Spatial pattern analysis of aggregation behaviour in Drosophila melanogaster
Aggregation, i.e. the tendency of individuals to gather in space and time, is a behaviour shown by many organisms. Animals typically show aggregation in the presence of common resources, to reduce predation risk or to engage in social interactions. Previous studies have shown that the fruit fly, Drosophila melanogaster, shows aggregation even in the absence of food and predators. This suggest that aggregation may result from some social interaction between flies. Like many other animals, social interactions in Drosophila are sex specific. For example, Drosophila females rarely show aggression towards each other and also prefer laying eggs near each other's eggs, which suggests that they may show some attraction towards each other. In contrast, males aggressively defend food patches from other males but can also form leks to attract females. Thus, if aggregation results from such social behaviours, then because of these differences, male and female flies may show different aggregation patterns. Interestingly, however, previous studies have not found any differences between aggregation patterns of male and female fruit flies. This lack of difference might be because previous authors measured aggregation using the distance between a fly and its nearest neighbour. They thus ignored other flies that may be in the fly's proximity. It is possible that males and females show similar distances to their nearest neighbour but show differences in the total numbers of neighbour. The distances to these neighbours may also be different for males and females. Such differences cannot be captured by nearest neighbour distances alone. Hence, to compare aggregation patterns of males and females in more detail, I used the "Ripley's K method" which takes into account all flies present near a fly by measuring the local density near it. If there is any aggregation shown by flies, then this local density will be large, on average, compared to a random, non-aggregated pattern. This method allows us to measure how many flies are near a fly and also allows us to test how this number changes with distance from the fly. If this method also shows no differences in aggregation pattern of males and females then it may indicate that aggregation is independent of sex specific social interactions between flies. However, if I find differences in the aggregation patterns of males and females then it may indicate that males and females interact differently amongst each other and aggregation might be a result of these different social behaviours.
Keywords: sexual dimorphism, social interactions, Ripley's K method.