The evolution and primary function of zebra stripes According to ontological evidence, zebras’ background coloration is black. The white stripping appears at a later developmental phase. The development of zebra stripping can thus be viewed as an evolutionary response (Egri et al. , 2011). Any discussion regarding the evolutionary purpose of zebra stripes is inevitably traced back to Darwin and Wallace. While Wallace suggested that zebra stripes developed as a camouflaging mechanism against attacks from predators in tall grass, Darwin refuted this explanation because zebras inhabited avannahs not grasslands.
Due to the significant curiosity surrounding zebra stripes, many alternative theories have been extended to explain the purpose zebra stripes evolved to serve. These theories fall into broad categories, for example some believe that zebra stripes provide camouflage through various mechanisms (including the stimulation of multiple visual effects, and by bearing close resemblance to woodland ecology), others defend the hypothesis that zebra stripes deter predatory attacks.
Other popular theories provide explanations and reasoning in favor of zebra stripes assisting thermoregulation, erving social purposes, and helping in evading ectoparasitic (parasites that leech on to the fur of zebras for purposes of nourishment) attacks. (Milius, 2014). Despite proliferating hypotheses regarding the purpose of zebra stripes, almost none of the hypotheses had been experientially confirmed until recently. The purpose of this paper is to comb through the hypotheses to discern between speculative reasons and science.
The paper also attempts to understand the evolution of stripes in an attempt to infer their function. Zebra stripes are a type of crypsis (or camouflage) was a opular hypothesis and was also speculated to be an evolutionary driver (How and Zanker, 2013). Possible explanations that have been offered in an attempt to answer how zebra stripes are a type of crypsis include that zebra stripes become larger, providing protection in certain hours of the day (such as twilight) or in grassy locales, or zebra stripes match a woodland ecology.
There has been little evidence to support that zebra stripes match their background. There has been a marginal correlation between stripping and the background at the equid subspecies level however said correlation has been ttributed to a correlation between stripping and tsetse and tabanid fly frequency. Moreover, given that zebra stripping is generally located around the ventral, a region that is not exposed to the background often, reduces the plausibility of the theory. Some animals have stripping as a means of hiding their young.
However, equids inhabit open areas and their offspring follow their parents around, rather than seeking refuge in their fur. The most popular mechanism that has been suggested in support of the crypsis theory is that zebra stripes cause a dazzle motion effect. The popularity of the dazzle motion hypothesis explains the scientific predilection towards the camouflage hypothesis. Motion dazzle effect believes that the stripes of zebras when in motion form a dazzle pattern making it difficult for the eye to accurately detect speed or direction of movement.
This effect inferentially can be said to provide protection against carnivorous animals seeking for prey and parasites seeking for a host. There has been some empirical evidence in favor of the dazzle motion hypothesis. However, the plausibility of the evidence is affected by a bias: zebra stripes when in motion ffect the human eye leading to misinterpretations of speed and direction. However to only test the working of this effect on humans is to infer that humans share an analogous visual mechanism to carnivores and tabanids (horseflies), which is not the case.
While the illusionary effect of zebra stripes is not debated, there is little understanding of what kind of effect it is and how it functions. Thanks to a recently conducted experiment that used a biologically stimulated algorithm to study the motion signals generated by different regions of zebras bodies when in motion. The experiment ascertained patterns that pointed to primarily two optical illusions, namely the wagon-wheel effect also known as temporal aliasing, and the barber-pole effect, which is similar to the aperture effect (How and Zanker, 2013). The wagon wheel effect is often featured in cinema.
It said to occur when increasing the rotational speed of a wagon wheel above a threshold causes it to seemingly spin in a reverse direction. Zebra stripes in motion thus bewilder the onlooker into believing that zebras are moving in a direction that is the opposite of the route they are actually undertaking. The barber- pole derives its namesake from adverts of barbershops. According to the barber-pole illusion, zebra stripes seemingly move vertically when in motion which results in the creation of a vertical movement when in fact movement is often horizontal.
It is important to note that while the optical illusions caused by the motion of zebra stripes is tested and verified, it is not yet clear what end the illusions serve. While it is clear that zebra stripes can indeed serve as a means of camouflage due to the optical illusions they induce, what they are providing camouflage against is not yet clear. The experiment assumed that effects generated could be directed towards the visual mechanism of predators or tabanaids, so either explanation is equally as likely to be plausible. The first hypothesis is closely tied in with the hypothesis of protection against predators hypothesis.
In consideration of the second theory, stripes functioning as a means of protection against predators, another recently conducted experiment investigates this through mechanisms such as camouflage, disruptive stripe coloring, making singling out individuals from a herd difficult, flight ability nd if striping is associated with the presence of larger predators such as hyenas, lions, tigers and wolves. The experiment in question is conducted on seven existing species of equids, out of which three sport black and white stripes, three have grey or brown coats and one (the African wild ass) only has thin stripes on its lower legs.
The experiment excludes horses because their evolutionary response to domestication separates them from the rest of the species. This experiment uses broad sampling to minimize the effects of phylogeny at the subspecies and species level (Caro et al. , 2014). Results reveal no correlation between presence of striping and protection against hyenas, but provided some protection from lion attacks. Zebras can fend off predators like hyenas by using their kicking and biting abilities. Striping however does not play a helpful role in fending them off though.
