Because fast-swimming cetaceans like dolphins don’t slow down enough for anyone to pick parasites off of them, scientists know even less about the lice that live on them. Fraija-Fernández, Aznar, and others at the University of Valencia’s Marine Zoology Unit study cetaceans with the help of a stranding network. When an animal comes ashore, researchers gather data on it as well as any other life forms it’s carrying. Between 1980 and 2016, they managed to study 176 striped dolphins and their whale lice. (When working with animals that can only be collected by chance, Fraija-Fernández and Aznar point out, “one must be patient.”)

Syncyamus aequus was the only louse species on the striped dolphins. And its numbers were sparse on these swift, streamlined hosts. Only about a quarter of the dolphins in the study had lice, compared to 100 percent of whales in some surveys. And while one whale can have thousands of lice onboard, infected striped dolphins carried fewer than five lice on average. More than a third of infected dolphins carried only a single louse. “We expected low infection levels,” the scientists say, “but not so low.”

The dolphins’ lice were smaller than lice living on whales. And they had found only a few places to tuck their little bodies into. Most lice were around dolphins’ blowholes, with some around their eyes or in the corners of their mouths. This was another sharp contrast to whales, where lice find abundant habitats. In addition to a whale’s skin folds, lice may live around the rough, whitish patches called callosities, or in a whale’s throat grooves, or even in the spaces between barnacles attached to a whale’s skin. Whales can carry three or more species of lice at once, each carving out its own territory.

Those bustling louse populations on whales have complex dynamics. Female whale lice can only mate right after they molt, so males jealously guard them during this period. They may chase away other males, or even lift up a female and carry her away. Males are larger than females in most whale lice species.

But in S. aequus, the researchers found, the opposite is true: Females are bigger. The researchers suspect that’s because there’s no competition between males. In fact, the lice barely see each other at all. A male louse living on a striped dolphin is lucky if it meets even one female louse in its life. Female lice may have evolved to be bigger so they can lay more eggs—assuming they ever find a mate.

Studying the parasites of whales and dolphins can help scientists answer questions about the whales and dolphins themselves. In 2005, for instance, researchers studied the population histories of right whales by looking at the mitochondrial DNA of their lice. In 2004, after a southern right-whale calf stranded and died, researchers found that it was infected only with humpback-whale lice—a tantalizing hint that the whale calf might have been nursed by a female of a different species.