Summary When approaching a landing surface, many flying animals use visual feedback to control their landing. Here, we studied how foraging bumblebees ( Bombus terrestris ) use radial optic expansion cues to control in-flight decelerations during landing. By analyzing the flight dynamics of 4,672 landing maneuvers, we showed that landing bumblebees exhibit a series of deceleration bouts, unlike landing honeybees that continuously decelerate. During each bout, the bumblebee keeps its relative rate of optical expansion constant, and from one bout to the next, the bumblebee tends to shift to a higher, constant relative rate of expansion. This modular landing strategy is relatively fast compared to the strategy described for honeybees and results in approach dynamics that is strikingly similar to that of pigeons and hummingbirds. The here discovered modular landing strategy of bumblebees helps explaining why these important pollinators in nature and horticulture can forage effectively in challenging conditions; moreover, it has potential for bio-inspired landing strategies in flying robots.

Fish can move freely through the water column and make complex three-dimensional motions to explore their environment, escape or feed. Nevertheless, the majority of swimming studies is currently limited to two-dimensional analyses. Accurate experimental quantification of changes in body shape, position and orientation (swimming kinematics) in three dimensions is therefore essential to advance biomechanical research of fish swimming. Here, we present a validated method that automatically tracks a swimming fish in three dimensions from multi-camera high-speed video. We use an optimisation procedure to fit a parameterised, morphology-based fish model to each set of video images. This results in a time sequence of position, orientation and body curvature. We post-process this data to derive additional kinematic parameters (e.g. velocities, accelerations) and propose an inverse-dynamics method to compute the resultant forces and torques during swimming. The presented method for quantifying 3D fish motion paves the way for future analyses of swimming biomechanics.

Fish make C-starts to evade predator strikes. Double-bend (DB) C-starts consist of three stages: Stage 1, in which the fish rapidly bends into a C-shape; Stage 2, in which the fish bends in the opposite direction; and a variable Stage 3. In single-bend (SB) C-starts, the fish immediately straightens after Stage 1. Despite fish moving in three-dimensional (3D) space, fast-start responses of adult fish have mainly been studied in a horizontal plane. Using automated 3D tracking of multi-camera high-speed video sequences, we show that both SB and DB fast-starts by adult female least killifish () often contain a significant vertical velocity component, and large changes in pitch (DB up to 43 deg) and roll (DB up to 77 deg) angles. Upwards and downwards elevation changes are correlated with changes in pitch angle of the head; movement in the horizontal plane is correlated with changes in yaw angle of the head. With respect to the stimulus, escape heading correlates with the elevation of the fish at the onset of motion. Irrespective of the initial orientation, fish can escape in any horizontal direction. In many cases, the centre of mass barely accelerates during Stage 1. However, it does accelerate in the final direction of the escape in other instances, indicating that Stage 1 can serve a propulsive role in addition to its preparatory role for Stage 2. Our findings highlight the importance of large-scale 3D analyses of fast-start manoeuvres of adult fish in uncovering the versatility of fish escape repertoire.

Trypanosomes are important disease agents of humans, livestock and cold-blooded species, including fish. The cellular morphology of trypanosomes is central to their motility, adaptation to the host’s environments and pathogenesis. However, visualizing the behaviour of trypanosomes resident in a live vertebrate host has remained unexplored. In this study, we describe an infection model of zebrafish (Danio rerio) with Trypanosoma carassii. By combining high spatio-temporal resolution microscopy with the transparency of live zebrafish, we describe in detail the swimming behaviour of trypanosomes in blood and tissues of a vertebrate host. Besides the conventional tumbling and directional swimming, T. carassii can change direction through a ‘whip-like’ motion or by swimming backward. Further, the posterior end can act as an anchoring site in vivo. To our knowledge, this is the first report of a vertebrate infection model that allows detailed imaging of trypanosome swimming behaviour in vivo in a natural host environment.

The recent boost in bird migration studies following the development of various tracking devices raised awareness of how detrimental attaching devices can be for animals. Such effects can occur during migration, but also immediately post‐release if the device impairs escape flight performance and, consequently, the bird's ability to evade predators. In this study, we investigated the effect of carrying a device on the escape flight speed and aerodynamic force production in a migratory passerine. We recorded upward‐directed escape flights of 15 male blackcaps. Each individual was tested without a tag, and when equipped with three different leg‐loop dummy tags with masses representing around 3%, 5%, and 7% of their body mass. The experiment was designed such that all individuals passed through all treatments in a randomized order. We found that two factors affected flight speed in roughly equal amounts: first, tagged escape flights had lower flight speeds compared to the control flights, irrespective of tag mass. Second, we found an effect of the total mass, that is, the sum of the masses of the individual bird and of the tag, with heavier birds being slower. In contrast, flight speed was not correlated with relative tag mass in percentage of body mass, the metric commonly used in ethical guidelines for tag attachment. Aerodynamic flight force production also depended on total mass, with heavier birds producing higher forces. But these flight forces did not differ between flights with or without a tag. We conclude that, when tagging birds, it is misleading to choose heavy individuals for tagging in order to minimize the tag mass as a percentage of body mass. This is particularly relevant in species for which body mass is not necessarily related to size, like migratory birds that accumulate large fat reserves. The lower escape speed in “tagged” flights could not be explained by differences in net flight force production, because these did not differ between flights with and without a tag. This suggests that the tag also affected pre‐flight take‐off dynamics, possibly due to a leg harness‐induced reduction in leg push‐off performance.