Black and white stripes rug8/1/2023 ( b) Motion maps generated from the same down-sampled images expanding from the central landing point. Yellow star illustrates a fictive landing site towards which a virtual fly is heading. Dashed squares illustrate horsefly views chosen at 4.0 m and 0.5 m distances. Illustration of the main test hypotheses. Even with featureless animal coats, contrast gain would ensure that there would be some (minimal) contrast on the coat to provide the optic flow that flies rely upon for executing smooth landings. The visual control of flight by sensing and responding to optic flow is an essential component of the visual ecology of flying insects and is considered to be strongly conserved across the arthropoda. As a fly, moving at a constant speed, approaches an object, the surface looms ever faster and the fly responds by reducing its speed to keep the rate of looming constant, resulting in a slow controlled landing. Optic flow is the pattern of apparent motion caused by relative movement between an observer and the scene. Specifically, stripes could interfere with the control of visually guided flight by disrupting optic flow. Given the striking appearance of zebras ( figure 1 a), it is often argued that regularly spaced stripes of zebra pelage somehow confuse the visual system of flying insects. The mechanism by which stripes exert these effects remains opaque. Parallel observations revealed that tabanids failed to decelerate when approaching zebras compared to horses and, anecdotally, flies were seen to bump into zebra pelage or fly past it. Recently, we showed that tabanid horseflies approach domestic horses wearing striped rugs as often as the same horses covered with black or white rugs but that far fewer tabanids actually landed on horses with these striped external appearances. (iv) In contrast with these ideas, stripes are an established potent force in reducing landings of biting flies based on experimental studies with striped artificial targets, horse models, human models, painted cows, and comparisons of live plains zebras with domestic horses. Moreover, experimental tests reveal no cooling benefits associated with striped objects or striped pelts. (iii) A third theme, that black and white stripes have different heat signatures that set up convection currents that cool zebras, could only operate under very restricted circumstances: over the animal's dorsum, not over its flank or legs, and, problematically, not under breezy conditions or when the animal moves, as these would negate convection currents acting anywhere on the body. Most damning, lions kill zebras more than expected from their abundance across 40 study sites in Africa suggesting that confusion is an unlikely functional explanation for stripes. (ii) Conjectures about stripes confusing predators are poorly supported by observations of plains zebras ( Equus quagga) fleeing in ways that do not enhance protean behaviour, nor obscure the outline of individual animals, and because they do not promote motion dazzle or cause lions to misdirect their attack. (i) Early ideas about stripes being a form of crypsis against predators have now been dismissed on grounds that large carnivores are only able to resolve stripes very close up and by experiments showing that zebra stripes are highly conspicuous to human observers. For convenience, they, and the evidence for and against them, can be collapsed into four principal themes. The unusual and striking colouration pattern of the three species of extant zebra ( Equus sp.) has generated many intriguing functional explanations over the last 150 years. Our data narrow the menu of fly-equid visual interactions that form the basis for the extraordinary colouration of zebras. Our observations that flies avoided checked patterns in a similar way to stripes refutes the hypothesis that stripes disrupt optic flow via the aperture effect, which critically demands parallel striped patterns. We found that flies avoided landing on, flew faster near, and did not approach as close to striped and checked rugs compared to grey. By recording and reconstructing tabanid fly behaviour around horses wearing differently patterned rugs, we could tease out these hypotheses using realistic target stimuli. misregistration of repeated features) of evenly spaced stripes. generation of false motion cues by straight edges), or through spatio-temporal aliasing (i.e. This could occur through disrupting the radial symmetry of optic flow via the aperture effect (i.e. A logical and popular hypothesis is that stripes interfere with optic flow patterns needed by flying insects to execute controlled landings. Of all hypotheses advanced for why zebras have stripes, avoidance of biting fly attack receives by far the most support, yet the mechanisms by which stripes thwart landings are not yet understood.
0 Comments
Leave a Reply.AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |