You can run--or you can hide: optimal strategies for cryptic prey against pursuit predators

We consider the optimal behavior of a cryptic prey individualas it is approached by a predator searching for prey. Althoughthe predator has not yet discovered the prey, it has an increasinglikelihood of doing so as it gets closer to the prey. Further,the closer the predator is to the prey when it discovers it,the more likely the predator will be to capture the prey. Thesearguments suggest that the prey should flee before the predatordiscovers it. However, the act of fleeing will alert the predatorto the presence of the prey and trigger an attack that mightnot have occurred otherwise. We capture these conflicting outcomesin a mathematical model, which we then use to predict the optimalbehavior of the prey and predator. We argue that the optimalstrategy for the prey is either to run as soon as they detecta predator approaching or to only flee in response to havingbeen detected by the predator. Running as soon as the predatoris detected is associated with low predator search speeds, alow nonpredation cost to running, a large advantage to the preyin initiating chases rather than reacting, limited ability tospot the predator at distance, a high ability to spot prey bythe predator, and a high probability that chases will be successful.The optimal strategy for the predator depends on whether itscurrent trajectory is taking it closer to or further from theprey. In the latter case, the predator should attack immediatelyon discovering the prey; in the former case, it should delayits attack until it reaches the point on its current trajectorywhere distance to the prey is minimized.     

Changes in haematological parameters in relation to prey switching in a wild population of harbour seals

1. Previous studies have found marked inter-annual variation in winter food availability, diet composition and body condition in a population of harbour seals ( Phoca vitulina L.) in Northeast Scotland. This study aimed to determine whether there were other physiological consequences of prey switching by comparing haematological parameters in years when the clupeids herring and sprat dominated the diet and in years when seals switched to alternative prey.
2. There were significant differences in leukocyte and erythrocyte parameters in relation to diet composition. In contrast, indices of body condition did not explain the variation in haematological parameters, suggesting that the observed changes did not result from differences in the energetic content of the prey.
3. Leukocyte counts were significantly higher after 'good' clupeid years, although the differences in mean counts were small. Such differences could have resulted either from immuno-suppression, for example because of differences in prey nutrient or contaminant levels, or from differences in the pathogen challenge resulting from geographical variations in water quality.
4. The differences in erythrocyte parameters were more marked, and there was evidence of widespread macrocytic anaemia when seals switched from clupeids to alternative prey. Such differences could result either from acclimation, as a result of prey-specific foraging strategies, or from differences in the nutritional quality of prey.
5. These results indicate that generalist predators such as the harbour seal may exhibit physiological responses to changes in the composition of their diet. These data highlight the need to consider the long-term physiological effects of variations in food availability on the population dynamics of generalist marine top predators. In particular, it is hypothesized that fish-induced anaemia may be responsible for declines in certain pinniped populations.     

Absence of intraspecific interference during feeding by the predatory ladybirds Chilocorus spp. (Coleoptera: Coccinellidae)

Abstract. 1. The hypothesis was tested that intraspecific behavioural interference does not adversely affect the feeding behaviour of adults of three predatory coccinellid species, Chilocorus nigritus (F.), C.bipustulatus (L.) and C.infernalis Mulsant, at densities found under field conditions.
2. Feeding rates on mature oleander scale Aspidiotus nerii Bouché were evaluated by two methods at various predator densities. Proportion of the population dispersing was also measured for one of the species.
3. Feeding rate did not decrease and dispersal did not increase with increasing predator density. No significant behavioural interference that might have reduced predatory efficiency was observed, counter to assumptions on which published interference models are based.
4. Results here help to explain the relative importance of parasitoids and predators in the effective control of red scale Aonidiella aurantii (Mask.). The results also provide guidelines for release of these bio-control agents.