John Lycett

Market forces predict grooming reciprocity in female baboons

We argue that grooming is a commodity that female primates can trade, either for itself or in exchange for other services (sensu biological markets theory) and that the decision to do either will depend on the degree of competition within a social group. We test this using data from four chacma baboon troops, living in two populations that differ markedly in the degree of contest competition. As predicted by the predominance of grooming dyads in which females are closely ranked there was, in all four troops, a positive correlation between the time invested by one partner and that by the other. In addition, as predicted, the allocation of time was more closely matched in troops where grooming could not be exchanged for anything else. In troops where resource competition was high, we found in one of two troops a positive relationship between rank distance and the discrepancy in time allocation, with the lower ranking of the partners contributing more grooming.

Longevity and the costs of reproduction in a historical human population

It has been argued that the priority that natural selection places on reproduction negatively affects other processes such as longevity and the problem posed by this trade‐off underlies the disposable soma theory for the evolution of human ageing. Here we examine the relationship between reproduction and longevity in a historical human population (the Krummhörn, north‐west Germany 1720–1870). In our initial analyses, we found no support for the hypothesized negative effects of reproduction on longevity: married women who remained childless lived no longer than women who reproduced and women who had few children lived no longer than women who had many children. However, more detailed analyses in relation to socio‐economic class revealed that the extent to which reproduction has an effect on longevity is a function of the level of economic deprivation. We found that, when possible sources of confound were controlled for (e.g. duration of marriage and amount of time spent in fecund marriage), there is an increasingly strong relationship between longevity and reproduction with increasing poverty.

MALE CONSORTSHIP BEHAVIOUR IN CHACMA BABOONS: THE ROLE OF DEMOGRAPHIC FACTORS AND FEMALE CONCEPTIVE PROBABILITIES

There is evidence for a general relationship between male dominance rank and mating success in primates, although the strength of this relationship differs among species. In chacma baboons (Papio hamadryas ursinus) male rank is found to be of more importance than in the other savannah baboon subspecies. However, even though the priority-of-access model explains the proportion of time spent in consortship for males of different rank in chacmas, highest-ranking males usually consort less often than expected. In this study, conducted in the Drakensberg Mountains of Natal and at De Hoop in the Western Cape, we investigated why dominant males in four study troops consorted only between 50% and 75% of days that they were expected to consort according to the priority-of-access model. Consortship success of highest-ranking males was primarily dependant on the number of available oestrous females in a troop. This was likely due to costs involved in consorting which limit the amount of days that a male could spend in consortship. Females pass through several cycles before conceiving and highest-ranking males were observed to consort more often on the conceptive cycle compared to the nearest nonconceptive cycle, but this was only true for males that were already resident for several months. Recently immigrated males that became highest-ranking often consorted during nonconceptive female cycles, while older, lower-ranking males consorted during the conceptive cycles. We propose that males with longer residency have more information about reproductive state of females and thus higher reproductive success than recently immigrated males.

Fission and troop size in a mountain baboon population

Abstract Baboon, Papio cynocephalus populations show a circumscribed range of troop sizes, dependent on features of the habitat that set limits on the ability of animals to balance their time budgets. To assess the proposal that a rising probability of troop fission is the mechanism underlying this it is first shown that (1) the distribution of troop sizes in two very different baboon populations, in the Drakensberg and at Amboseli, are not described by the null model of random allocation of individuals to troops (the Poisson process); and (2) data on troops undergoing or attempting fission support the hypothesis that the probability of fission increases with increasing troop size. A model was constructed that generated a distribution of troop size from an underlying, ‘pre-fission’ distribution by varying two parameters: the probability that a troop of given size will split, and the relative sizes of the two daughter troops that result if fission occurs. Both the Drakensberg and the Amboseli populations were adequately described by the model and it is concluded that the distribution of troop size in both populations is governed by fission in the manner proposed by Dunbar (1992, Behav. Ecol. Sociobiol. 33, 35–49). The model also indicates that, relative to Amboseli, the probability of fission in the Drakensberg is very high, with all troops of more than 28 members eventually splitting. The hypothesis that low food availability accelerates fission in mountain baboons through the decreased foraging efficiency of larger troops was tested. Troops of a wide range of sizes allocated very similar amounts of time to foraging, foraged with the same efficiency, and travelled approximately the same distances to do so. It is concluded that fission in mountain baboons, being exacerbated neither by predation nor by inefficient foraging, may be caused directly by the inability of individuals to service social relationships in growing troops in the face of small amounts of time available for social interaction.

Cohort size and the allocation of social effort by female mountain baboons

Dunbar (1992,Behav. Ecol. Sociobiol.33, 35-49) argued that constraints on social time limited the size to which savannah baboon, Papio cynocephalustroops in any given population could grow before fissioning. Since this should be reflected in population structure, we have elsewhere (Henzi et al. 1997a, Anim. Behav. 53, 525-535) constructed a model, based on a rising probability of fission, that fits the observed distribution of troop sizes of mountain baboons, P. c. ursinusin the Drakensberg mountains of South Africa and which predicts that the probability of fission will rapidly increase once a troop has more than 23 members (or 8.7 females). We test this prediction in this paper. Since Dunbar argued that females will drive fission once they cannot engage in the grooming necessary to sustain alliances, we compared the grooming interactions of adult females from four troops in the Drakensberg mountains. The mean female grooming clique size reached an asymptote at 7.4 females, so that females in cohorts of eight or more no longer attempted to groom all other females, and mean grooming bout length declined as the cohort grew to 7.9 females and then increased again. These values are coincident with the female cohort size predicted by our model of troop growth and fission. We argue that females attempt to groom all other females as well as sustain closer relationships with a few females through longer bouts of reciprocated grooming. When the demands of grooming all other females reduce bout length to a point when no reciprocated bouts are possible, female clique size is capped. As a troop continues to grow, the mechanical difficulties involved in gaining access to grooming partners leads to a reduction in the diversity of grooming relationships. This weakening of the total female network, as cliques become more differentiated, is likely to facilitate fission. We conclude that our data provide the first within-population validation of Dunbars hypothesis concerning the mechanism underpinning fission. In the Drakensberg, where there is no advantage to female coalitions, we propose, as an amendment, that females will leave a troop not to escape local competition, but to follow a male with whom they have a close friendship.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Maternal investment in mountain baboons and the hypothesis of reduced care

