Lateralized behaviors benefit individuals by increasing task efficiency in foraging and anti-predator behaviors []. The conventional lateralization paradigm suggests individuals are left or right lateralized, although the direction of this laterality can vary for different tasks (e.g. foraging or predator inspection/avoidance). By fitting tri-axial movement sensors to blue whales (Balaenoptera musculus), and by recording the direction and size of their rolls during lunge feeding events, we show how these animals differ from such a paradigm. The strength and direction of individuals’ lateralization were related to where and how the whales were feeding in the water column. Smaller rolls (≤180°) predominantly occurred at depth (>70 m), with whales being more likely to rotate clockwise around their longest axis (right lateralized). Larger rolls (>180°), conversely, occurred more often at shallower depths (<70 m) and were more likely to be performed anti-clockwise (left lateralized). More acrobatic rolls are typically used to target small, less dense krill patches near the water’s surface [], and we posit that the specialization of lateralized feeding strategies may enhance foraging efficiency in environments with heterogeneous prey distributions.

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Figure 1 Lateralized feeding in blue whales. Show full caption (A) Artist rendition of the two types of lunge feeding strategies from side-on orientation (i.e. X–Z plane) — barrel and side-rolls. The top schematic (1) shows a left-sided barrel roll where the whale rotates a full 360° during prey capture. The bottom graphic (2) shows a right-sided roll, where the whale rotates less than 180° during the feeding event. The estimated angle of visual range is shown as a white cone and demonstrates that during the left-side roll, the whale’s right eye is directed towards the prey until the lunge (mouth opening) is initiated. (B) Distributions of the maximum rolls angles for left (yellow) and right (blue) roll directions. The dashed line at 180° represents the separation of classification of roll types (barrel rolls above, and side rolls below 180°, respectively). (C) Heat-plot showing the size of individual whales’ rolls as a function of depth. The majority of side rolls (≤ 180 degrees) are performed deeper than 70 m, whereas larger barrel rolls usually occur in the top 70 meters of the water column. Three rolls greater than 400° were excluded from (B) and (C) for clarity. (D) Distribution of the observed laterality indexes of individuals (red) (n = 49) and expected laterality indexes assuming no individual-level lateralisation (blue) (see Supplementary Information). These two distributions differ significantly from one another (χ2 = 38.9, df = 1, p < 0.001). (E) Relationship between the laterality index of an individual and the mean size of its rolls. The larger the size of an individual’s rolls, the more likely it was to be left lateralized. (F) Relationship between the mean depth an individual was feeding at and its laterality index. Individuals feeding at shallower depths were more likely to be left lateralized. In (E) and (F), shaded regions represent the 95% confidence intervals for the fitted regression lines. Blue whales (n = 63 individuals) exhibited stereotyped maneuvers during lunge feeding events (n = 2,863 lunges in total; 45 ± 5.3 (mean ± SE) lunges from each individual, Figure S1 A in Supplemental Information , published with this article online). Immediately before a whale opened its mouth to capture prey, it made a rolling movement around its longest axis ( Figure 1 A). Two types of rolling behavior were associated with these lunges — ‘side-rolls’ and ‘barrel-rolls’. Smaller side-rolls consisted of the whale rotating ≤ 180° in one direction during the feeding lunge, followed by a rotation in the opposite direction to its initial rotation (i.e. non-complete rotation) ( Figure 1 A). In contrast, larger, more acrobatic ‘barrel-rolls’ consisted of a uni-directional roll past the horizontal (i.e > 180° rotation) ( Figure 1 A,B). While the majority of side-rolls were performed deeper than 70 m, the majority of barrel-rolls were performed in the upper 70 m of the water column ( Figure 1 C).

where R r and L r are the numbers of rolls that an individual made to the right and left, respectively. At the population level, the distribution of laterality indices differed significantly from what would have been expected assuming no individual-level lateralization (χ2 = 38.9, df = 1, p < 0.001; These rolls also have directionality, occurring when a whale initially rolls to the left or right. To assess whether rolls were lateralized at the population and individual levels, we calculated a laterality index (LI) for each individual that made 10 or more rolls (n = 49 individuals). The LI of each individual was calculated aswhere Rand Lare the numbers of rolls that an individual made to the right and left, respectively. At the population level, the distribution of laterality indices differed significantly from what would have been expected assuming no individual-level lateralization (χ= 38.9, df = 1, p < 0.001; Figure 1 D). There were both more individuals that were left and right lateralized in the population than would be expected by chance ( Figure 1 D). At the individual level, 28 of the 49 individuals we measured had absolute laterality indices that differed significantly from chance ( Figure S2 ). Of these, there were significantly more right-lateralized individuals than left-lateralized individuals (binomial test, n = 21, N = 28, P = 0.006; Figure 1 D).

s = 0.78, n = 55, p < 0.001; s = –0.40, n = 49, p = 0.005; Individuals were consistent in the size of their rolls to the left or right (Spearman Rank Correlation: r= 0.78, n = 55, p < 0.001; Figure S2 B); some whales made consistently larger rolls whereas some made consistently smaller rolls. Individuals that made larger rolls were more likely to be left lateralized, whereas individuals that made smaller rolls were more likely to be right lateralized (Spearman Correlation, r= –0.40, n = 49, p = 0.005; Figure 1 E). Further, individuals’ laterality indexes were related to the mean depth at which feeding occurred (Pearson Correlation, R = 0.37, n = 49, p = 0.009; Figure 1 F). The shallower the depth at which individuals fed, the more likely individuals were to have a negative laterality index. There was no evidence, however, that individual whales made consistently the same sized rolls above or below 70 m (see Supplemental Information ).