We had scarcely discovered the Iberian lynx’s louse before it vanished. The creature was identified in 1997 when an adult louse and a nymph (a baby louse) were removed from the pelt of a dead lynx; it was later named Felicola isidoroi. At the time, Iberian lynxes were in severe decline as a result of a virus in rabbits, their prey. With this decline, the domain of the isidoroi louse dwindled and its habitat fragmented, as lynxes were less likely to cross paths with one another.

When conservationists took Iberian lynxes into captive breeding programs, they deloused them, dewilding the natural environment that is a lynx. Even if the lynxes hadn’t been deloused, their lice may not have survived captivity. Wildcats in enclosures tend to overgroom and pace. These can be fatal events for a louse that’s scratched off or dislodged by fretful movement.

A given lynx likely does not mourn his lice (they itched him!), so why should we care about the lynx’s louse? As it turns out, we have good reason to pay heed to vanishing lice. Some biologists argue that parasites and their hosts should be viewed as one entity, because their interaction drives the evolution and health of both species. Their relationship may even render whole ecosystems more resilient. Perhaps, then, if we can’t save all endangered species, we should focus on saving those animals that host unique and rare clingers-on, both on the skin and within. Several parasites have been shown to aid immunity. The intestinal worms inside some fish sop up noxious heavy metals from the tissue of their carriers. A few parasites have been called “ecological puppeteers” for the ways they change the behavior of their hosts and alter the ecosystem—for example, prompting crickets to jump into streams, where insect-eating fish eventually flourish; or making moose more feeble, therefore supporting wolf packs.

Parasites can also teach us about their hosts. Because lice evolve alongside their hosts, the genome of a species-specific louse is an index card showing the history of its carrier species: former abundances of the larger animal, its adaptations and demographics. With the loss of the isidoroi louse, science lost a means of exploring the lynx’s past.

Another lesson we can learn from the lynx and its louse is that a host need not become extinct for the creatures that subtend it to disappear. We’re certain to have shared the world with other things that have died out, unmet and un-observed, altering entire ecosystems in the process. Every organism exists in relation to other organisms, and even the tiny, the irksome and squirming, warrant attention.

This article appears in the March 2019 print edition with the headline “A Parasitic Relationship.”

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