Zebras are black with white stripes. The presence of stripes introduces an exceptionally assertive type of visual stimulation into the young animal’s world and this difference from other, un-striped animals is emphatically not trivial. Anyone who has watched zebras attentively will have become aware that eyes have to work overtime to accommodate to their every movement.
Each time an animal changes the angle of its body the eye registers the vertical stripes actually getting wider or narrower (depending on how close to broadside it is) while horizontal stripes, instead, keep a similar width. Research has shown that individual visual nerve cells in mammals are programmed to respond to certain primary properties of the seen environment and different cells register different properties. Thus, the process of seeing is essentially modular and is mediated by large batteries of specialized visual receptors.
Research has also shown that black and white stripes can trigger an identifiable selection of these specialized visual neurons, notably those that register: (a) brightness contrast; (b) color contrast; (c) edge contrast; (d) edge orientation; (e) spatial frequency; (f) direction of motion; and (g) temporal flicker. In real life it is likely that this battery of receptors plays their narrow roles as part of the machinery of seeing, helping eyes and brains to extrapolate such useful information as directional movements and distance and thus contribute to making sense of the total environment.
Somehow, zebras have evolved a pattern that isolates such visual responses from their normal environmental contexts. Black and white stripes represent a sort of abstraction of what eyes are hungry to see! Strong physiological responses can be stimulated in mammalian eyes by patterns that successfully isolate the particulate, component-specific properties of cells within the visual cortex.
Stripes, particularly vertical stripes, trigger responses in early-stage, motion-selective neurons, which are functionally tuned to register the direction of motion. It is this specialization of visual neurons that is central to the proposal that the overall striping of zebras has evolved in direct response to the separate sensibilities of these neurons. Also central to this concept is the detachment of striping from individuality, manifest in the observable conditioning, of young zebras, to respond indiscriminately, and with every sign of being ‘attracted’ to stripes, regardless of their ‘owner’.
As it matures, the young zebra seems to find almost any fellow zebra attractive and they can even be drawn towards zebras of another species (behavior that is particularly frequent among Plains Zebras Equus quagga and Grévy’s Zebras E. grevyi in Laikipia in Kenya). More important, especially for its general application to the evolution of visual signals of all sorts, the suggestion here is that visual signals are effectively ‘designed’ in the eyes of their beholders – in this case, the eyes of other zebras.
Central to understanding the evolution of stripes is relating their beginnings to their function in the behavioural repertoire. There may have been some shifts over time but some continuity must be expected. Stripes could, of course, have derived from the alteration of some pre-existent pattern with stripes aligning themselves out of, say, a scatter of cryptic spots and squiggles, such as are common on many juvenile mammals and birds that need to hide from predators.
Additionally, or alternatively, striping in zebras could have begun with exaggeration of some detail of surface morphology that was significant in the behavioural repertoire of the zebras’ common ancestor. A scan of the behaviour of contemporary zebras offers hints as to what that detail might have been and suggests an explicitly social function for stripes. The clue comes from one of the few easily observed signs of ‘friendliness’ among zebras.
Skin-nibbling is integrated into the single most important affiliation in zebra society, namely the mother–young bond. Directly and indirectly, that bond is likely to provide the social glue that holds zebra societies together. Furthermore, zebras do not nibble at random; they approach, nose to nose and prefer the point where the neck meets the shoulder above all other sites, although the root of the tail and lower rump are also popular. Of course, mothers, partly because they are so much taller, have little choice but to nibble the base of their offsprings’ manes and backs (which in itself might have established a very early taste for being nibbled on the shoulder and explain its universality among equids). Young foals, instead, cannot reach their mothers’ shoulders and must be content with her legs, flanks and face, but the preference for nibbling shoulders is evident in the interactions of similar-aged foals.
In adopting mutual skin-nibbling as a bonding device zebras resemble some primates and many other social mammals that have appropriately shaped incisors in both jaws. The most striking feature of the neck-shoulder junction is that the skin has to be fairly loose there to accommodate to a lot of neck movement. This looseness gives rise to vertical folds or skin wrinkles, which would be unremarkable were it not for the fact that many organisms respond to, or need, just such visual markers to help direct their species-specific behaviour. When such attention-attracting focii become ‘target areas’ it is not uncommon for them to be visually advertised.
Therefore, ancestral equids would have been conforming with a very widespread phenomenon had they evolved light and dark vertical streaks to exaggerate and advertise the vertical skin folds on their shoulders. Knowing that all equids still target the shoulder for grooming and that the majority of species (always or sometimes) have striped shoulders makes it plausible that this preferred ‘target area’ was the starting point for stripes.
Projecting that connection back into evolutionary time to suggest that stripes spread out and away from the shoulders, making the entire animal one huge, three-dimensional ‘target area’ (which living zebras, indeed, are), is completely consistent with the functional behavior of living zebra
