Skip to main content

Monkeying Around

Nonhuman Models of Human Behavior

When a professor invites you to spend the summer hanging out with monkeys, you say yes. Colt Halter, senior Psychology student, wasn't just monkeying around, though. A summer spent observing and befriending numerous species of monkeys became the basis for his Honors thesis, in which he explored stress levels and mothering styles in rhesus macaques.

Primate behavior isn't the only behavior that has interested Colt. Starting out at BYU, Colt wanted to be a mental health counselor or a marriage and family therapist. Fascinated by human development, Colt wanted to know the “why” behind certain human behaviors. In PSYCH 387: Primate Behavior taught by Dr. Dee Higley, Colt finally began to see the origins of human behavior through their primate ancestors. Colt threw himself into the class with an ignited passion, and that passion did not go unnoticed. Halfway through the semester, Dr. Higley invited Colt to research at the California National Primate Research Center (CNPRC) in Davis, California for the summer. The center houses thousands of monkeys, and researchers there investigate behavior, genetics, hormones, and so much more. Colt quickly agreed.

At CNPRC, Colt’s days began at 5 am. Dressed in his stuffy scrubs in the often 100-degree heat, he would spend most of the day observing and recording behaviors of rhesus macaques on an ethogram (a checklist of behaviors). From outside the enclosure, he would track the movements of the monkeys as they roamed their one- to two-acre enclosures.

When he wasn't recording behaviors, he would assist with go-no go tests to test the cognitive function of titi monkeys. Scientists at the center believe that pair bonding, or monogamous mating, is correlated with increased cognitive inhibition, or the self-control necessary to not do whatever you want to do. The test is basically “Simon Says” for the monkeys; they are prompted to press buttons that are green but withhold pressing when the buttons are red.

Though the monkeys were the research subjects, they ensured that the interns knew who were really the top bananas. One of Colt’s jobs was to collect urine samples from titi monkeys, used to monitor protein levels, cortisol, and other hormones. He would reward them with peanuts after doing so, and he believes the titis quickly used this strategy to their own gain by refusing to urinate when the peanut amount was insufficient.

Beyond his monkey business, Colt attended weekly seminars held by elite scientists in the fields of behavioral genetics and molecular neuroscience. It was over the course of those seminars that Colt decided to officially change his major to Psychology and focus on the biological basis of behavior. This change has ultimately resulted in his acceptance to a PhD program studying Clinical Neuropsychology at Wayne State University in Detroit, Michigan.

Colt used data from his time at CNPRC to form the basis of his Honors thesis, which synthesizes hormone levels, behaviors, and parenting styles. When deciding to join the Honors Program, the opportunity to write a thesis was, surprisingly, the main attraction for Colt. To complete his thesis, Colt used longitudinal data to get a comprehensive view of primate behavior. In 2012, researchers at CNPRC measured stress-induced plasma cortisol (SIPC) as an indicator of anxiety levels in infant rhesus macaques. High SIPC is correlated to individuals with high stress responses. They took these measurements before, during, and after the infants were removed from their mothers for 25 hours. Seven years later, Colt was able to observe those infants as mothers themselves. The monkeys who had higher SIPC levels as infants were less sensitive mothers: when their offspring would wander away, the infants had to return to find the mother rather than the mother venturing off to find the infant. There was less maternal cradling, where the mother hugs, protects, and carries their offspring, as well.

Colt’s primate research allows for a nonhuman model for human behavior. Dr. Higley, Colt’s Honors thesis mentor, has been able to model the effect of alcohol addiction and serotonin-mediated temperament by using Colt’s nonhuman primate models. Colt may have opened up a window into human behavior, demonstrating how a mother’s temperament can affect offspring. Even though it began as monkey business, Colt’s research is a success.

Colt Halter is a Psychology major from Clinton, Utah. After graduating in August 2021, he is now attending Wayne State University in Michigan, where he is pursuing a PhD in Clinical Neuropsychology. His Honors thesis, “Stress-induced plasma cortisol concentrations in infancy are associated with later parenting behaviors in female rhesus macaques (Macaca mulatta),” is published in Developmental Psychobiology and can be accessed here. When he isn't hanging out with monkeys, Colt enjoys playing guitar, running half-marathons with his wife, and helping out with behavior therapy as a registered behavior technician.