Abstract It has been argued that female mammals should terminate expensive forms of infant care earlier as habitat quality declines. More recently it has been shown that among a variety of mammalian species, early termination of care is also associated with highly favourable conditions. In this paper we present data on maternal investment decisions among baboons (Papio cynocephalus ursinus) inhabiting the Drakensberg Mountains of South Africa, and compare these with data from East African baboon studies. Mothers in the mountain habitat face a set of environmental conditions where the problem of resource allocation to offspring is expected to be particularly acute. We begin by using the model of Altmann (1980) of maternal time budgets to demonstrate that mountain baboon mothers experience greater perturbations to their activity budgets while suckling than do mothers in other populations. They also provide consistently greater levels of care to their infants and do so in the absence of any form of overt conflict over access to the nipple. Although this investment results in a relative lengthening of the interbirth interval (IBI), it is accompanied by relatively higher infant survival. We argue that factors that influence the maternal strategy adopted by mountain baboons include slow infant growth rates and a lack of predation in the habitat which influences probability of offspring survival beyond the immediate postnatal period. We suggest that both “care-dependent” sources of mortality (e.g. female reproductive condition, the amount of milk transferred to offspring) as well as “care independent” sources of mortality (e.g. predation, infectious disease) should be considered in studies of parental investment.

Mobile phones as lekking devices among human males

This study investigated the use of mobile telephones by males and females in a public bar frequented by professional people. We found that, unlike women, men who possess mobile telephones more often publicly display them, and that these displays were related to the number of men in a social group, but not the number of women. This result was not due simply to a greater number of males who have telephones: we found an increase with male social group size in the proportion of available telephones that were on display. Similarly, there was a positive relationship between the number of visible telephones and the ratio of males to females. Our results further show that the increased display of telephones in groups with more males is not due to the ostensive function of these devices (i.e., the making and receiving of calls), although single males tended to use their phones more. We interpret these results within the framework of male-male competition, with males in larger group sizes functioning in an increasingly competitive environment. This competitive environment is suggested to be akin to a lek mating system in which males aggregate and actively display their qualities to females who assess males on a number of dimensions. We suggest that mobile telephones might be used by males as an indicator of their status and wealth (sensu “cultural ornaments”).

Female baboons do not raise the stakes but they give as good as they get

We used data from four chacma baboon, Papio cynocephalus ursinus, troops, living in two populations, to test the raise the stakes (RTS) strategy of reciprocity. Female baboons did not raise the stakes either within or across grooming bouts. Instead they time-matched grooming contributions and divided grooming into short episodes. In addition, analysis of the grooming behaviour of frequently versus infrequently grooming dyads did not reveal differences in grooming patterns predicted by the RTS strategy. We suggest time constraints preclude the escalation of grooming bout length as required by RTS; the data were more consistent with a strategy of give as good as you get. However, this strategy could not explain all the patterns observed, and we conclude that biological market theory represents a more appropriate framework for investigating female grooming dynamics than dyadic games based on the iterated prisoners dilemma. We suggest that competitive altruism among individuals acts as a market force influencing an individuals value as a grooming partner. Copyright 2000 The Association for the Study of Animal Behaviour.

Abortion rates reflect the optimization of parental investment strategies.

Parental investment decisions in human beings, including infanticide, have usually been considered in relation to the postnatal survival probabilities of their children. A number of factors which influence parental ability and willingness to invest in offspring have been identified from these studies. Here we argue that at least some of the same factors which influence investment decisions postpartum also affect the decision to terminate a current pregnancy through voluntary abortion. We show that both female age and marital status influence the probability of abortion, with the key variable being the likelihood of future marriage. Thus, abortion procedures extend a womans ability to manage her reproduction into the prenatal period.

Whose Life Is It Anyway? Maternal Investment, Developmental Trajectories, and Life History Strategies in Baboons

Levels of investment, interbirth intervals, and offspring survival vary across populations of savannah baboons in relation to habitat quality. In this chapter, we use detailed data from two populations of South African chacma baboons (De Hoop in the Western Cape and the Drakensberg Mountains in Kwa-Zulu Natal) to investigate the constraints on offspring ability to embark on independent foraging trajectories. These analyses reveal that both offspring independence and probability of survival are contingent on both timing of birth and habitat predictability. At De Hoop, where births are nonseasonal but the availability of weaning foods is contingent on rainfall, infants are at greater risk of making expensive “mistakes,” (i.e., embarking on independent trajectories that cannot always be sustained). This contrasts with the Drakensberg, where the seasonal nature of births and food resources constrain infants more tightly with respect to achieving independence, but at the same time reduce their mortality risk. We extend these analyses to other Papio populations across Africa, revealing that the relationship between parental investment and offspring survival is a complex one, reflecting both the ease with which offspring can assume an independent lifestyle and the balance between intrinsic and extrinsic sources of mortality.