Question

The following passage is adapted from a scientific essay on evolutionary biology.

For over a century, evolutionary biologists debated the adaptive function of the zebra's distinctive black-and-white stripes. The classic hypothesis, famously championed by nineteenth-century naturalists, proposed that stripes functioned as a form of camouflage. By blending into the shimmering heat waves and tall grasses of the African savannah, zebras were thought to confuse predators like lions. However, this theory struggled under closer scrutiny; lions are visual hunters, but they primarily detect prey by movement and scent rather than static visual patterns. Furthermore, zebras are noisy and social animals, making quiet concealment highly unlikely, as they rarely attempt to hide from predators.

To address these shortcomings, researchers in the mid-twentieth century proposed an alternative explanation: the stripes served a social function, facilitating group cohesion or individual recognition within herds. While plausible, this hypothesis lacked empirical support. Zebras indeed exhibit complex social behaviors, but other closely related equids (such as wild asses) show similar herd dynamics without requiring high-contrast markings. Consequently, the social cohesion theory remained largely speculative, failing to explain why zebras specifically evolved such unique coats.

A significant breakthrough came when researchers began investigating the sensory systems of blood-sucking insects rather than large carnivores. In a series of controlled experiments, scientists discovered that tabanids (biting flies) are highly sensitive to polarized light. Biting flies use polarized light reflections from dark animal hides to locate hosts. High-contrast striped patterns disrupt this light reflection, creating an optical illusion that makes it difficult for flies to land successfully. Comparative studies across equid species soon confirmed that striping patterns correlate strongly with regions of high biting-fly activity, providing a robust ecological explanation for the trait. By sequencing their evidence from predator-avoidance failures to physical insect-landing experiments, scientists shifted the paradigm of zebra stripe research.

In the context of the passage's argument, the author's discussion of wild asses and other unstriped equids functions primarily to do which of the following?