Scientists explain why birds get sex on the brain in the spring
7 August 2009
BBSRC-funded researchers at the University of Oxford have discovered how birds sense the lengthening days of early spring and time when they breed, solving a 70-year mystery.
Light receptors deep in the brains of birds detect the changing day length and trigger the seasonal development of the reproductive system. If all goes well chicks hatch six to eight weeks later.
The research, published in the journal Current Biology and funded by BBSRC and the Wellcome Trust, has pinpointed the identity of the light receptors in chicken and Japanese quail in a deep part of the brain called the hypothalamus.
"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 of the Nuffield Laboratory of Ophthalmology at the University of Oxford, who led the work. "By timing their mating to the changing seasons, birds can make sure that there will be enough food around for their chicks.
"For example, rooks eat worms, and they have their offspring early in spring when the soil is still moist and the worms are at the surface. Finches will have their chicks later in the year to match when there are seeds around to feed them."
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.
"Eyes can respond very well to light in fractions of a second, which is crucial for vision. But detecting dawn and dusk requires measuring changing levels of light over long periods of time, a different thing altogether. This is what the light receptors do in a deep part of the bird’s brain called the hypothalamus," says Professor Foster.
Since the initial research in the 1930s, a whole cascade of events have been identified that lead to the production of hormones which stimulate the reproductive response in birds. But the identity of the initial deep-brain light sensors have remained a mystery. It has remained one of Professor Foster’s research interests for 25 years since he started his PhD.
In this new work, Professor 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.
The team showed in lab tests that when this chicken gene is active in nerve cells, the nerve cells generate electric signals in response to light. Finally, they demonstrated that chicken VA opsin is present in the hypothalamus.
"We have shown that chickens have a pigment called VA opsin, that it reacts to light, and that it is around in exactly the part of the bird’s brain that we know responds to day length," says Dr Stephanie Halford, first author on the study. "This evidence is as strong as we can get that we’ve pinpointed the light receptors that tune birds’ responses to the changing seasons."
Having identified the chicken VA opsin gene, the researchers have now found VA opsins in amphibians, reptiles and other birds. They believe this deep-brain mechanism for sensing the change in day length and coordinating seasonal breeding is seen 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. This does not mean that mammals do not respond to the seasons. In sheep for example, the eye regulates production of the hormone melatonin, which is released at night. The longer the night, the longer the pattern of melatonin release. The increased release of melatonin with the shortening days in the autumn triggers reproduction and ultimately the arrival of lambs the following spring. Although the initial light receptor is different in mammals, the same structures in the brain are still involved in regulating responses to the passing seasons.
"Humans are seasonal animals too," says Professor Foster. "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."
Notes to editors
‘VA opsin-based photoreceptors in the hypothalamus of birds’ by Stephanie Halford and colleagues is to be published in the journal Current Biology with an embargo of 20:00 BST (UK time) / 12:00 ET (US) on Thursday 6 August 2009.
The research was funded by the Wellcome Trust and the Biotechnology and Biological Sciences Research Council.
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