Attractivity is mediated by chemosensory cues generally present in urine. Also known as pheromones. If you block the ability to smell, females tend not to engage in these behaviors quite as frequently. These behaviors have typically been studied in lab. It has recently been found that behaviors elicited in lab are different than in the wild.
Rats live in colonies.
Almost all females in colony wil come into behavioral estrus at the same time. Females tend to have thier offspring all at the same time. Might promote the survivability of the offspring. Females also tend to modulate the receptive phase of the cycle differently in the wild than the lab. During receptive phase, rats engage in lordosis.