Why birds sing

2009-08-07 11:55

Cape Town - Researchers at the University of Oxford have discovered a light-sensitive spot in the brains of birds that turns them on to start singing and have sex.

"When you hear birds singing in the springtime, it's a light-sensitive molecule deep in their brain that's triggered this reproductive event," says Professor Russell Foster, who led the work at the university's Nuffield Laboratory of Ophthalmology.

His team's findings have solved the decades-long mystery of why birds know it is time to find a mate, Oxford's press office said in a statement on Thursday.

Light receptors located deep in a bird's brain - in an area known as the hypothalamus - detect changing day lengths and trigger the seasonal development of the reproductive system. If all goes well, chicks hatch six to eight weeks later.

"Exactly how birds sense the arrival of spring has been a long-standing problem. In the 1930s, it was shown that birds surprisingly don't use their eyes to measure the increasing number of hours of sunlight.

"Instead, a deep part of their brain registers day length. This is possible because bird skulls and brain tissue let a lot of light through, so that a significant amount of light still reaches deep parts of the brain."

Light-sensitive molecule

Foster and colleagues identified a gene in the chicken genome that encodes a novel light-sensitive molecule.

"The molecule belongs to a family of photopigments called VA opsins that were first discovered in fish. These light-sensitive molecules are not involved in vision, but detect slow changes in the light environment such as dawn and dusk."

Having identified the molecules in chickens, the researchers then found VA opsins in amphibians, reptiles and other birds.

They believe this deep-brain mechanism for sensing the change in day length and co-ordinating seasonal breeding occurs in a whole range of species, across all vertebrates except mammals.

"Mammals may have gone through a 'bottleneck' in their evolution when the whole lineage was nocturnal and light receptors outside the eye were lost."

However, Foster says humans are seasonal animals too.

"The number of births, suicides, murders and instances of child abuse turn out to vary with the time of year.

"For example, up until the 1940s, there was a 30% variation in the number of births in Spain at different times of the year, with most seen in the spring," he says.

- SAPA