Context dependent variation in aggression and mating behaviour in the pygmy halfbeak (Dermogenys collettei): a study of wild population Piotr Michalak Degree project in biology, Master of science (2 years), 2021 Examensarbete i biologi 30 hp till masterexamen, 2021 Biology Education Centre, Uppsala University, and Department of Zoology, Stockholm University Supervisor: John Fitzpatrick External opponent: Carolina Segami 1 Contents 1 Introduction 3 2 Methods 6 2.1 Study species . 6 2.2 Video collection and selection process . 7 2.3 Scoring . 9 2.4 Environmental variables . 10 2.5 Statistics . 10 3 Results 12 3.1 Group size and composition . 12 3.2 Aggression . 12 3.3 Mating . 17 3.4 General activity . 19 4 Discussion 21 4.1 Social factors and environment . 21 4.2 Conclusions . 24 Abstract To understand animal behaviour, it is important to consider the environment in which it occurs. The environment, consisting of both abiotic factors and social context, is usually highly variable and leads to variation in individual’s and group’s behaviour. To better understand the environmental influences on behaviour of pygmy halfbeaks (Dermogenys collettei), a small live-bearing fish, I viewed videos of shoals of wild halfbeaks in Singapore. I investigated effects of environmental variation (water depth, canopy cover and water vegetation) and social environment (group size and male to female sex ratio) on halfbeaks’ aggression and mating be- haviours. I found that environment had little effect and most variation between studied shoals was probably due to social factors. I found some evidence for aggres- sion increase in larger shoals, primarily in males. Sex ratio had different relation with aggression for individual sexes and mating behaviours decreased when sex ra- tio became more male biased. This study shows that halfbeaks probably modify their behaviour in relation to social environment. I also show that these changes are similar to those described in other species, which strengthens the validity of using halfbeaks to study social interactions. 2 No animal or plant can live in a vacuum. John Maynard Smith (1975) 1 Introduction When considering animal behaviour, it is paramount to understand the context in which it occurs. Without it, we can provide little insight into why animal behaves a certain way. The environment of the animal is a complex, variable net of abiotic and biotic factors. These factors generate selective pressures that manifest themselves on both the level of an individual and a whole populations. If consistent on the evolutionary timescale, it can lead to species divergence (Rundle and Nosil 2005). This can operate only if there is individual, heritable variation of the trait in the population. Additionally, because the environment is highly variable, individuals usually show a range of behavioural responses. Abiotic environment can have a direct influence on animal behaviour. Factors like light intensity, temperature and ground topography can all play a role in influencing animal behaviour. Importantly however, abiotic factors influence multiple species in the same are and in turn, these influence each other. For example, trees might block the sunlight desired by an animal, possibly influencing their general predation risk. As light intensity can lead to higher visibility of prey, it influences predator’s and in consequence, prey’s behaviour. Under higher possible predation risk in turn, prey might decrease its activity to become less conspicuous. Guppies (Poecilia reticulata) under higher light intensity show lower number of aggressive behaviours that could be visible to potential predators (Endler 1987). Multiple night foraging species decrease their activity under bright moonlight (Lima and Dill 1990). Other factors that reduces prey visibility, like water vegetation for fish, also has been showed to impact behaviour of prey. In bluegills (Lepomis macrochirus) and fathead minnows (Pimephales promelas), fish tended to prefer areas densely covered by plants when predators where present, presumably as a refuge (Savino and Stein 1989). However, in dense vegetation one might expect fewer social interactions, as forming groups becomes difficult and visibility of conspecifics is also reduced. Through modifying their behaviour depending on the environment, organisms try to maximize their fitness and find an evolutionary trade-off between negative factors (like predation susceptibility (Dill 1987)) and other activities. Predation can influence a scope of life-history traits (eg. Reznick and Endler 1982). Food distribution is also influenced by the environment and can have surprising effect on a fundamental aspect of species and populations ecology. Emlen and Oring (1977) showed, how different distribution of