[Show abstract][Hide abstract] ABSTRACT: In many hierarchical animal societies, dominant individuals control group membership owing to their power to evict subordinates. In such groups, the presence of subordinates, and therefore group stability, is continually dependent on subordinates being tolerated by dominants. The dominant decision to tolerate or evict is, in turn, dependent on the costs and benefits to dominants of subordinate presence. We investigated the effect of subordinate presence on dominants in the female dominance hierarchy of the dwarf angelfish Centropyge bicolor, using both observations of natural groups and experimental removals of subordinates. We found that the presence of subordinates had no effect on dominant access to resources, as measured by dominant foraging rates and home range areas, nor on dominant fitness, as measured by growth rates and spawning frequencies. Our results suggest that the presence of subordinates has a neutral effect on the current fitness of dominants, so that dominants have no great incentive to evict subordinates. We discuss the possibility that tolerance of subordinates might be further explained by considering future fitness, as dominant females in these haremic protogynous angelfish stand to inherit the male position, whereupon subordinate females change from potential competition to useful mates.
[Show abstract][Hide abstract] ABSTRACT: Social groups are often structured by dominance hierarchies in which subordinates consistently defer to dominants. High-ranking individuals benefit by gaining inequitable access to resources, and often achieve higher reproductive success; but may also suffer costs associated with maintaining dominance. We used a large-scale field study to investigate the benefits and costs of dominance in the angelfish Centropyge bicolor, a sequential hermaphrodite. Each haremic group contains a single linear body size-based hierarchy with the male being most dominant, followed by several females in descending size order. Compared to their subordinate females, dominant males clearly benefited from disproportionately high spawning frequencies, but bore costs in lower foraging rates and greater aggressive defence of their large territories. Within the female hierarchy, more dominant individuals benefited from higher spawning frequencies and larger home ranges, but displayed neither higher foraging rates nor spawn order priority. However, dominance in females was also linked to aggressiveness, particularly towards immediate subordinates, suggesting that females were using energetically costly aggression to maintain their high rank. We further showed by experimentally removing dominant females that the linear hierarchy was also a social queue, with subordinates growing to inherit higher rank with its attendant benefits and costs when dominants disappeared. We suggest that in C. bicolor, the primary benefit of high rank is increased reproductive success in terms of current spawning frequency and the prospect of inheriting the male position in the near future, which may be traded off against the cost of aggressively defending rank and territory.
[Show abstract][Hide abstract] ABSTRACT: Competition between individual group members is a key force in shaping social group structure and size. In animal societies, within-group competition may be structured by linear dominance hierarchies, which can be stable if there are minimum differences in competitive ability between adjacently ranked individuals. This requirement constrains maximum group size because only a certain number of clearly differentiated ranks can be fit into the range of competitive abilities between the top and bottom of the hierarchy. We investigated this hypothesis in the body size-based linear dominance hierarchy of the angelfish Centropyge bicolor. Unlike in previous studies, we found that maximum group size in natural C. bicolor groups was not always strictly limited by the range of possible body sizes and the average size difference between adjacent ranks. Oversized groups displayed a compressed body size hierarchy with smaller size differences between adjacent ranks, less effective regulation of subordinate foraging rates to maintain size differences, and greater spatial segregation between adjacently ranked individuals. Our results suggest that when spatial segregation compromises the regulatory mechanisms that maintain clear size hierarchies, groups can become larger than expected by slotting more individuals into a compressed size hierarchy. However, we also found that oversized groups tended to fission into smaller groups, suggesting that they are transient entities and that ultimately the group size limits imposed by the need to maintain a well-defined hierarchy are unavoidable. Copyright 2010, Oxford University Press.
