Do fishes with higher brain to body mass ratio have a greater tendency to live in groups than in pairs or alone?
Introduction
The social brain hypothesis was proposed to explain the relationship between brain to body mass ratio and social behaviour (Dunbar, 2009). This hypothesis suggests that group size and complexity act as a driver for brain size, resulting in primates having greater encephalization than other mammals. In this experiment, we chose to classify social organization within three different groups: solitary, in pair or in group. To assess whether encephalization differs between groups, we observed the social organization of every species we encountered and compared their brain to body mass ratio using fish data base (www.fishbase.org). We hypothesize that fishes living in pairs would have a bigger ratio than the ones living alone, but a smaller one than those living in groups.
Hypothesis: Fishes with higher encephalization (i.e brain to body mass ratio) have a bigger tendency to live in group than in couple, than alone.
Material and methods
We used a camera to follow each species of fish for 2 minutes (when possible). With the videos, we were able to identify the species later and to tell if they were spending their time alone, in pairs or in groups. Using fishbase.org, we had access to a big data base including brain to body mass ratio for the most part of the species we encountered. Over the 57 species we have identified, 34 were used for this study, and the rest couldn’t be used because of insufficient information on brain size. Encephalization was calculated by dividing the brain weight in milligrams (mg) by the body weight in grams (g). For statistic, we used the R program. We made “t.test” comparing each social behaviour brain to body mass ratio. Data were collected in El Quseir, Egypt between the 22nd and 29th of April 2017.
Figure 1. Barplot of social behaviours with brain/weight ratio. T.test comparing social status show a significant difference between social status group-solitary: P-value= 0.00028 and group-couple: P-value=0.00813 but not between solitary-couple: P-value=0.074
Blue spotted stingray
Discussion
Masked Puffer fish
Results
Through the different species we observed, we found a significant correlation between brain/body weight ratio. As predicted, many interactions are related to a more developed brain. A solitary fish has a smaller encephalization than fishes living in pair and even smaller than fishes living in group. The results (Figure 1.) were significantly different when comparing group to solitary (p-value 0.00028) and group to couple (p-value 0.00813). Even though we can see a difference and the p-value is really close to 0.05, we found no significant differences between solitary and couple (p-value 0.074).
killian Vaucher & Simon Ioset
The brain size might be used as a factor to predict whether fishes would more likely live in group, in pairs or alone.
According to (Pérez-Barbería et al., 2007), there is a coevolution relationship between sociality and brain size in mammalian orders (Ungulates, Carnivore, Primates). Social organization might need more cognitive performances because individuals have to predict the behaviour of others to maintain social cohesion (Pérez-Barbería et al., 2007). Living in group is costlier in term of brain use but it brings advantages to the members of the group. Reduced predation, offspring care and resource acquisition are a few examples of the benefits of living in a group (Pullian and Caraco 1984).
References
Dunbar RIM. 2009. The social brain hypothesis and its implications for social evolution. Annals of Human Biology, 36(5): 562 572
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Fishbase.org. (2018). Search FishBase. [online] Available at: http://fishbase.org/search.php [Accessed 12 Jan. 2018].
Pérez-Barbería FJ, Shultz S, Dunbar RI. 2007. Evidence for coevolution of sociality and relative brain size in three orders of mammals. Evolution, 61:2811–2821.
Pullian, H. R., and T. Caraco. 1984. Living in groups: is there optimal group size? Pp. 122–147 in J. R. Krebs and N. B. Davies. eds. Behavioural ecology: an evolutionary approach. Sunderland, MA