Female twins suffer a disadvantage if they are born with a twin brother, a study suggests.
Scientists studying a population of wild sheep have found that female lambs with a twin brother were 10 per cent lighter at birth compared with those with a twin sister.
The researchers at the Universities of Edinburgh and Cambridge studied a wild population of Soay sheep on the island of Hirta, St Kilda.
On average, the females with male twins were also less likely to survive their first winter, and they had fewer lambs over their lifetime.
In contrast, male sheep were found to be unaffected by the sex of their twin.
Researchers believe that male embryos may be more effective in competing for nutrients in the womb, to the detriment of their female counterparts.
The research also suggests female twins may have their development hampered by exposure to their twin brother’s hormones, such as testosterone.
Male and female embryos have different needs at early stages of development. Our findings show that conflict between male and female siblings can arise very early in life, potentially with long-term consequences.