Monday, August 1, 2011

Paper: Socialism rules in the rock hyrax

Recent studies of humans show that more socialist countries, in which the assets are more evenly spread among the citizens, are better places to live in. Theses countries demonstrate higher longevity, less crime, less health problems and so on. In contrast, in these countries there are more suicides and more alcoholism. The explanation is that in such countries people direct violence towards themselves, and not towards other people. In countries with large gaps between the rich and the poor, all suffer: it's not only the poor who are victims of violence and health problems.

In my latest paper, Adi Barocas and me looked for the effects of social network structure on longevity in the rock hyrax. Our long term study allows us to follow individuals throughout their lives and determine their time of death. We found that groups vary in their equality of social associations: some groups are more "egalitarian", while in others there are highly connected individuals alongside peripheral ones. Our main finding is that hyraxes living in more egalitarian groups lived longer, which means that such groups present some advantages for their members. More technically, if the variance in strength centrality was high the hyrax had lower chances to survive. Interestingly, we found no such effect in the individual level. In other words, in the socially skewed groups, it's not necessarily the more central individuals that survived longer. Social inequality affected all members. Therefore, if we are looking for the mechanism that generates this outcome, I believe that it is not related to predation. If predation was the issue, we would expect to find that less connected individuals died sooner, but that was not found. Instead, I think that less "socialist" groups lead to higher stress, that may affect all members.
Rock hyraxes in Ein Gedi. Photo: Giora Ilany

There comes the question of why some groups cannot maintain social equality. I think it may be the result of a few dominant individuals, who are usually aggressive and increase the stress in the group, but this stress affects them too. As we continue to follow this population, we may be able to find the reasons for social inequality in some groups. It is worth to mention that the same group may change during the years, becoming less or more equal, and that is probably a result of its composition of members.
The higher groups have lower variance in centrality than the lower groups. Node size is proportional to centrality. 

In other findings, we show that group members survived better than solitary males, again demonstrating the advantages of sociality. We also found that individuals in smaller groups survived better than those living in larger groups. This shows that sociality has its limits, and groups wich are too large are not advantageous for their members. Group size was not related to variance in centrality.

Overall, I think that our results add to the growing literature showing the effect of sociality on fitness, as previously found in baboons and dolphins. In addition, this study is one of the first to examine the effects of social structure in the group level.