Biology subjects

Dunbar, R. I. M.

Publications and source records attributed to Dunbar, R. I. M..

13 recordsLinked to original sources

bioRxiv2025

Casual Versus Fractal Modularity in Mammalian Social Organisation

Multilevel social systems in birds and mammals have attracted considerable interest, not least because they share some similarities with the natural structural organisation of human societies. I use a fractal analysis of a taxonomically wide range of species reported to have multilevel social systems to show that these in fact involve two very different phenomena: those that are fractally modular and those that are casually modular. The first have a strictly fractal structure identical to that found in human societies; the second lack formal structural coherence and are closer in form to casual herds. The former are associated with larger neocortices than the latter, suggesting that they may be cognitively more demanding. Understanding the evolution of multilevel systems and the cognition that underpins them obliges us to adopt a more nuanced approach to social evolution.

animal behavior and cognition↗

bioRxiv2025

What Relative Neocortex Size Tells Us About Social Evolution

Primate evolution has been dominated above all by increases in brain size over time. Here I show that two separate pressures have been responsible for substantial changes in the major structural features of the primate brain. Lineages that have adopted pairliving are associated with a differential increase in the volume of the subcortical brain. This mainly reflects an increase in body size and a consequent need to invest in cerebellum size in order to manage large body masses in three-dimensional arboreal environments, a trend that continues through the great apes. The second trajectory is associated with a switch to group-living, and is associated with a progressive increase in neocortex volume. This seems to have occurred in three distinct waves corresponding to stepwise increases in social group size. These trends are not associated with phylogeny, but represent taxonomically mosaic evolution driven by individual species exposure to new kinds of habitats.

animal behavior and cognition↗

bioRxiv2025

Maternal time budgets of feral goats at high latitudes in northwest Europe.

Reproduction is energetically expensive for mammals, and especially so during lactation. In large-brained mammals such a primates, females invest heavily in additional feeding to fuel the costs of lactation. The evidence is more mixed in ruminant ungulates. We examine the foraging costs of lactating female feral goats living under environmentally challenging conditions in the northwest of Scotland where these effects are most likely to be exaggerated. We show, using data from three separate studies, that, compared to matched non-lactating females, lactating females do increase the time devoted to foraging, but only to a limited extent that is well below the theoretical requirements of their kids. Although they do not alter their diet, lactating females spend more time in shelter and at lower altitudes in order to reduce thermoregulatory costs. At high latitudes, the rate at which kids grow is such that females cannot afford to extend lactation much beyond two months. This will inevitably set a limit on ungulates capacity to produce large-brained offspring. HighlightsO_LILactation is costly, obliging females to increase food intake or draw on body fat C_LIO_LIAt high latitudes, goats increase foraging, but not enough to meet kids demands C_LIO_LIFemales do not forage on richer foods, but they do try to minimise thermal costs C_LIO_LIAs a result, females have high mortality after weaning when forage quality is poor C_LI

animal behavior and cognition↗

bioRxiv2025

Insectivorans and Carnivorans Exhibit the Same Social Brain Relationship as Primates

It has previously been thought that the social brain hypothesis applies in its full quantitative form only to primates. Using more appropriate brain indices and less error-prone statistical methods, we here show that it does in fact apply to both Insectivorans and Carnivorans, as well as to elephants and orcas. While Insectivorans form a grade on their own, reflecting a simple transition from small-brained solitary social systems to slightly larger-brained pairbonded monogamy, the Carnivorans map directly onto the primate socio-cognitive grades, dividing between loose sociality and more bonded sociality paralleling the pattern observed in monkeys. In addition, the subgroupings of species with multilevel social systems (including hyaenids elephants and orcas) map across the grades in the same way as do primates with similar societies. This suggests that common principles and trajectories underlie social evolution in mammals.

animal behavior and cognition↗

bioRxiv2024

Impact of Climate, Habitat and Scale on the Population Dynamics of Feral Goats on the Isle of Rum, NW Scotland

Although feral goats are an invasive species renowned for their ability to survive in degraded habitats, their capacity to occupy high latitude habitats is severely restricted. I analyse long term data on the lifehistory and demography of a feral goat population on the Isle of Rum, NW Scotland, in relation to both longterm variation in climatic variables and within-population variation in environmental variables. While exhibiting many features characteristic of ungulate lifehistory, goats are especially sensitive to variations in thermal conditions, especially during winter. This is compounded by the fact that, at the latitude of Rum, goats give birth in mid-winter, even though this imposes significant stress on both mother and kid. Longterm patterns in population growth rates are correlated with winter temperature and the NAO index, with little evidence for density-dependent effects (except in respect of fertility). In addition, there was evidence that the presence of a large sympatric red deer population was limiting the goats capacity to increase by denying them access to preferred foraging habitat. Nonetheless, their unusual sensitivity to the thermal environment implies that the goat population will increase significantly with progressive climate warming.

