14 DAY TRIAL // Mothers are known to be especially attuned to their child howls. But the folk wisdom has recently been scientifically proven.
A research team focused on mouse mothers, which display a significantly more powerful brain response to the distress calls of infant mice than virgin females do. The research underlines the link between hormonal, behavioral and call receptiveness changes.
When young mouse pups are separated from their mothers, they begin emitting high-pitched ultrasound calls at around 65 kilohertz, which a human ear cannot hear. "Some scientists have named these noises 'distress calls', but that's been controversial because some people say it's more a physiological response than an emotional response," explains Robert Liu at Emory University in Atlanta, Georgia, US.
In the wild, a mother mouse will follow these ultrasound calls to get to her crying offspring. Mother mice display twice as much interest in these calls than in a 20 kHz tone (which can be detected by the human ear).
Male mice that had fathered multiple litters presented the same reaction to the offspring's distress calls. Instead, virgin females focused similarly to both types of sounds, while bachelor males ignored both types of sound.
The team placed probes in the mice' left auditory cortex, the brain's zone involved in sound processing. The brain activity was measured while sets of pup cries were played through loudspeakers. In the maternal mice, the brain's activity reached the peak in 16.6 milliseconds following the start of the distress call, while in virgin mice it took about 50% more (21.2 ms).
The auditory neurons were also found to form more coordinated synapses in mother mice than in virgins, increasing the mother's capacity to react to her babies' cries.
But what causes this reaction is still unknown; probably mothers associate the offspring's noises with brain reward chemicals, like dopamine. But the motherhood's boosted auditory power was also linked to hormones. For example, higher levels of estrogen in the midshipman fish increases its ability to pick up mating calls.
The team also wants to investigate the brains of male mice to see if they, too, present altered brain activity after fathering multiple litters.