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.

Paper: Who Spreads Parasites in Lizards?

Little is known about how parasites are spread through in host populations. Spread patterns should depend on host contact patterns. Social network analysis provides great tools for modeling parasite and pathogen transmission in host populations. Theoretically, highly connected individuals are in greater risk of being infected.

In a recent paper in the Journal of Animal Ecology, Fenner et al. describe the infection patterns of a tick and a nematode in the pygmy bluetongue lizard (Tiliqua adelaidensis), a solitary lizard from South Australia. These lizards don't move much, staying in burrows most of the time.

A relative: the blotched bluetongue lizard

The authors used 3 plots representing sub-populations in the same area. In the only plot in which ticks were found on lizards, lizards that had more or closer neighbors, i.e. more connected, had more ticks. Stable resident hosts were more important than dispersers in influencing tick distribution.

For nematodes there was support for the role of dispersers - infected hosts were more connected to dispersers. This suggests there are different transmission pathways for different parasites, possibly due to differences in parasites survival times.

To summarize, this paper shows the strength of network analysis in exploring alternative hypotheses about the dynamics of infection patterns. It also exposes once again the dependence of the result on the way the network was defined - in this case the distances between burrows of lizards.

Paper: Network Analysis of Songbird Dialects

Today I am glad to present a paper by Yoktan et al. to which I have contributed the network analysis. This work describes the different dialects of songs of the orange-tufted sunbird in Israel. More than 100 years ago, sunbirds were found only in the southernmost parts of the Rift Valley in Israel, in the Arava and some oases to the north, such as Ein Gedi. The Zionist settlement in many small villages (kibbutz & moshav) along the Rift allowed the expansion of sunbirds as gardening in these settlements introduced many species of ornithophilous plants.

In this study we have recorded singing male sunbirds in many locations along the Rift and analyzed the spectrogram of their trill component. We found that each location had a slightly different dialect, and built a network of locations according to their dialects. We used data of the distance between each two dialects as the basic matrix. Since we had a full matrix, we had to set some threshold in order to remove some of the ties and get a meaningful network. We set the threshold to be the largest distance that still allowed all locations to be connected as one component, in order to be able to relate each location to the others.

The trills in different locations

The network of locations by song distances revealed locations that are "connected", i.e. their songs are relatively similar, while other locations were "disconnected" - their songs were quite different from each other. This network shows that there are "communities" of locations with similar songs (we determined the communities using the Girvan-Newman algorithm). Some of these communities consist of locations from the same geographical region, while others have a mix of locations from different regions. The locations in the Arava valley, the natural habitat of the sunbirds before the expansion, all belong to the same community. From there began the habitat expansion to the north. Three of the most isolated locations in the network indeed represent villages in the extreme north, suggesting these sunbirds do not interact much with more southern populations. Also interesting are the locations along the dead sea: sunbirds were present in Ein Gedi before the settlements, and their songs closely resemble the Arava songs. However, the songs of nearby Kaliya and Mitzpe Shalem, which were settled in the 1970s', are found in a different community, together with northern settlements, suggesting that these two locations were inhabited by sunbirds coming back from the north, and not directly from Ein Gedi.

The network of locations according to sunbird songs

Another interesting finding is that network centrality, which depicts how central were locations of singers in the network, was negatively correlated with genetic variability. This fact implies that the most central locations in terms of song dialects, which are Ein Gedi, Bet Zera and Sede Eliezer, host established populations of sunbirds which resist intruders. These locations are probably a source of dispersal to other places.

Overall, our results support the historical processes hypothesis of dialect formation, which predict song dialects of nearby locations which were occupied at the same time to be similar. This work illustrates the power of network analysis in describing not only social relations between animals, but also other types of relations. I believe it could be useful for many other types of analyses. 

Degree Centrality - Mini Review

While reviewing the fast accumulating literature about animal social networks might be an overwhelming task, I would like to try and focus some posts on some basic network measurements and what we have learned from them so far. Degree is the most basic measurement for an individual in a network. In a binary network it is the number of ties an individual has with others. In a weighted network it is the sum of all individual's tie weights, which means that two individuals may have the same number of connections, but one of them may have stronger connections than the other.

So what do we know about animals in terms of degree? Lusseau and Newman (2004) showed that in a bottlenose dolphins population in New Zealand there is no assortative mixing by degree, i.e. dolphins having high degree do not preferentially attach to other dolphins with high degree. This is opposed to findings in humans, where assortative mixing is common. The dolphin network suggests that preferential attachment is not strong in their network evolution, meaning that the network was not formed by the connection of new members to central members. An additional finding of this study is that the network is robust to the loss of high-degree members. In other words, there are many redundant paths in the network, allowing it to withstand removal of central figures.
In a newer paper Lusseau et al. (2006) report some degree homophily in a bottlenose dolphin population in Scotland. It is difficult to say if the difference in these results reflects a real difference in the social structure of different dolphin populations or is an artefact of some methodological inconsistencies.

Croft et al. (2005) did find assortative interactions in a study of multiple guppy populations and one three-spined stickleback population. The degree of individual fish was positively correlated with the average degree of their network neighbors. The authors suggest that could cause faster spreading of information, but also of pathogens, among populations. Although this study was performed on multiple populations, we'll need more data in order to state that assortative interactions are common in animals.

Guppies

In another study, on captive pigtailed macaques, Flack et al. (2006) observed "policing" behavior by some members of a group. They showed that in the presence of policing the average degree of grooming and playing networks was higher. Policing also affected assortative mixing, where less policing meant increased assortativity. This is the first study to show the effect of individual removals on the degree of other individuals in the network. However, since this study was done on captive animals it has limited power in describing wild social structures.

A different kind of study compared the networks of onagers and Grevy's zebras (Sundaresan et al. 2007), and found that onagers have higher degree than zebra, i.e. they associate with more partners. Interestingly, when testing only preferred associations (after statistically testing which associations are more common than expected by chance), such differences were absent. This shows that these two species do not differ in the amount of significant associations each individual has.

Wolf et al. (2007) studied social networks of the Galapagos sea lion. I have reviewed a recent work by them in an earlier post. They found that males had lower average degree than females. The authors suggest that since females are less aggressive than males and show more fidelity to a specific location they may be able to form more associations.

As can be seen, we have only began to scratch the surface of what we can learn from the degrees of individuals. The described papers currently do not lead to any conclusion regarding other species, or even other populations. I will end this post here and continue in part 2.