Abstract
Central place foraging theory (CPF) has been used to predict the optimal patch residence time for air-breathing marine predators in response to patch quality. Humpback whales (Megaptera novaeangliae) forage on densely aggregated prey, which may induce drastic change in prey density in a single feeding event. Thus, the decision whether to leave or stay after each feeding event in a single dive in response to this drastic change, should have a significant effect on prey exploitation efficiency. However, whether humpback whales show adaptive behavior in response to the diminishing prey density in a single dive has been technically difficult to test. Here, we studied the foraging behavior of humpback whales in response to change in prey density in a single dive and calculated the efficiency of each foraging dive using a model based on CPF approach. Using animal-borne accelerometers and video loggers attached to whales, foraging behavior and change in relative prey density in front of the whales were successfully quantified. Results showed diminishing rate of energy intake in consecutive feeding events, and humpback whales efficiently fed by bringing the rate of energy intake close to maximum in a single dive cycle. This video-based method also enabled us to detect the presence of other animals around the tagged whales, showing an interesting trend in behavioral changes where feeding duration was shorter when other animals were present. Our results have introduced a new potential to quantitatively investigate the effect of other animals on free-ranging top predators in the context of optimal foraging theory.
Highlights
Predators should modify their foraging behavior to efficiently exploit prey whose density and availability dynamically changes over time
We hypothesize that, 1) the gain function within a single foraging dive will show a diminishing rate of energy intake and 2) humpback whales will leave the prey patch when the total rate of energy intake (En) in a single dive cycle is maximized: En in a single dive cycle is maximized when it overlaps with the rate of energy intake (Ec) (Fig 1B)
Recent study of humpback whales using a ship-mounted echo-sounder revealed that the foraging decisions of humpback whales are driven by both prey depth and density
Summary
Predators should modify their foraging behavior to efficiently exploit prey whose density and availability dynamically changes over time. Foraging behavior of rorqual whales was previously studied by attaching multi-sensor digital archival tags onto whales, in combination with prey distribution data measured using shipmounted echo-sounders [12, 22] This method succeeded in providing many fruitful insights on the whales’ foraging strategy across patches, but the spatiotemporal resolution was not high enough to detect the change in prey density on a single-dive scale. We defined the energy gain per unit time over a single dive as a currency optimized by humpback whales and the travel time, post-surface recovery time, and change in rate of energy intake in relation to change in prey density over time as constraints Under this condition, we hypothesize that, 1) the gain function within a single foraging dive will show a diminishing rate of energy intake (diminishing return) and 2) humpback whales will leave the prey patch when the total rate of energy intake (En) in a single dive cycle is maximized: En in a single dive cycle is maximized when it overlaps with the rate of energy intake (Ec) (Fig 1B). ; investigated how humpback whales adjust their foraging duration in response to presence/absence of other individuals
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