Girls who have twin brothers may be at a disadvantage in life after losing out in the womb, new research claims.
A study of wild sheep found that female lambs with male twin siblings were 10% lighter at birth than those with twin sisters, were less likely to survive their first winter and had fewer offspring over their lifetime.
The findings show that male embryos out-compete females for nutrients when they are together in the womb, scientists believe.
A female twin may also be damaged by exposure to her twin brother's hormones.
Lead researcher Dr Peter Korsten, from the University of Edinburgh, said: "Male and female embryos have different needs at early stages of development, and this means that the female embryos may lose out to their brothers. Our findings show that conflict between male and female siblings can arise very early in life, potentially with long-term consequences."
The research, funded by the Natural Environment Research Council (NERC) and the European Commission, is published in the Royal Society journal Biology Letters.