2020 updates by Cathy Bickel
A female gerbil in heat exhibits stereotypical behavior. She will dash up to her partner, male or female, and touch or nearly touch the other gerbil with her nose. Then she will dash to the other side of the tank and pause, turning her hindquarters toward the other gerbil, squatting slightly, and flexing the muscles on her back and hindquarters.
A male will follow her and mate, whereas a female partner will most likely ignore her. Occasionally, a female partner will mount the female in heat, or the female in heat will lose her patience and mount the other gerbil.
In the rare case where a male-female pair does not mate successfully on the first try, the female seems to come into heat again in about four days.
Heat cycles of females housed together seem to follow no predictable pattern. Females in heat will occasionally become aggressive toward their cagemate. Extra toys and diversions may help. If your clan includes tanks of males, try to keep them beyond scent of females in heat, as this can provoke aggression between otherwise harmonious males.