ecology↗

bioRxiv2024

Significance of Evolutionary Lags in the Primate Brain Size/Body Size Relationship

INTRODUCTIONThe original brain lag hypothesis proposed that primate brain evolution depended on spare energy derivative of savings of scale enabled by increasing body size. Deaner & Nunn [1] concluded that, in fact, there was no evidence for a brain lag. However, their result may have been due to a number of possible confounds in their analysis. METHODSI revisit their analysis to test for potential confounds using updated datasets. I also ask how primates paid for the energy costs incurred by changes in brain and body mass, and whether the impetus for these changes was predation risk. Finally, I ask whether the observed patterns explain the brain/body size ratio trajectory observed in fossil hominins. RESULTSI show that using statistically more appropriate statistics and updated data yields a significant brain lag effect. However, contrary to the original brain lag hypothesis, the brain/body ratio does not converge back on the allometric regression line, but continues to evolve beyond it. Increases in brain size are correlated with exploiting large group size rather than body size as the principal defence against predation risk, with significant growth in brain size (but not body size) only being possible if species adopted a more frugivorous diet. Finally, I show that hominins followed a similar trajectory from an australopithecine baseline that fell on the relevant allometric regression. CONCLUSIONThe brain lag effect is much more complicated than the original hypothesis proposed, with a distinctive switch from body to brain over evolutionary time.

evolutionary biology↗

bioRxiv2022

Processing of Different Social Scales in the Human Brain

For an individual to lead a healthy and fulfilling social life, it is essential to have relationships with multiple people who are at different levels of emotional closeness. Based on ethological, sociological and psychological evidence, social networks have been divided into five scales of emotional closeness, gradually increasing in size and decreasing in emotional proximity. Is this division also reflected in different brain processes? During functional MRI, participants compared their emotional closeness to different members of their social network. We examined the brain area that was differentially activated for levels of emotional closeness, and found that its vast majority (78%) showed preference for people who are closest to participants, including the temporoparietal junction, middle temporal gyrus, precuneus and dorsomedial prefrontal cortex. A different system, which includes the medial temporal lobe, retrosplenial cortex and ventromedial prefrontal cortex showed preference for all other social scales. Moreover, we found a significant correlation between brain responses to emotionally close people and smaller spaces (room, building) as well as between emotionally distant people and larger spaces (neighborhood, city). Finally, brain activity at the default mode network (DMN) was associated with social scale preference, such that its subnetwork DMN A, related to social processing, showed preference to closer social scales, while DMN C, related to spatiotemporal processing, showed preference to farther social scales. Our results show that the cognitive processing of a few intimately close people differs from the rest of the social network, emphasizing their crucial role in social life. SignificanceWe divide the people in our lives according to levels of emotional closeness, called social scales, ranging from the few people who are the closest to us, in which we invest most of our social efforts and time (support clique), to the farthest level of [~]150 acquaintances. Here, we used neuroimaging to investigate the brain processing of different social scales. We found that the area of cortex dedicated to the support clique is much larger than that of all other scales and encompasses different brain regions. Interestingly, this division is similar to the one between processing of small and larger spaces, and processed by different subregions of the default mode network. Our study emphasizes the importance of close relationships in our social lives as found in the brain.

neuroscience↗

bioRxiv2021

Morningness-eveningness assessment from mobile phone communication analysis

Human behaviour follows a 24-hour rhythm and is known to be governed by the individual chronotypes. Due to the widespread use of technology in our daily lives, it is possible to record the activities of individuals through their different digital traces. In the present study we utilise a large mobile phone communication dataset containing time stamps of calls and text messages to study the circadian rhythms of anonymous users in a European country. After removing the effect of the synchronization of East-West sun progression with the calling activity, we used two closely related approaches to heuristically compute the chronotypes of the individuals in the dataset, to identify them as morning persons or "larks" and evening persons or "owls". Using the computed chronotyes we showed how the chronotype is largely dependent on age with younger cohorts being more likely to be owls than older cohorts. Moreover, our analysis showed how on average females have distinctly different chronotypes from males. Younger females are more larkish than males while older females are more owlish. Finally, we also studied the period of low calling activity for each of the users which is considered as a marker of their sleep period during the night. We found that while "extreme larks" tend to sleep more than "extreme owls" on the weekends, we do not observe much variation between them on weekdays. In addition, we have observed that women tend to sleep even less than males on weekdays while there is not much difference between them on the weekends.

evolutionary biology↗

bioRxiv2020

Female Dispersion Is Necessary, But Not Sufficient, For The Evolution of Monogamy in Mammals

