Review written by Laura W. Hirschfield, UG ‘24 (Neuroscience)
Scientists have continued to debate for centuries the reasons behind why the zebra got its stripes. The zebra’s pattern is rare in the animal kingdom, intensely expressed in both sexes, conspicuous, and yet, its utility is not apparent. Unlike the many other hoofed mammals, called ungulates, that also occupy the east African savannah, the stripes do not provide camouflage for any of the habitats zebras occupy.
Over the years, evolutionary biologists have put forth many hypotheses in an attempt to explain this distinctive trait, but many lack further scientific exploration. The ectoparasite (anti-parasite) hypothesis has the most empirical support, and is centered around the observation that zebras are attacked less by biting flies than the other ungulates they live with, such as gazelle and eland. This is evolutionarily advantageous because biting flies weaken animals by causing blood loss and spreading disease. Stripes have been hypothesized to affect the flies by interfering with their visual and navigational systems, making it more difficult for them to locate, identify, or land on their striped targets, but the mechanism behind this effect has yet to be explained.
A team of researchers in the Rubenstein lab in the department of Ecology and Evolutionary Biology at Princeton University set out to conduct a straightforward experiment examining the ectoparasite hypothesis to understand the mechanism behind zebra stripes serving as a parasite repellent tool. Their article addresses the many gaps in the literature on the anti-fly benefits of stripes by crafting their experimental procedure to resolve potential confounds. This includes restricting the experiment to close range at which flies can see the stripes, and the use of real pieces of animal hide (pelts) to maintain the complexity of the stripes as hair (and not paint). In addition, the pelts of skin were salt-cured to isolate visual aspects of the pelt as the driver of the decision to land, instead of something like smell. The study did this not only comparing zebra and impala pelts, but also comparing different zebra species stripes, as they range from wider in Plains zebras to narrower in Grevy’s zebras.
The experiment took place at the Mpala Research Center (MRC) in Laikipia County, Kenya. 400cm2 sections were taken from the rumps of two different zebra species, plains and Grevy’s zebras (see photos below), and impala (an ungulate found in abundance in Kenya), and any attached tissue was removed from the pelts. Pelts were then salt-cured and sun-dried to rid them of any smell or property outside of appearance. Local biting flies (of the Stomoxys species) were trapped and kept out overnight to ensure they were hungry prior to the experiment. 15-25 flies were released from a petri dish placed in the center of the experimental area, and after the flies settled, the zone and stripe location positions were noted (see diagram). After a 1-minute interval, the top of the experimental box was tapped to initiate the next landing-choice subtrial, for a total of 10 subtrials. This was repeated to compare all of the different combinations of plains, Grevy’s, and impala pelts.
The results of this study found that flies strongly prefer impala skin over the skin of any zebra species; no difference in preference between zebra species, with flies being more likely to land on black stripes in both zebra species. These findings suggest that the visual appearance of the stripes is what deters the flies, and not a smell or taste preference. In addition, this study adds biological validity to all past research on the topic that used artificial stripes. Furthermore, by restricting the starting distance of the flies to the hides in this study, researchers were able to determine that while flies were equally as likely to be in the zebra or impala approach ranges, they showed a strong preference for actually landing on the impala pelt. This tells us that the visual effects of the stripes must be occurring at a very close range (30cm or less).
Overall, the use of real zebra hides proved most important when comparing the plains versus Grevy’s fly preference. Past research using oil tray traps found a significant difference between wide and narrow stripes, but with the natural variation of stripe width across the two species actually being tested, no difference is found. Taken together, the study effectively used real zebra pelt samples to show that visual close-range (30 cm or less) mechanisms are able to operate without any long-range mechanisms to discourage flies from landing on zebras. Grevy’s zebras are endangered, while the plain population remains stable, so studying the differences between the two, such as in their stripe patterns, may help determine what is leading to the decline of one species over another. Zebras are an important part of the African savanna ecosystem, and thus it’s important that we understand how their unique pattern has helped maintain their survival.
The original article discussed here was published in Nature Scientific Reports on November 3, 2022. Please follow this link to view the full version.
