The sex hormone oestrogen plays a crucial role in a wide variety of human emotional responses, say experts.
Oestrogen has many functions, say experts
It not only has a part in generating feelings of sexual desire, claim scientists, but is also at the root of other types of arousal - producing alertness or even fear.
Experiments on mice suggest that without oestrogen, the animals still have these responses, but they are less finely tuned.
The researchers, from Rockefeller University in New York, say a primitive "general arousal" system which needs the sex hormone to operate may be key to normal human responses.
The research team took normal mice, and compared the way they react to fear or sexual stimuli with mice engineered to lack important genes.
These genes produce receptors on the surface of cells which allow oestrogen to bind and influence the way the cell works.
Mice lacking a particular type of oestrogen receptor were significantly less responsive to the same stimuli - highlighting the need for oestrogen in this process.
Combining this finding with data from other similar experiments appeared to reveal the presence of a "general arousal" mechanism - perhaps a priming mechanism that made the body more sensitive to stimuli.
The system would be important in how the body responds to hunger and thirst and pain, as well as fear and sexual desire.
The researchers say that understanding arousal is one of the most important areas of research.
"Besides the status of arousal as a 'holy grail' in neurobiology, its deficits can contribute to disorders of cognition, and its erosion can account for some of the mental difficulties during ageing."
They suggested that harnessing a "general arousal" mechanism could "enhance vigilance", and help develop anaesthetic techniques.
British researcher Dr Keith Kendrick from the Babraham Institute in Cambridge said that oestrogen had a number of important functions aside from controlling libido.
"Sex steroids play an important role in memory," he said, "They appear to optimise the way that neural connections respond to stimuli.
"If you don't have these, the systems work, but not very well."
The research was published in the Proceedings of the National Academy of Sciences.