"I just want to know why he did it, Cindy."

"He's a man. Men have affairs."

"He's my husband!" Professor Susan Alcock glanced at the picture on her desk, the one taken when she and her spouse had dressed up like gorillas for a Halloween party. In the photo, they hadn't put on their masks yet, and Susan now thought she looked much younger then, even though Halloween was only three months ago. She turned away from their stupidly grinning faces and slammed her file cabinet shut. "Besides, he's not a man. He's a smartass gibbon. I hate him!"

Cindy Ricocho, a postdoc from down the hall who had won Susan's friendship by bringing her a plate of fudge when she once had a stress attack, pushed the office door closed with her foot, so the undergrads in the lab wouldn't see that their boss was upset.

"Can't be a gibbon," Cindy said. "They're monogamous."

"No, I read a paper. They caught one on camera sneaking around behind his mate's back. Just like I caught Harmon. I can't believe he taped it and then left it in the VCR!"

"He probably thought you didn't know how to work the VCR."

"I still think he thinks my doctorate is only an honorary degree."

Cindy puffed out her cheeks. "So what now? Are you going to confront him?"

"I don't know." Susan pulled at her bottom lip, a nervous habit she had acquired the day she got married. "I just want to know why he did this to me."

"Have you noticed any changes in him? Besides the fooling around, of course."

"No! That's what gets me. He's been as sweet as ever."

"You two still go at it?"

"Not as much. But it's been a gradual decline. It's not as if our sex life slammed to a halt."

"You've been married for how long? Four years now?"

Susan glared at her. "I know what you're going to say."

Divorce usually happens between the second and fourth year of marriage, peaking in the fourth. It happens across cultures, and western cultures are not immune.

"Harmon and I aren't getting a divorce," Susan said. "We're still happily married - until he meets with an unfortunate accident, that is."

"But the same reasoning applies to cheating as well. Now that Harmon has had four years with you, he feels secure enough to fly away and have fun with someone else. Spread his genes around."

Susan leaned back in her chair and sighed. "I know . . . I know, I know. I know. And I've thought about it. But I feel there's something else at work here. I watched that tape twice. He was really enjoying himself with her. He never liked it that much with me. There's something I'm missing about the reasons behind his infidelity."

"Something biological or cultural?"

"I don't know."

"Can I ask you a question? Why do you want to know?"

"Don't judge me, all right?" Susan wiped her hands down her face and struggled to remain sane. "Because understanding a problem gives you distance from it. When you have distance from a problem, you can deal with it." She stared at the photograph on her desk again and wished she could cause it to spontaneously combust by the force of her will. "I don't know how I'm going to deal with this."

"Blame it on our species," Cindy said with a comforting tone. "When you think of it, how many other species have sex just for fun?"

"Dolphins. Pygmy chimps. A few others," Susan answered.

"What about the other end of the spectrum?"

"A completely monogamous species? Albatrosses. I have a calendar of them." She pointed at it hanging on the wall. "And no one's ever seen a beaver cheat on its mate."

"Beavers?" Cindy almost laughed.

"An unfortunate coincidence." Susan looked between her legs and grimaced. "At least I know my beaver has never cheated."

"The girl Harmon was with - is she married?"

"Who knows? Who cares? It's not her in particular he likes. Now that I think back on it, I'm sure there've been others."

"Maybe Harmon likes slutty women. Men are attracted to women for purely sexual reasons more than women are to men."

"Yes, attraction is the problem." Susan touched her cheek.

"No, no, no! That's not what I meant. Susan, you're beautiful!"

"I'm older than her. Age changes a woman's face."

"Faces aren't the only thing men key in on," Cindy argued.

"I have acne too. That means problems with reproduction as well."

"Again with faces? There are other things men look for in a woman."

"You're right," Susan droned listlessly. "They gauge a woman's age, health, design quality, reproductive history, testosterone vs. estrogen ratio . . ."

"Come on. Men like boobs!"

"Mine are smaller than hers." Susan grabbed her forehead and moaned with frustration. "This is getting me nowhere! I'm trying to act so rational . . . I'm never going to understand why that bastard cheated on me."

"You don't have to understand his reasons," Cindy said. "All you have to do is decide what you're going to do about it."

Burning inside, Susan tossed her head back and shouted at the ceiling. "I want him to love me! I want him to want me - only me, a complete and utter longing. How do I get that? How do I make him ignore all other women?"

"Quiet down. They'll hear outside. I don't think that's possible."

Susan snapped, "Who says?"

"No one, I guess." Cindy kept her voice calm. "I only meant that some things can't be changed. Relationships aren't straight paths. Sometimes you can't go back to the way things were."

"I don't want to go back to the way we were. I want to fix that. I want to change what he was - what he is - so that he desires no one in the world but me."

"That may be unrealistic."

"Oh yeah? How does a woman attract a man?"

"What do you mean?"

"Through signals, right?" Susan leaned forward and slapped her arms on her desk. Ideas were popping into her head faster than she could count them. "She attracts a man by the way she looks, the way she walks, and how she speaks to him, right?"

"I guess so. So?"

"Women fake those signals all the time. When a woman wants to attract a man, she makes herself appear better than she actually is. She dyes her hair. She covers her face with makeup. She wears high heels. She flirts. What I'm saying is that she figures out what a man wants and gives it to him, right? So, therefore, if I can figure out exactly what Harmon wants, I can make him want only me."

Cindy looked as if afraid to speak. "Susan, men aren't puzzles to solve."

"They're animals, right?" Susan jumped up and began pacing as she spoke with conviction. "Animals are simply biological machines, right? If you push the right buttons, you can get a machine to do what you want it to do. So all I have to do is figure out how hard I have to smash down Harmon's buttons."

