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Contaminant exposure is known to alter behaviour in a wide range of contexts, including animal contests. We exposed hermit crabs to sublethal concentrations of copper, a common contaminant in coastal ecosystems. When hermit crabs engage in shell fights over empty gastropod shells they adopt two distinct roles. Attackers initiate the fight and perform bouts of vigorous shell fighting, while defenders remain withdrawn into their shells until the encounter is resolved. This role asymmetry means that the two opponents make different strategic decisions during the contest and we utilized an experimental design that allowed us to assess the effect of copper exposure independently on each role. We found that exposure of attackers to copper reduced the number of fights that were initiated, leading to fewer fights in total where the defender was evicted. Of attackers that did fight, those that were exposed to copper were also less likely to evict the defender, but this effect varied with the size difference between the crabs. During the fight, copper exposure caused attackers to rap less vigorously. When defenders were exposed to copper, however, this had no effect on the defender's chance of resisting eviction. Thus, we show for the first time that exposure to a contaminant can alter the outcome of asymmetric contests via differential effects on each of the two roles.

Internally fertilizing hermaphroditic animals show a bewildering variety of mating behaviours and mechanisms and many species have reciprocal sperm exchange. When matings are frequent and costly, partners are predicted to donate more sperm when they receive more sperm (i.e. conditional sperm exchange), but this has been shown experimentally only in sea slugs, free-living flatworms and possibly cestodes. We studied mate choice and sperm reciprocity patterns in the hermaphroditic land snail Succinea putris . Mate choice was random with respect to shell size. However, in matings where the partners were of unequal size, significantly more matings were observed where a small active individual mated on top of a larger inactive individual than the reverse. We suggest that this may be the result of mating on a vertical substrate or hanging underneath a horizontal substrate, because under these conditions it may be easier for a larger, inactive individual to carry a smaller, active one, than vice versa. The number of sperm transferred during mating was highly variable and was not related to the size of the donor, the size of the recipient, the size difference between the two partners or mating duration. Twelve of the 87 matings involved unilateral sperm exchange. There was no evidence for conditional sperm digestion. There was a significant, but weak positive relation between the numbers of sperm donated by the two partners. Hence, it remains unclear whether sperm exchange in S. putris is conditional or not.

An animal's ability to win intrasexual conflicts is often determined by the combined influence of several different traits, such as size, experience, residency and prior injury. When each of two (or more) asymmetries favours a different rival, the outcome of a conflict will reflect each asymmetry's contribution to overall status. We investigated the interacting effects of size difference and prior loss of forelegs (a common injury in nature) on the ability of males to win intrasexual conflicts in Trite planiceps, a New Zealand jumping spider (Salticidae). When both rivals were intact, the probability of the larger rival winning increased with the size difference between rivals. When rivals were of matching size, injury was a strong predictor of outcome; intact spiders beat rivals missing one or both forelegs and spiders missing one foreleg beat rivals missing both forelegs. During conflicts between spiders that differed both in size and in injury state, these two variables had combined effects on outcome. The tendency for larger rivals to win increased with size difference, but the probability of the larger rival winning diminished if the larger rival of the pair was the more injured. We used inverse prediction from logistic regression models to quantify the size advantage that is sufficient to offset the disadvantage of prior injury.Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Abstract. A field experiment offering wild passerine birds three different sizes of pastry bait was performed to investigate whether the birds could use the size of the baits as a label for palatability. The predator population avoided medium-sized baits when they were made unpalatable with quinine and mustard powder if the size difference between the baits was large enough. The limits of this size-distinguishing ability varied with experience. Birds that learned a large size difference were better able to distinguish a smaller difference. The birds appeared both to learn with experience of the task, and to resample each day rather than generalizing from one day to the next. They did not appear to be able to detect quinine by sight or smell. The consequences of the ability to use size as a label of palatability are discussed in the context of a model-mimic relationship and the frequently smaller size of Batesian mimics.

This paper examines the choice of prey size by captive pied kingfishers. Experiments consisted of simultaneous choices between two prey items of different sizes presented at three depths. The pied kingfishers chose the larger prey in the majority of trials. The probability of choosing the larger prey (‘selectivity’) was positively correlated with the relative size difference between prey. Selectivity increased with increased prey depth. Latency to capture was not correlated with relative size difference but increased with increased prey depth. These results suggest that comparison between prey items was more important than absolute prey size in the kingfisher's prey choice. Some unique foraging and visual problems of fish-eating birds are discussed.

Fights between male Euophrys parvula, a New Zealand salticid spider, consist of a number of discrete ‘stages’, of increasing intensity. Two aspects of these contests were investigated: (1) relative body size of the two opponents, and (2) the presence of a ‘resource’ (a female model). In 92% of all contests where a size difference existed, the larger of the two opponents won. Contest intensity (measured as the highest intensity behaviour elicited during the contest) was inversely correlated with relative body size of the two opponents (measured as carapace width). Contests escalated further when the female model was present. No relationship was found between contest duration and either relative body size or the presence of the model. It is suggested that for contests in which a number of behaviours of different intensity are used, contest intensity may be a better estimate of contest cost than duration. The results are discussed in terms of theoretical models of contest behaviour.

Dryomyza anilis males defend the small carcasses on which females oviposit as well as the females themselves during copulation and oviposition. Contests over territories and females, were compared to assess the way different asymmetries affect the outcome of a struggle. In contests over females with a lot of eggs the rate of take-over was lower than in contests over immature females, but the size of the female or her size relative to the owner did not affect the outcome. In territorial disputes and territories was the size difference between the opponents. Fifty-three per cent of the contests over females and 39% of territorial struggles resulted in a take-over. In take-overs, owners were smaller than copulating males or territory owners on average. Take-overs are part of a process in which resource owners continue to change until one is strong enough to defend the resource successfully. Contests over females lasted significantly longer than those over territories, which suggests that for males females are more valuable than territories. This is to be expected as territories are of value because they ultimately increase the probability of obtaining females. A positional advantage or the higher payoff of the owner in disputes over females may be the reason for the lower rate of take-over in female defence than in territorial struggles between opponents of similar size.