Recently, 25 sea slugs were served an underwater buffet to study their tastes. Offered a choice of brine shrimp and hydroid polyps (small coral-like organisms), the slugs opted for polyps that had swallowed the shrimp—a clever caloric twofer. Using a novel feeding strategy, they were jumping the line in the food chain to devour their prey and the prey of their prey. Researchers dubbed this kleptopredation, using the Greek word, kleptes, for thief or cheater.

Of course, the criminal trope is only a metaphor; human standards of fair play hold no sway in the animal kingdom. (Though some have taken the term full circle to call out human rapacity: The Daily Kos cited the 2017 Republican tax bill as a case in point.) And it turns out that real kleptopredation is an environmentally friendly strategy. The slug, a gaudily colored species of nudibranch, uses hydroid colonies for shelter. By eating polyps with full bellies, it’s able to consume far fewer of them, thus preserving its habitat. Hydroids may enjoy their meals less, but in the long term it’s a win-win for both parties.

Indeed, we humans might take a cue from sea slugs to concentrate our own use of resources. It’s the paradoxical case for building dense cities instead of leafy suburbs and growing our food on industrial-scale farms. Perhaps there really is honor among thieves.

This article appears in the March issue. Subscribe now.