food can lead to different observed mating systems. If resources are not uniformly distributed and can be monopolised, potential for multiple matings increases as more individuals can be found in the same 3 region. This in turn might influence the potential for sexual selection and differential reproductive success observed in such groups. Environmental factors have also an impact on social conditions. In turn the social environment is a critical factor further modifying behaviour. Differences in group struc- ture and size lead to different levels of competition for food and mates, and different predation risk. Operational sex ratio (OSR), the ratio of sexually active males to females, is expected to be a result of various factors, like potential for resource monopolization and ability of animals to explore that potential (Emlen and Oring 1977). OSR influences social behaviour and strength of sexual selection. As the ratio becomes more skewed towards one sex, the frequency and intensity of intrasexual competition and aggression has been showed to increase (Grant and Foam 2002; Weir et al. 2011). Although the pat- tern, observed here in Japanese medaka Oryzias latipes (a shoaling fish), is true for both female-female and male-male aggression, the increase is usually stronger within the male sex(Grant and Foam 2002). The OSR and increased competition also leads to changes in courtship. More males competing seems to correlate with decrease intersexual inter- actions per individual male, but with increased number of female interactions per single female (Clark and Grant 2010). In extreme cases, in some fish, change in OSR can lead to the sex change of an individual - a complete overhaul of sexual strategy (Ross 1990). Group size is also an important factor influencing social and sexual behaviours. In gup- pies, bigger groups show increased levels of male-male aggression (Seghers and Magurran 1991; Price and Helen Rodd 2006). However, population of guppies that were more prone to schooling (forming uniform groups) showed decreased levels of aggression - as predicted if schooling is considered an anti-predator response (Seghers and Magurran 1991). As our understanding of the importance of environmental and social factors increases, it becomes more important to our understanding of behavioural variation to combine data both from laboratory experiments and observations in the wild. One of the organisms used to study social interaction in the laboratory is pygmy halfbeak (Dermogenys collettei). It is a small, live-bearing fish that shows high levels of male-male aggressions and acomplex courtship behaviour (Greven 2010). It is used to study female mate choice (Ogden et al. 2020; Reuland et al. 2019) and shoaling behaviours (Ho et al. 2015). Despite it being relatively common and being used as a model for studying behaviour, little is known about its behaviour in the wild (Greven 2010). Especially little is known about how the environmental factors that influence social behaviours in halfbeaks. It is also not known, precisely which factors influence halfbeak behaviour (for example, what type of predation halfbeaks face (Ho et al. 2015)), and therefore it is not clear which environmental factors are most relevant. Moreover, halfbeaks in the wild were shown to behave differently than in the laboratory (Greven 2010). In this study, I attempt to increase our understanding 4 of halfbeak behaviour in the wild by looking at videos of wild populations taken at twelve different sites in Singapore, in their natural habitat. These twelve sites differ bothin abiotic and biotic environment and I expected these factors to influence the differences in observed behaviour. I looked into aggressive and mating behaviours, because they can be easily discerned in the videos. I also try to estimate general levels of activity in the videos. These three aspects of behaviour can be viewed as similar to three facets of individuals temperament (aggression, sociability and activity), that are known to be influenced by environmental factors and stressors like, predation risk (Réale et al. 2007; Heinen-Kay et al. 2016). While the studies of wild halfbeaks are limited, they have been shown to share several life history traits with poeciliid fishes (e.g. guppies,Poecilia ( reticulata) (Reznick et al. 2007). Both groups of fish are live-bearing, and engage in frequent social interactions, establish dominance hierarchies and exhibit mate choice (Greven 2010). Drawing from knowledge of other species, I therefore expect environmental factors to have predictable effects in halfbeaks. One of the potential hypotheses is that halfbeaks in different environ- ments would encounter different predation risk influenced by environmental factors. As mentioned previously,