Passive Prey Discrimination in Surface Predatory Behaviour of Bait-Attracted White Sharks from Gansbaai, South Africa

Between the years 2008 and 2013, six annual research expeditions were carried out at Dyer Island (Gansbaai, South Africa) to study the surface behaviour of white sharks in the presence of two passive prey: tuna bait and a seal-shaped decoy. Sightings were performed from a commercial cage-diving boat over 247 h; 250 different white sharks, with a mean total length (TL) of 308 cm, were observed. Of these, 166 performed at least one or more interactions, for a total of 240 interactions with bait and the seal-shaped decoy. In Gansbaai, there is a population of transient white sharks consisting mainly of immature specimens throughout the year. Both mature and immature sharks preferred to prey on the seal-shaped decoy, probably due to the dietary shift that occurs in white sharks whose TL varies between 200 cm and 340 cm. As it is widely confirmed that white sharks change their diet from a predominantly piscivorous juvenile diet to a mature marine mammalian diet, it is possible that Gansbaai may be a hunting training area and that sharks show a discriminate food choice, a strategy that was adopted by the majority of specimens thanks to their ability to visualize energetically richer prey, after having been attracted by the odorous source represented by the tuna bait.

Effects of sensory mode in prey discrimination and predatory behaviour of rock lizards

An essential part of foraging ecology is to understand the processes of detection, recognition and discrimination of prey, as well as the sensorial modalities involved. Often, predators do not rely on a single sensory system but on multiple interacting senses. Specifically, lizards mainly use vision and vomerolfaction for prey pursuit. Here, we used an experimental approach to study how the Carpetan rock lizard, Iberolacerta cyreni, responds to different types of stimuli (chemical, visual, or both combined) from two prey species. The number of individuals approaching the prey and the number of attacks differed between treatments, however, we did not find differences in latency time, number of individuals attacking the prey or number of tongue flicks. Our results suggested that visual cues combined with chemical stimuli enhanced detection of both prey species and that prey discrimination occurred posteriorly and independently of using any or both types of stimuli.

Prophylactic Avoidance of Hazardous Prey by the Ant Host Myrmica rubra

Ants are the hosts of many microorganisms, including pathogens that are incidentally brought inside the nest by foragers. This is particularly true for scavenging species, which collect hazardous food such as dead insects. Foragers limit sanitary risks by not retrieving highly infectious prey releasing entomopathogenic fungal spores. This study investigates whether similar prophylactic strategies are also developed for food associated with weak or delayed risks of fungal contamination. We compared, in Myrmica rubra ant colonies, the retrieval dynamics of dead flies that were (1) conidia-free, (2) covered with a low amount of Metarhizium brunneum entomopathogenic conidia or (3) recently fungus-killed but not yet sporulating. Foragers mostly avoided fungus-killed prey and delayed the retrieval of conidia-covered flies. A second sanitary filter occurred inside the nest through a careful inspection of the retrieved prey. Ultimately, ants mostly consumed conidia-free and conidia-covered flies, but they relocated and discarded all fungus-killed prey outside of the nest. Our study confirms that, as a host of generalist entomopathogenic fungi, Myrmica rubra ants have developed a prophylactic avoidance and a differential management of prey depending on their infectious potential. We discuss the functional value as well as the possible cues underlying pathogen avoidance and prey discrimination in ants.

Pathogen avoidance and prey discrimination in ants

Insect societies have developed sanitary strategies, one of which is the avoidance of infectious food resources as a primary line of defence. Using binary choices, we investigated whether Myrmica rubra ants can identify prey that has been artificially infected with the entomopathogenic fungus, Metarhizium brunneum . We compared the ants' foraging behaviour towards infected prey at three different stages of fungus development : (i) prey covered with fungal conidia, (ii) prey freshly killed by the fungus and (iii) sporulating prey. Most foragers retrieved a corpse covered with a high number of spores but they consistently avoided a sporulating prey and collected less prey that had recently died from fungal infection. Furthermore, ant responses were highly variable, with some individuals retrieving the first prey they encountered while others inspected both available prey before making a decision. Workers were not repelled by the simple presence of fungal conidia but nevertheless, they avoided retrieving cadavers at later stages of fungal infection. We discuss how these different avoidance responses could be related to: differences in the ants’ perceptive abilities; physico-chemical cues characterizing fungus-infected prey or in the existence of physiological or behavioural defences that limit sanitary risks associated with potentially contaminated resources.