Full-text · Article · Jun 2010 · Behavioral Ecology
[Show abstract][Hide abstract] ABSTRACT: Central to our understanding of social group formation and maintenance is the question of how within-group conflict resolution is achieved in the face of asymmetrical competition over resources and reproduction. A crucial yet implicit assumption of many conflict resolution models dealing with reproductive skew is that subordinates have perfect knowledge of the extent of conflict between themselves and their dominants, enabling behavioural responses on an individual rather than evolutionary scale. However, a mechanism enabling subordinates to accurately assess their relative conflict levels has yet to be empirically demonstrated. Here, we show in the angelfish Centropyge bicolor that the rate of overt mild aggression from dominants to subordinates acts as a signal of increasing rank conflict. The clarity of this signal can be reduced by spatial segregation, causing subordinates to be less able to respond appropriately by regulation of their foraging rates. A reduced signal ultimately leads to a less well-defined dominance hierarchy and destabilization of the social group. Our study suggests that, contrary to previous suggestions, dominant aggression rates play a crucial role as an accurate information signal required for the evolutionary stability of skew models.
Full-text · Article · May 2010 · Proceedings of the Royal Society B: Biological Sciences
[Show abstract][Hide abstract] ABSTRACT: In a recent paper, we showed that leadership arises from individual behavioral differences in pairs of foraging stickleback (Gasterosteus aculeatus). Foraging data from randomly combined pairs of fish were analyzed using Markov Chain models to infer the individual movement rules underlying joint behavior. While both fish responded to partner movement, bolder individuals were the least responsive and showed greater individual initiative. Shy partners were more faithful followers and were also found to bring about greater leadership tendencies in their bold partners. The ability of such followers to inspire bolder fish suggests that leadership may be dependent on individual temperament differences, reinforced by social feedback.
[Show abstract][Hide abstract] ABSTRACT: In many animal groups, certain individuals consistently appear at the forefront of coordinated movements [1-4]. How such leaders emerge is poorly understood [5, 6]. Here, we show that in pairs of sticklebacks, Gasterosteus aculeatus, leadership arises from individual differences in the way that fish respond to their partner's movements. Having first established that individuals differed in their propensity to leave cover in order to look for food, we randomly paired fish of varying boldness, and we used a Markov Chain model to infer the individual rules underlying their joint behavior. Both fish in a pair responded to each other's movements-each was more likely to leave cover if the other was already out and to return if the other had already returned. However, we found that bolder individuals displayed greater initiative and were less responsive to their partners, whereas shyer individuals displayed less initiative but followed their partners more faithfully; they also, as followers, elicited greater leadership tendencies in their bold partners. We conclude that leadership in this case is reinforced by positive social feedback.
[Show abstract][Hide abstract] ABSTRACT: Abstract 1. The costs and benefits of behavioural care of offspring can often be easily quantified through observations and experiments. Other forms of parental investment, on the other hand, are usually less amenable to cost–benefit analysis.
2. Here, the costs and benefits are estimated for protective egg coating by a chrysomelid beetle, Cryptocephalus hypochaeridis, where the female spends a considerable amount of time adding extra structural components to each of the eggs after laying them.
3. Adding this protective coating was very costly, both in terms of material and energy used: the mass of the extrachorion is equivalent to half the mass of the egg, and water loss and energy expenditure while coating the egg is equivalent to half what would be lost while laying a further egg.
4. Choice tests with egg predators demonstrated that these high costs are offset by benefits in terms of protection against predation: whereas uncoated eggs are readily eaten by predators, coated eggs are always rejected.
[Show abstract][Hide abstract] ABSTRACT: Non-random mating has been observed in many species of beetle, where larger or heavier males have a greater mating success. This difference in male mating success could be through direct competition between males, or female choice. We examined non- random mating in the leaf beetle Cryptocephalus hypochaeridis (L.) (Coleoptera Chrysomelidae). In the field, we found that suc- cessfully mating males were significantly heavier than unsuccessful males. We also found that the flowers in which we found beetles were significantly taller and wider than unoccupied flowers. However, we found no relationship between flower morphol- ogy and the mass of male occupants, suggesting that females are actively choosing the larger males. In the laboratory, females were found to show no preference for male size, and mated randomly. This suggests that mate choice in C. hypochaeridis is de- pendent upon cues other than flower size and height or male mass. We discuss what these cues might be, and how our results re- late to the mating strategies of chrysomelid beetles. We also describe the mating behaviour of C. hypochaeridis.
Full-text · Article · Jan 2006 · Bulletin of Insectology