birth for another year.^[24][28] Males reach puberty at about four to five years, but they are usually unable to reproduce because of social constraints and wait until they are about eight to ten years old.^[14] Average lifespan in the wild is 15 years.^[41]

Communication

Adult geladas use a diverse repertoire of vocalizations for various purposes, such as: contact, reassurance, appeasement, solicitation, ambivalence, aggression, and defense.^[42] The level of complexity of these vocalizations is thought to be near that of humans.^[43] They sit around and chatter at each other, signifying to those around that they matter, in a way, to the individual "speaking". To some extent, calls are related to the status of an individual. In addition, females have calls signaling their estrus. Geladas communicate through gestures, as well. They display threats by flipping their upper lips back on their nostrils to display their teeth and gums, and by pulling back their scalps to display the pale eyelids.^[44] A gelada submits by fleeing or presenting itself.

In 2016, a research group at the University of Michigan found that gelada vocalizations obey Menzerath's law, observing that calls are abbreviated when used in longer sequences.^[45][46]