Observational learning and irreversible starvation in first-feeding zebrafish larvae: is it okay to copy from your friends?

Starvation is one cause of high mortality during the early life stages of many fish species. If larvae do not learn to feed, or if no food is available during early stages, irreversible starvation occurs and larvae reach the Point of No Return (PNR), the developmental period/age when they will not feed even if food is available. Fish larvae may learn to how to feed by observing conspecifics or through personal/individual experience with prey items that are encountered. We examined food acquisition in first-feeding zebrafish larvae to determine the impact of delayed feeding and identify the time of irreversible starvation and the PNR. Next, we examined how feeding ability, and the PNR, is altered by either observational learning or previous experience, to determine which paradigm facilitates successful feeding.Our data indicate that zebrafish larvae learn to feed, with the PNR at 7-8 days postfertilization (dpf). Exposure to prey items immediately after hatching (3-5 dpf) results in the highest survival rates. Zebrafish larvae learning to feed by observing conspecifics also had high survival, though the PNR was not changed. In contrast, previous experience with prey items caused an earlier PNR and reduced survival. Overall, these results that indicate feeding is a learned behavior in zebrafish larvae and interacting with/observing conspecifics during the early larval period is a better predictor of feeding ability than previous experience with food.

Eavesdropping grey squirrels infer safety from bird chatter.

When multiple species are vulnerable to a common set of predators, it is advantageous for individuals to recognize information about the environment provided by other species. Eastern gray squirrels (Sciurus carolinensis) and other small mammals have been shown to exploit heterospecific alarm calls as indicators of danger. However, many species-especially birds-emit non-alarm auditory cues such as contact calls when perceived predator threat is low, and such public information may serve as cues of safety to eavesdroppers. We tested the hypothesis that eavesdropping gray squirrels respond to "bird chatter" (contact calls emitted by multiple individuals when not under threat of predation) as a measure of safety. We compared vigilance behavior of free-ranging squirrels in the presence of playbacks of bird chatter vs non-masking ambient background noise lacking chatter after priming them with a playback recording of a red-tailed hawk (Buteo jamaicensis) call. Squirrels responded to the hawk call playbacks by significantly increasing the proportion of time they spent engaged in vigilance behaviors and the number of times they looked up during otherwise non-vigilance behaviors, indicating that they perceived elevated predation threat prior to the playbacks of chatter or ambient noise. Following the hawk playback, squirrels exposed to the chatter treatment engaged in significantly lower levels of vigilance behavior (i.e., standing, freezing, fleeing, looking up) and the decay in vigilance behaviors was more rapid than in squirrels exposed to the ambient noise treatment, suggesting squirrels use information contained in bird chatter as a cue of safety. These findings suggest that eastern gray squirrels eavesdrop on non-alarm auditory cues as indicators of safety and adjust their vigilance level in accordance with the vigilance level of other species that share the same predators.