When she stopped, he could still call to mind some of her tunes. Whales had songs, of course, but birds must have been the first musicians. Unless dinosaurs too had made music. He seemed to recall something about big hollows in certain hadrosaur skulls, inexplicable except as sounding devices. The sound one of those would have made was interesting to try to imagine. He even hummed a bit, testing how it would feel in his own big barrel of a chest.

“So was that the bird, or you?” he asked when she took a pause.

“We are the same,” she said.

After a while she said, “Mozart’s pet starling once revised a phrase he wrote. The bird sang it after he played it on the piano, but changed all the sharps to flats. Mozart described it happening in the margin of the score. ‘That was beautiful!’ he wrote. When the bird died, he sang at its funeral, and read a poem to it. And his next composition, which the publisher called A Musical Joke, had a starling style.”

“Nice,” Wahram said. “It’s true that birds always look intelligent.”

“Not doves,” she said. But then, in a dark tone: “You can either have high specific intelligence or high general intelligence, but not both.”

Wahram didn’t know what to say to that; the thought had turned her suddenly grim. “Well,” he said. “We should whistle together.”

“So we’ll have both?”

“What?”

“Never mind. All right.”

So he went back to the Eroica, and this time she whistled along, in an avian counterpoint or descant to the melodies. Her parts fit his in the manner of internal cadenzas, or jazz improvisations, and at Beethoven’s more heroic moments, which came pretty frequently, her additions rose to a furious pace of invention, sounding as if the bird inside her had been driven into a fit by Beethoven’s audacity.

They whistled some very stirring duets. It definitely passed the time in ways that it hadn’t passed before. You needed the gift of time, he thought, to explore a pleasure like this. He could go through all the Beethoven he knew; and after them, the four symphonies of Brahms, so noble and heartfelt; also the last three symphonies of Tchaikovsky. All the great parts of the soundtrack of his oh-so-romantic youth. Meanwhile Swan was up for anything, and her augmentations added a wild baroque or avant-garde touch to the tunes, additions that often amazed him. The piercing quality of her sound must have carried a long way up and down the tunnel, and sometimes the sunwalkers would slow down and walk just ahead of them, bouncing in time to the music, even whistling themselves, inexpertly but enthusiastically. The finale of Beethoven’s Seventh was particularly successful with them as marching music; and when they got up after a rest to take up their walking again, the sunwalkers often requested the horn cry that began Tchaikovsky’s Fourth, then its first theme, so full of the feeling that there was a fate ruling them now, a fate dark and grand.

At the end of one of their shared performances of Beethoven’s Ninth, they all shook their heads in wonder, and Nar turned back and said, “Sirs, you certainly are good whistlers! What tunes!”

“Well,” Wahram said. “Those are Beethoven.”

“Oh! I thought they called it whistling.”

“We thought you were making them up,” Tron added. “We were impressed.”

Later, when the three youths had gotten ahead, Wahram said, “Are all the sunwalkers like that?”

“No!” Swan said, annoyed. “I told you, I’m a sunwalker myself.”

He did not want her annoyed. “Tell me, do you have anything else interesting added to your brain?”

“I do.” She still sounded sour. “There’s an earlier AI, from when I was a child, put in my corpus callosum to help deal with some convulsions I was having. And a bit of one lover—we thought we’d share some of our sexual responses and see where that led us. Which was nowhere, as it turned out, but I presume that bit is still in there. And there’s other stuff too, but I don’t want to talk about it.”

“Oh dear. Is it confusing?”

“Not at all.” Grimmer and grimmer she sounded. “What, don’t you have anything in you?”

“In a way. I suppose everyone does,” he said reassuringly, though in fact he had seldom heard of a brain with as many interventions as hers. “I take some vasopressin and some oxytocin, as recommended.”

“Those both come from vasotocin,” she said authoritatively. “There’s just one amino acid of difference between the three. So I take the vasotocin. It’s very old, so old it controls sex behavior in frogs.”

“My.”

“No, it’s just what you need.”

“I don’t know. I feel fine with the oxytocin and vasopressin.”

“Oxytocin is social memory,” she said. “You don’t notice other people without it. I need more of it. Vasopressin too, I suppose.”

“The monogamy hormone,” Wahram said.

“Monogamy in males. But only three percent of mammals are monogamous. Even birds do better than that, I think.”

“Swans,” Wahram suggested.

“Yes. And I am Swan Second Swan. But I’m not monogamous.”

“No?”

“No. Except I’m faithful to endorphins.”

He frowned, but assumed she was joking and tried to go along. “Isn’t that just like having a dog or something?”

“I like dogs. Dogs are wolves.”

“But wolves are not monogamous.”

“No. But endorphins are.”

He sighed, feeling he had lost her point, or that she had. “It’s the touch of the beloved that stimulates endorphins,” he said, and left it at that. You couldn’t whistle the end of the “Moonlight Sonata.”

That night, as they slept in the tunnel on their little aerogel mattresses under their thin blankets, he awoke to find that Swan had moved, and was sleeping against him back to back. The resulting flood of oxytocin relieved his sore hips a little; this was how one could read it. Of course the urge to sleep with someone, the pleasure in sleeping with someone, was not exactly synonymous with sex. Which was reassuring. Across the floor indeed the three ferals were curled together like kittens. The tunnels were warm, often too warm, but right on the floor it got cold. Very faintly he heard her purring. Feline genes for same—yes, he had heard of it—people said it felt good, very like humming. Feel pleasure, purr, feel better: a positive feedback into more pleasure, loop, loop, loop, all at the pace of breathing, it sounded like when he listened to her. A different kind of music. Although he knew very well that sick cats sometimes purred at a momentary relief, or even as if hoping to feel better, trying to jump-start the loop. He had lived with a cat who had done that near its end. A fifty-year-old cat is an impressive creature. The loss of this ancient eunuch had been one of Wahram’s first losses, so he remembered its purr near the end as particularly pitiful, the sound of some emotion too crowded to name. A good friend of his had died purring. So now this purr from Swan gave him a little shiver of worry.

Down the tunnel after a sleep, groggy and dim. The morning hour. Whistle the slow movement from the Eroica, Beethoven’s funeral music for his sense of hearing, written as it was dying inside him. “ ‘We live an hour and it is always the same,’ ” he recited. Then the slow movement of the first of the late quartets, opus 127, variations on a theme, so rich; as majestic as the funeral march, but more hopeful, more in love with beauty. And then the third movement that followed was so strong and cheerful it could have been a fourth movement.

Swan gave him a black look. “Damn you,” she said, “you’re enjoying this.”

His bass croak of laughter felt good in his chest, a little hadrosauric. “ ‘Danger to him was like wine,’ ” he growled.

“What’s that?”

“The Oxford English Dictionary. Or that’s where I saw it.”

“You like quotations.”

“ ‘We have come a long way, we have a long way to go. In between we are somewhere.’ ”