When more is less: the fitness consequences of predators attacking more unpalatable prey when more are presented

In 1879, Fritz Müller hypothesized that mimetic resemblance in which defended prey display the same warning signal would share the costs of predator education. Although Müller argued that predators would need to ingest a fixed number of prey with a given visual signal when learning to avoid unpalatable prey, this assumption lacks empirical support. We report an experiment which shows that, as the number of unpalatable prey presented to them increased, avian predators attacked higher numbers of those prey. We calculated that, when predators increase attacks, the fitness costs incurred by unpalatable prey can be substantial. This suggests that the survival benefits of mimicry could be lower than Müller proposed. An important finding is, however, that these costs decline in importance as the total number of available prey increases.     

Experienced chicks show biased avoidance of stronger signals: an experiment with natural colour variation in live aposematic prey

An important factor for understanding the evolution of warning coloration in unprofitable prey is the synergistic effect produced by predator generalisation behaviour. Warning coloration can arise and become stabilised in a population of solitary prey if more conspicuous prey benefit from a predator's previous interaction with less conspicuous prey. This study investigates whether domestic chicks (Gallus gallus domesticus) show a biased generalisation among live aposematic prey by using larvae of three species of seed bugs (Heteroptera: Lygaeidae) that are of similar shape but vary in the amount of red in the coloration. After positive experience of edible brownish prey, chicks in two reciprocal experiments received negative experience of either a slightly red or a more red distasteful larva. Attacking birds were then divided into two treatment groups, – one presented with the same prey again, and one presented with either a less red or a more red larva. Birds with only experience of edible prey showed no difference in attack probability of the two aposematic prey types. Birds with experience of the less red prey biased their avoidance so that prey with a more red coloration was avoided to a higher degree, whereas birds with experience of the more red prey avoided prey with the same, but not less red coloration. Thus, we conclude that bird predators may indeed show a biased generalisation behaviour that could select for and stabilise an aposematic strategy in solitary prey. This revised version was published online in July 2006 with corrections to the Cover Date.     

Learning of colour and position cues in domestic chicks: Males are better at position,females at colour

Male and female chicks were trained to discriminate between two boxes for food reinforcement. The correct box was indicated by a colour cue (red or brown) and a position cue (right or left). After learning, the colour and the position cues were dissociated: the right-left location of the two boxes was alternated between trials according to a semi-random sequence.The chicks were thus retrained to discriminate either on the basis of colour (irrespective of position) or on the basis of position (irrespective of colour). There were no sex differences, during training, with both position and colour cues. However, during re-training females performed better on the colour learning task and males performed better on the position learning task.     

Conspecifics as informers and competitors: an experimental study in foraging bumble-bees

Conspecifics are usually considered competitors negatively affecting food intake rates. However, their presence can also inform about resource quality by providing inadvertent social information. Few studies have investigated whether foragers perceive conspecifics as informers or competitors. Here, we experimentally tested whether variation in the density of demonstrators ('none', 'low' and 'high'), whose location indicated flower profitability, affected decision-making of bumble-bees Bombus terrestris. Bumble-bees foraged on either 'simple' (two colours) or 'complex' (four colours) artificial floral communities. We found that conspecifics at low density may be used as sources of information in first flower choices, whereas they appeared as competitors over the whole foraging sequence. Low conspecific densities improved foragers' first-visit success rate in the simple environment, and decreased time to first landing, especially in the complex environment. High conspecific densities did not affect these behavioural parameters, but reduced flower constancy in both floral communities, which may alter the efficiency of pollinating visits. These results suggest that the balance of the costs and benefits of conspecific presence varies with foraging experience, floral community and density. Spatio-temporal scales could thus be an important determinant of social information use. This behavioural flexibility should allow bumble-bees to better exploit their environment.     

Avian cestodes affect the behaviour of their intermediate host Artemia parthenogenetica: an experimental study

The brine shrimp Artemia parthenogenetica (Crustacea, Branchiopoda) is intermediate host for several cestode species whose final hosts are waterbirds. Previous field studies have shown that brine shrimps infected with cestodes have a bright red colour and are spatially segregated in the water column. However, the ethological mechanisms explaining such field observations are unknown. Changes in appearance and behaviour induced by trophically transmitted parasites have been shown to increase the risk of predation by the final host. In this experimental study, we compared the behaviour of uninfected Artemia and those infected by avian cestodes. We found that parasitised individuals behave differently from unparasitised ones in several ways. In contrast to uninfected individuals, infected brine shrimps were photophilous and showed increased surface-swimming behaviour. These observations suggest that the modified behaviour (in addition to the bright red colour of the majority of the infected individuals) results in infected brine shrimps becoming more vulnerable to avian final hosts, which facilitates parasite transmission. We discuss our results in terms of the adaptive nature of behavioural changes and their potential implications for the hypersaline ecosystem.