Given the lukewarm evidence found in favor of the hypothesis, providing protection against predators such as lions does not appear to be the primary function of zebra stripes and it is unlikely that this motive was an evolutionary driver to stripping. Furthermore, in certain parts of Africa lions hunt zebra n large numbers that exceed their abundance, which makes the hypothesis less plausibly the evolutionary driver. Another hypothesis that attempts to explain the function of zebra stripes concerns heat regulation.
In the same experiment that tested the predator hypothesis, this hypothesis was also tested by checking for correlation between the stripping of zebras and their inhabitation of ecological settings with an average maximum temperature of twenty-five degrees to thirty degrees Celsius. Heat drive was also refuted since there was little to no correlation between stripping at a species or ubspecies frame and living in humid habitats. First of all, most equids tend to inhabit open savannahs which tend to face humid temperatures, as Darwin observed.
Secondly, both stripped and non-stripped equids primarily tend to seek out shade as a means of managing the heat. The heat regulation hypothesis was supported by the fact that stripes acted as convection centers, due to their white and black stripping. However as observed in two sub-species of equids, E. zebras and E. burchelli, stripes that run along the dorsal absorb heat instead of reflecting it. While the stripping of certain subspecies ay indeed reflect heat and subsequently help in its management, it does not seem to be the universal purpose of stripping.
The social cohesion hypothesis was also tested by the same experiment (Caro et al. , 2014) by using average group size, and maximum herd size as indicators of the role played by striping in aiding the nature of social interactions. Some researchers believe that zebra stripes serve as a means of identification amongst zebras while others believe they help in the creation of herds and bonds between herd members. They also believe that stripes play a role in determining choice of mate and in shaping he grooming habits of zebras. The results of the experiment however reveal that there is no association between stripping and group density.
Stripes also definitely do not help in identification because domestic horses and un-stripped equids are also capable of recognizing individuals without the aid of visual clues. However, it is important to note that the indicators used only applied to a portion of the social cohesion hypothesis and the rest of the hypothesis remains untested. The equid social system can best be understood as a single equid male living in a polyandrous pack or a non-polyandrous pack with ight-knit bonds. The latter type of social system tends to have fewer members.
E. grevyi, E. africanus, E. iang and E. hemionus tend to form the second type of social systems whereas E. zebra, E. burchelli and E. przewalskii tend to form the former type of social systems. The most supported and proven hypothesis has been that zebra stripping provide protection against attacks by tsetse and tabanid flies. Before delving into the mechanisms of how zebra stripping provides for protection against flies, it is important to understand the need for such protection or in other words, why re equids so susceptible to potential attacks so as to develop an evolutionary defense against them?
Unlike artiodactyls (hoofed animals) and their domesticated cousins, horses, zebras have shorter and less dense hair, which increases their vulnerability to tsetse and tabanid bites. In the event of being bitten, zebras will suffer greater blood loss. Furthermore tabanids are also the carrier of pathogens that adversely affect equids, especially sub-species living in subSaharan Africa. According to existing literature, these diseases include but are not limited to influenza, African horse sickness, anemia, and trypanosmiasis.
These diseases are deadly to all equids and are carried by all tabanids, which makes tabanids harmful to all species of equids. It is worth noting that some species are harmed more than others depending on physiological factors. For example, as previously discussed a domesticated horse will be less harmed if bitten by a tabanid than a sub-specie say, E. burchelli. In fact, both undomesticated and domesticated horses are often frequently bitten by tabanids, unlike other equids. Given that all equids stand to be bitten by all flies, this hypothesis already applies at a species level.
Evidence supporting the, zebra stripping being an anti-fly denfense mechanism, hypothesis makes it the most plausible. First and foremost, the frequency of tsetse and tabanid flies biting zebras to the point of drawing blood is extremely low, especially in the sub-specie, G. morsitans. The spread of the diseases carried by tsetse and tabanid flies is also exceptionally low in zebras. After gaining an understanding of why tsetse and tabanid flies pose an imminent threat to zebras and why the anti-fly defense hypothesis is the strongest amongst the rest of the hypotheses, it is important to also understand the mechanisms of its operations.
It has been suggested that tsetse and tabanid flies tend to prefer landing on solid surfaces rather than stripped surfaces, which is what prevents them from landing on the fur of zebras. This occurs because tabanids tend to be attracted towards horizontally polarized light, not the polarization reflecting light of zebra stripping. Tabanid attraction towards horizontally polarized light can be attributed to its water-based habitation; tabanids use horizontal polarizing light to discover water sources. The light also helps tabanid females to better find mates and hosts.
Tabanids are also less likely to be attracted to the color white which explains the coloration of zebras. Hence the evolution of zebra stripping developed in opposition to the evolution of tabanid’s attraction to horizontal polarizing light and solid colors. There is no one right or wrong hypothesis, just hypotheses that are weak and hypotheses that are well-substantiated and supported. On the basis of an analysis of all the presented hypotheses, it seems most likely that zebra stripes developed as an evolutionary response to attack by tsetse and tabanid flies.