Explanations for the evolution of monogamy in mammals typically emphasise one of two possibilities: monogamy evolves when females are overdispersed (such that males cannot defend more than one female at a time) or when males provide a service to the female. However, the first claim has never been directly tested. I test it directly at three levels using data from primates and ungulates. First, I show that the females of monogamous genera do not have territories that are significantly larger, either absolutely or relatively, than those of polygamous genera. Second, using both the Mitani-Rodman and Lowen-Dunbar inequalities, I show that, given their typical day journey lengths, males of most monogamous species could easily defend an area large enough to allow them to monopolise as many as 5-10 females if these ranged solitarily. Finally, I use a model of male mate searching strategies to show that, unlike the males of socially-living polygamous species, the opportunity cost that monogamous males incur is typically more than five times the reproductive success they have by being obligately monogamous. This suggests that the selection pressure dissuading them from pursuing a roving male strategy must be very considerable.

animal behavior and cognition↗

bioRxiv2020

The Evolution of Self-Control

The capacity to inhibit prepotent actions (inhibitory self-control) plays an important role in many aspects of the behaviour of birds and mammals. Although a number of studies have used it as an index of foraging skills, inhibition is also crucial for maintaining the temporal and spatial coherence of bonded social groups. Using three different sets of comparative data, we show that, across primate species, the capacity for self-control correlates better with the demands of social contexts than with the demands of foraging contexts, whereas a more generalised capacity for causal reasoning correlates better with foraging contexts. In addition, we confirm the Passingham-Wise Conjecture that the capacity for self-control is unique to anthropoid primates. These results suggest that the capacity for self-control most likely evolved because it was crucial for the evolution of bonded social groups. Significance StatementThe capacity for self-control has commonly been viewed as an index of foraging skills. In fact, it plays a much more important role in the social domain by enabling groups of animals to maintain social cohesion as they travel through time and space. In this respect, it is particularly important for species that live in stable bonded social groups (congregations). We show that, in this respect, it is uniquely characteristic of the anthropoid primates, in contrast to other kinds of reasoning tasks such as causal reasoning on which primates often perform no better than other birds and mammals.

animal behavior and cognition↗

bioRxiv2018

Lag Effects in Primate Brain Size Evolution: A Re-Evaluation

The question as to whether there is a lag between brain and body mass evolution was ostensibly solved two decades ago by Deaner & Nunn (1999) who used phylogenetic methods to show that there was no evidence to suggest that changes in brain size lagged behind changes in body size. However, their assumption that body size would always change ahead of brain size is open to question. In addition, many of their datapoints are confounded by grade shift effects. A reanalysis of their data controlling for these confounds shows that there is in fact a strong lag effect, but that the direction of the lag is the reverse of that originally assumed: brain size typically changes first, and does so under selection from changes in group size. The data suggest that it takes about 2.0 million years for body size to converge back onto the conventional allometric relationship with brain size. In the meantime, species that have increased brain size are likely to incur a significant energy cost that must be met from elsewhere. I show that they seem to do so by changing to a more nutrient-rich diet.

animal behavior and cognition↗

bioRxiv2018

Defending the Undefendable: Male Territorial Behaviour and Mating System in Monogamous Primates

It has been suggested that monogamy evolves when females forage alone and are overdispersed, such that males cannot defend more than one female at a time. I test the underlying assumption that the females of monogamous anthropoid primates are overdispersed in three different ways, and compare the results with data for several polygynous primate genera. First, I show that monogamous primates do not have per capita territories that are significantly larger than those of polygynous taxa. Second, given their day journey length and the Mitani-Rodman equation (Mitani & Rodman 1979), males of most monogamous species could easily defend areas large enough to allow them to monopolise 5-6 females. Finally, I use a model of male mate searching strategies to show that, unlike the males of polygynous species, the males of monogamous species would sire more offspring by adopting a roving male form of polygyny when females are dispersed. The opportunity cost that monogamous males incur is typically more than five times the reproductive success they have by being obligately monogamous, suggesting that the selection pressure preventing them from pursuing a roving male strategy is very considerable. Given that biparental care always follows the adoption of monogamy in primate evolution, the only viable explanation for monogamy would seem to be either high predation risk or high infanticide risk.

ecology↗

bioRxiv2018

Group Size as a Trade Off Between Fertility and Predation Risk: Implications for Social Evolution

Cluster analysis reveals a fractal pattern in the sizes of baboon groups, with peaks at [~]20, [~]40, [~]80 and [~]160. Although all baboon species individually exhibit this pattern, the two largest are mainly characteristic of the hamadryas and gelada. We suggest that these constitute three pairs of linear oscillators (20/40, 40/80 and 80/160), where in each case the higher value is set by limits on female fertility and the lower by predation risk. The lower pair of oscillators form an ESS in woodland baboons, with choice of oscillator being determined by local predation risk. Female fertility rates would naturally prevent baboons from achieving the highest oscillator with any regularity; nonetheless, hamadryas and gelada have been able to break through this fertility glass ceiling and we suggest that they have been able to do so by using substructuring (based partly on using males as hired guns). This seems to have allowed them to increase group size significantly so as to occupy higher predation risk habitats (thereby creating the upper oscillator).

ecology↗