Tales from Tanzania: So, why do zebras have stripes, like, for real?

Today we saw dozens of very photogenic zebras. This got David and me talking: What came first, the zebra or the stripes?

baby Zebra

There are plenty of theories as to why the zebra has stripes – some more fantastical than others.

According to one African tale, the zebra was once pure white. One very hot day, the zebra went down to a watering hole to drink, but the watering hole was being guarded by a baboon bully who didn’t want to share. The zebra and the baboon got into a fight over the water and during the course of the fight, the zebra backed into the baboon’s fire and tripped. The hot branches left black scorch marks on the zebra’s hide.

While that tale, and others like it, are certainly fun to hear, they aren’t terribly scientific. So what says science?

One popular theory posited is that the stripes act as camouflage, but zebras graze in the open in relatively short grass. The stripes aren’t really making them blend into much of anything.

Another theory is that the stripes make it harder for a predator to pick out an individual from the group. Scientists originally believed that the stripes, when viewed at a distance, would simulate heat waves. Thus, that close-up would be confusing, because all the zebras would blend together. In reality, lions are the main predators of zebras and they generally hunt at night. A zebra’s stripes would, in fact, make them more distinct and visible in the moonlight.

The flip side of this theory says that the stripes help zebras recognize individuals in the larger group. This is actually supported by research, which shows that captive zebras prefer standing next to a patterned wall rather than a solid colored wall, but not 100 percent understood.

A relatively new theory that seems to have some merit is that the zebra’s stripes make excellent fly repellant. Cattle (and zebra too) are more susceptible to illness caused by the tsetse fly, which bite as a reaction to movement and dark colors. In fact, the tsetse fly prefers to bite through dark clothing even when a light, bare arm is available.

To test this theory, one study looked at horsefly bites on horses. They set up five “targets” for the horseflies — one black, one white, one grey, one with horizontal stripes and one with vertical stripes. The great target and the target with the vertical stripes had the least number of bites.

Why? The horsefly, like most insects with compound eyes (including the tsetse fly) has the ability to see polarized light. This improves their ability to see solid and dark colors, allows them to “see” heat signatures and enables them to track moving objects (especially dark ones). Because of these specially adapted eyes, the zebras’ stripes would make them disorienting to the horsefly. The stripes, hypothetically, break up the shape of the zebra and make it harder for the fly to focus in to bite.

While this all sounds great in theory, scientists haven’t conducted these same tests on actual zebras in Africa, but they are pretty positive the results would be the same.

Kwa heri ya kuonana!

Tales from Tanzania: The banks of the Nile that divide two crocodiles

In a previous post, I mentioned that beetles are my favorite invertebrates — which sort of begs the question, “What’s your favorite vertebrate?” And that’s an easy one: Alligator mississippiensis or the American alligator. I am lucky that we live in Houston, close to alligators, and even luckier that we have a juvenile alligator in our live animal collection.

For now though, I am super excited to see a Nile crocodile. I have been assured that we will get a look at one today, as we travel through the Grumeti River area in the Western Corridor of the Serengeti National Park.

nile_crocodileUntil about two years ago the number of crocodilian species numbered 23. Was a new species discovered? The answer is kinda, but not really. Modern science proved what the ancient Egyptians already knew: The “Nile crocodile” is actually two species: the Eastern Nile crocodile (Crocodylus niloticus) and the Western Nile crocodile (Crocodylus suchus). The smaller, more docile Western Nile crocodile was commonly used in Egyptian culture (mummified, kept at temples, etc.) while evidence shows that Egyptians knew the larger, more aggressive Eastern Nile crocodile was to be avoided. DNA evidence comparing living crocodile populations and mummified crocodiles proved the distinct difference in the two species and brought the total number of crocodilians up to 24, of which nine are currently threatened or endangered.

What’s more, the same DNA study also showed that the Eastern Nile crocodile was more closely related to four species in the Caribbean than it was its neighbor, the Western Nile crocodile.

The Eastern Nile crocodile is the second largest extant reptile in the world. The largest accurately recorded male was shot in Tanzania and measured 21 feet, 3 inches and weighed approximately 2,400 pounds. In fact, we are visiting Tanzania at the perfect time to get a glimpse of these ambush predators as it is the dry season and they will be nesting.

I don’t want to get too close, however, as 63 percent of the 250 to 750 Nile crocodile attacks on humans per year prove fatal!

Bonus fact: A group of crocodiles is called a bask.