Ontogeny

Ontogeny, or development, is how a behavior develops and changes during an animal’s lifetime.  In this case, how a rat’s laughter-like behavior is shaped as it matures.

How A Rat Grows Up Changes How It Vocalizes

If rats are isolated before and during maturation, they are more likely to exhibit laughter-like ultrasonic calls.  However, if they are socialized with other rats after this period of isolation, their calling patterns shift and they are less likely to emit the 50-kHz ‘laughter’ calls (Burgdorf and Panksepp, 2000). This could be due to increased need to call out to others (who are not readily nearby to attend to the needs of the rat). Below is a small video of rat pups feeding. They are calling out to their mother occasionally, their vocalizations alerting her to any distress on their part, and alerting other pups.

Growing up, rat pups emit 40-kHz distress calls, which signal that maternal care is needed.  These cries might be analogous to the 22-kHz cries adolescent and adult rats emit when anxious.  In contrast, 50-kHz adult calls are most likely not anxiolytic ('anxiolytic' means 'decreasing anxiety'), rather, indicative of positive affect (Burgdorf & Panksepp, 2001). Adult rats do not emit these 50-kHz calls when stressed, but they do emit them when they are being tickled. This indicates both change over the course of the rat's life, as well as a unique vocalization for laughter-like behavior.