Visual odometry of Rhinecanthus aculeatus depends on the visual density of the environment.

Distance travelled is a crucial metric that underpins an animal's ability to navigate in the short-range. While there is extensive research on how terrestrial animals measure travel distance, it is unknown how animals navigating in aquatic environments estimate this metric. A common method used by land animals is to measure optic flow, where the speed of self-induced visual motion is integrated over the course of a journey. Whether freely-swimming aquatic animals also measure distance relative to a visual frame of reference is unclear. Using the marine fish Rhinecanthus aculeatus, we show that teleost fish can use visual motion information to estimate distance travelled. However, the underlying mechanism differs fundamentally from previously studied terrestrial animals. Humans and terrestrial invertebrates measure the total angular motion of visual features for odometry, a mechanism which does not vary with visual density. In contrast, the visual odometer used by Rhinecanthus acuelatus is strongly dependent on the visual density of the environment. Odometry in fish may therefore be mediated by a movement detection mechanism akin to the system underlying the optomotor response, a separate motion-detection mechanism used by both vertebrates and invertebrates for course and gaze stabilisation.

Avoidance of different durations, colours and intensities of artificial light by adult seabirds.

There is increasing evidence for impacts of light pollution on the physiology and behaviour of wild animals. Nocturnally active Procellariiform seabirds are often found grounded in areas polluted by light and struggle to take to the air again without human intervention. Hence, understanding their responses to different wavelengths and intensities of light is urgently needed to inform mitigation measures. Here, we demonstrate how different light characteristics can affect the nocturnal flight of Manx shearwaters Puffinus puffinus by experimentally introducing lights at a colony subject to low levels of light pollution due to passing ships and coastal developments. The density of birds in flight above the colony was measured using a thermal imaging camera. We compared number of flying shearwaters under dark conditions and in response to an artificially introduced light, and observed fewer birds in flight during 'light-on' periods, suggesting that adult shearwaters were repelled by the light. This effect was stronger with higher light intensity, increasing duration of 'light-on' periods and with green and blue compared to red light. Thus, we recommend lower light intensity, red colour, and shorter duration of 'light-on' periods as mitigation measures to reduce the effects of light at breeding colonies and in their vicinity.

Shearwaters know the direction and distance home but fail to encode intervening obstacles after free-ranging foraging trips.

While displacement experiments have been powerful for determining the sensory basis of homing navigation in birds, they have left unresolved important cognitive aspects of navigation such as what birds know about their location relative to home and the anticipated route. Here, we analyze the free-ranging Global Positioning System (GPS) tracks of a large sample (n = 707) of Manx shearwater, Puffinus puffinus, foraging trips to investigate, from a cognitive perspective, what a wild, pelagic seabird knows as it begins to home naturally. By exploiting a kind of natural experimental contrast (journeys with or without intervening obstacles) we first show that, at the start of homing, sometimes hundreds of kilometers from the colony, shearwaters are well oriented in the homeward direction, but often fail to encode intervening barriers over which they will not fly (islands or peninsulas), constrained to flying farther as a result. Second, shearwaters time their homing journeys, leaving earlier in the day when they have farther to go, and this ability to judge distance home also apparently ignores intervening obstacles. Thus, at the start of homing, shearwaters appear to be making navigational decisions using both geographic direction and distance to the goal. Since we find no decrease in orientation accuracy with trip length, duration, or tortuosity, path integration mechanisms cannot account for these findings. Instead, our results imply that a navigational mechanism used to direct natural large-scale movements in wild pelagic seabirds has map-like properties and is probably based on large-scale gradients.

Wake sorting, selective predation and biogenic mixing: potential reasons for high turbulence in fish schools.

There has been debate about animals' contribution to ocean circulation, called biomixing, or biogenic mixing. The energy input of schooling fish is significant but the eddies may be too small; so energy is dissipated as heat before impacting oceanic structure. I suggest that high turbulence caused by some very large aggregations of small animals has an important impact via a more direct ecosystem feedback process than overall ocean circulation. In the model presented here, large schools exhibit cooperative behavior and act like giant sieves grading zooplankton through individual swimmer's wakes, which focus the best prey in predictable positions. Following schoolers exploit these patterns. Then schools leave, in their wakes, chaotic turbulence enhancing growth of the smaller zooplankton and phytoplankton which has been graded out by the school. The result is a different community structure of plankton than would exist without such biomixing. Changes to plankton abundance and community structure on oceanic scales over the past century are correlated to overfishing and are consistent with this concept.