"But people are more complex than that. Yes, maybe you can figure out how to attract Harmon on some basic biological level, but that doesn't mean he still won't be attracted to other women as well. We're not insects. We don't give off chemical signals to overwhelm prospective mates. We can only play the odds and hope for the best."

"Life is so stupid! God, why can't I simply take the monogamy gene out of a beaver and insert it into Harmon's genome? That would be great! But I don't know enough about genetic engineering to get the results I need. No one knows enough! Damn it, why wasn't I born thirty years in the future?"

Pulling out a chair, Cindy begged Susan to sit down and calm down.

"All right, if it'll help to work out your frustrations, let's be hypothetical about all this," Cindy said. "What you need is a signal that Harmon can't resist, a signal that'll make Harmon desire only you, right? A signal that'll spur a FAP in Harmon."

"A FAP?" Susan sat, but she was far from calm.

"You know, a fixed action pattern. Like yawning, for example. Once someone starts yawning, it becomes an automatic response that they can't resist. It's a sign stimulus too, right? When you see someone yawn, you feel the need to yawn yourself. Even just talking about someone opening their mouth and sucking in a big moaning breath makes you want to do the same. All you have to do is to find a similar type of signal that'll spur a sexual FAP in Harmon."

Susan nodded. "But yawning can be suppressed if you try hard enough. But you're right, what I need . . ." Susan yawned. "Excuse me. What I need is to find a sexual sign stimulus and then figure out a way to strengthen it. Help me think, Cindy. What animals use sign stimuli?"

"When a goose sees a ball, it'll try to nest on it. The roundness of the ball reminds it of an egg."

"No, no, no! I need something like . . . something like bacteria. Or something that feeds on bacteria. Like a worm!" Susan lunged for the books piled up on her office shelf, yanked one away, and hurriedly flipped through its pages. Stopping in the middle, she pointed at the text. "There! Caenorhabditis elegans!"

Cindy brightened. "I know where you're heading - the bull!"

"You heard about it too? The one that refused to stud?"

"They injected genetically altered C. elegans into its brain, and it humped every cow on the farm!"

"Not every cow," Susan said. "Only the cows it had been introduced to within a few hours after the injection. Specific cows! It couldn't care less about the other ones. The C. elegans modified the bull's sexual preference."

"That's what you want," Cindy cheered. "You want to be Harmon's only cow!"

"Exactly!"

"Oh wow! Wouldn't it be great if it could work on people? On Harmon? If this wasn't just hypothetical?"

Susan became very settled and quiet, and as she leaned across her desk, she pushed her hair behind her ear with a calculated thrust, and then she cast a devious little smile Cindy's direction as she said, "Why does it have to be just academic?"

"Because," Cindy said, carefully, "it's never been tried on humans."

"Not yet."

Cindy's eyes widened. "Susan, what are you thinking?"

"What do you think I'm thinking?"

"No . . . you can't mess around with this stuff, Susan. You can't inject Harmon just because you feel like it."

"I want him to want me, Cindy."

"I know you do, but this isn't the way to do it."

"Why? Would you turn me in?"

Cindy looked as if she was going to be sick; then after a long moment, she bowed her head and exhaled, and said with a fragile voice, "No, I guess I wouldn't."

Susan slapped her thighs. "Then it's settled."

It wouldn't be difficult to get her hands on the modified C. elegans. The researcher who made them was against patenting organisms and would readily give them to her if she claimed she wanted to use them for a legitimate project.

"Are you sure about this?" Cindy asked.

"How do you mean?"

"How can you be satisfied knowing Harmon doesn't love you by his own free will? Love requires free will, or what is it worth? Why do you want him anyway, Susan? He betrayed you. Don't you want to take an ice pick and plunge it through his heart? This man cheated on you. Can you ever be satisfied knowing you tricked him into wanting you again?"

Susan was blunt. "We all play tricks on one another."

Injecting Harmon was easy. Susan simply got him drunk and pushed the needle into his neck after he had passed out. She half considered injecting his testicles for good measure, but feared he'd notice and wake up.

She stayed with him all night, sitting on the edge of the bed, and waited for him to come around. She had a brief moment of panic when she realized she'd have to make sure that she would be the only woman he would see for the next few hours. Otherwise, the sign stimulus wouldn't take full effect, or worse, he'd be attracted to some other woman. That meant keeping him from going to work. Finding some rope in the garage, she tied him spread-eagled on the bed.

He awoke, groggy. "Suzy? What are you doing?"

"Hush, my dear." She straddled him. She made him look directly at her face. "This is an experiment."

"Suzy . . . I'm all tied up!"

"I know, so be quiet. I'll explain later."

"Suzy!"

Susan came into the lab around noon, smiling, and motioned to Cindy to follow her into her office, where she closed the door, plopped down into her chair, and then picked up the Halloween photograph from her desk and tossed it in the trash can.

"So?" Cindy asked. "How did it go?"

Susan acted as if uninterested, but then she burst out laughing. "It worked!"

"You're kidding? He wants you now?"

"He wants only me. He's transfixed! I even tried to get him to watch a porno movie and he threw the whole VCR into the fireplace!"

"It worked? I can't believe it worked!"

"I confronted him about the affair. He called the girl and told her it was over. He called up two other girls I didn't know about as well. Then we had sex for three hours straight. I was barely able to walk! He says he can't wait for me to come home again."

"I really can't believe it! This is great."

"He said that his life would be a living hell without me."

"Fantastic!"

Susan stood and headed for the door. "Follow me, will you? I need someone to help me take the suitcases out of my car."

"Suitcases?" Cindy frowned.

A small but distinct smile blossomed on Susan's face. "I've left the bastard," she said. "He'll never have me again."