Medical News Today: Why are scientists painting stripes on cows?

Why do zebras have stripes? Rudyard Kipling’s Just So Stories inform us that the stripes developed because the zebra stood in the “slippery-slidy shadows” of trees. Scientists, however, have other ideas.

They have not firmly established why the zebra is adorned with such an elaborate and impressive design, but theories include thermoregulation, confusing predators, communicating with other zebras, and camouflage.

Although there is little evidence to support these explanations, one theory does have some experimental backing — some scientists believe that the black and white stripes of a zebra protect it from biting insects.

As just one example, a 2019 study found that horses wearing coats with striped patterns attracted fewer flies than both horses without any covering and horses wearing materials without stripes.

With growing support, it now seems reasonable that a zebra’s stripes function as an insect repellant. This fact, of course, is interesting in its own right, but can this information be useful, too?

Using stripes as protection

Biting flies are a serious concern for livestock owners because they affect cattle’s behavior and can lead to economic losses. The presence of biting flies reduces feeding and bedding down time, which can affect the animals’ development.

Also, when biting flies are present, cattle tend to bunch together to reduce the risk of sustaining bites. This bunching behavior increases heat stress and the risk of injury. Additionally, it can reduce weight gain and negatively affect milk production.

Some researchers estimate that biting insects in the United States cost the dairy and beef industry more than $2 billion each year.

If a zebra’s stripes can help it minimize fly attacks, could similar stripes also help cattle? A group of researchers recently set out to explore this question, and they published their results in the journal PLOS ONE.

Testing the stripes

To investigate, the scientists enlisted the help of six pregnant Japanese Black cows. They painted each cow in one of three ways: black and white stripes to resemble a zebra, only black stripes, or no paint at all (the control group).

As Japanese Black cows are naturally black, the animals with the painted black stripes did not appear much different than normal. However, this intervention was to ensure that any repelling of flies was not due to the smell of paint.

The researchers then observed the cows, counting their fly-repelling behaviors. These included beating the ears, throwing the head, and stamping the legs, as well as tail flicks and skin twitches. The team also took photos of the side of each animal to record the number of biting flies that were present. Lastly, they placed sticky transparent sheets on the ground next to each animal to trap flies in the area so that they could determine the species of the insects.

The scientists found significantly fewer flies on the legs and body of the painted cows compared with the others — about half the number. There were no differences between the control group and the group with painted black stripes.

When they assessed the fly-repelling behaviors, the researchers found that the control cows carried out an average of 53 behaviors in 30 minutes, while the black stripe cows carried out 54.4. In contrast, the black and white cows only carried out 39.8 behaviors in each 30-minute window. Overall, the authors conclude:

“The results of our study showed that the numbers of biting flies on black-and-white painted cows were significantly fewer than those on the all-black and black-striped cows.”

This unusual but seemingly effective technique might help save the cattle industry money. In addition to this financial benefit, it might help reduce the use of pesticides. Insects are quick to adapt to chemicals that humans design to kill them — according to the authors, they “often evolve resistance to a new pesticide within about a decade after its introduction.”

The authors continue, “This work provides an alternative to the use of conventional pesticides for mitigating biting fly attacks on livestock that improves animal welfare and human health, in addition to helping resolve the problem of pesticide resistance in the environment.”