Beni followed the Doctor through the colorful market, jostling with the six-legged, two-trunked inhabitants of Thind. Humanoids were rare on Thind, but not completely alien, and while the multi-hued Thind regarded them with curiousity, no one stood in their way as the Doctor led Beni deeper into the city.
"What is that smell?" Beni asked as they passed by a stall loaded with pots and pans. A Thind in a swirling blanket of reds and yellows waved one about in one trunk, extolling its virtues to an assembled crowd.
"I hope it's not unpleasant," said the Doctor.
"Not at all," Beni replied. "A bit floral."
"Well, then you probably won't be too put out when I tell you it is intestinal gas."
"Oh." Beni sniffed. "Well, it's a lot more pleasant than what humans emit."
"That's because you've evolved to recognize your own toxic matter, not theirs. They use that smell to identify rank. Most of the Thinds you've met so far are commoners. You may have noticed the Thind is a herd race, hmm?"
Beni's professional ire rose to the surface. "Doctor, I'm an interstellar archeologist. I'm perfectly capable of recognizing..."
"Yes, sorry," said the doctor. "They evolved a smell that lets them identify their own herd. It's light enough that it doesn't obstruct their ability to detect predators. Or prey."
"Prey?"
"The Thinds will eat meat, if presented to them. They're omnivores, descended from a casual scavenger ancestor. Most of them prefer a plant-based diet, and because most herds grazed in different locations, they all have slightly different internal flora. A sort of tribal identification system." He glanced up the hill at the vast castle of white stone that seemed to point up and away from the marketplace. "The leadership of the Thinds has started to evolve in their own way, based on post-merchantile economies. They've started to master capitalism." The Doctor frowned momentarily. "Poor dears."
"Why 'poor dears'?"
The doctor shrugged. "They get the cream of the society. The very best. Which means they eat a lot of meat. The top of the Thind hierarchy ceases to smell much like the tribe from which they came. Instead, they start to smell like each other. It's a terrible affliction, being at the top of the hierarchy, slowly pulling away from the people you claim to represent." He shrugged. "It's especially terrible for the Thind leadership, because of the belly burps."
"I don't understand."
"Bernie, this lightly floral scent you're sniffing is due to diet, a mostly vegetarian diet. The leadership eats a high-meat, high-fat, high-protein diet. The smell is terrible."
"You mean?"
"Yes, Beni. All great Thinds stink alike."