Biology subjects

Dunbar, R. I. M.

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

3 recordsLinked to original sources

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↗