Women who have just finished ovulating are better at detecting snakes than at other times of their menstrual cycle, according to an unusual study that sheds light on in-built reflexes for survival.
Nobuo Masataka of Japan's Kyoto University tested 60 healthy women of child-bearing age at three different phases of their cycle.
They were simultaneously shown nine pictures, one of which was a snake among flowers while the others were only of flowers.
The volunteers were tested on how quickly they spotted the serpent.
The fastest women were those who were in the so-called luteal phase of their menstrual cycle, or the stage that immediately follows ovulation.
The study strengthens theories that we have a "fear reflex," or an innate response to threat signals such as potentially poisonous snakes, Masataka believes.
Previous research has found this trait among infants aged as young as eight months and even among non-human primates.
Among women, the reflex seems to be influenced by hormone levels at a stage when they could be pregnant and thus be more protective towards their foetus, the study suggests.
"It could contribute to women's ability to increase their vigilance towards biologically relevant threatening stimuli around themselves during this period of possible pregnancy," it says.
The paper was published on Thursday in the British journal Scientific Reports.