The original experiments with rats applied the following protocol: A male rat was placed into an enclosed large box with four or five female rats in heat. He immediately began to mate with all the female rats repeatedly until he eventually became exhausted. The females continued nudging and licking him, yet he did not respond. When a novel female was introduced into the box, he became alert and began to mate once again with the new female.  This phenomenon is not limited to  common rats.The Coolidge effect is attributed to an increase in dopamine levels and the subsequent effect upon an animal's limbic system. In a study conducted by Fiorino et al., male rats were used to study the role of the mesolimbic dopamine system on their sexual behaviour. In their experiment, microdialysis was used to monitor dopamine efflux from the nucleus accumbens during three stages of sexual behaviour, these included: copulation, sexual satiety, and the reinitiation of sexual behaviour. Behavioural testing for the Coolidge effect consisted of several phases including, copulation with a female, reintroduction to the same female, access to the same female, introduction to a novel female, and copulation with the novel female. During these phases, dopamine and its metabolites were monitored. Results from this study found that overall there was a significant increase in dopamine efflux in response to both the first female and the second female. During copulation with the first female, concentrations of dopamine in these male rats showed a significant increase, however, when the same female was presented again, a significant increase in dopamine was not observed. When a novel female was presented, initially, there was a small increase in the levels of dopamine, however, after continued copulation with the novel female, a significant increase in dopamine levels was observed. From these results, they concluded that an increase in mesolimbic dopamine efflux is associated with the appetitive and consummatory stages of sexual behaviour in male rats. Their data also suggest that stimuli associated with a novel female may increase dopamine transmission in a rat that is sexually satiated, and hence have a role in the reinitiation of sexual behaviour.Additional studies have also provided further evidence for the role of the nucleus accumbens on sexual behaviour in rats. In a study conducted by Wood et al., male rats were divided into three conditions, and were presented with a cotton ball laden in either saline (control group), estrous vaginal smear of a familiar female (experimental group), and estrous vaginal smear of a novel female (experimental group). In this experiment, the role of the nucleus accumbens was characterized through recording neuronal activity of single cells in this area of the brain. Results from this study showed a greater proportion of neuronal activation when initially presented with a novel estrus stimulus in comparison to familiar estrus stimulus. Subsequent presentations of the novel estrus stimulus did not show an increase neuronal activity.

Question: How did the rat respond when a new female was added?
Answer: became alert and began to mate once again with the new female