Raising sons is an exhausting experience that leaves killer whale mothers less likely to produce more offspring, a new study suggests.According to the research, each living son cut a mother's annual likelihood of successful breeding - a calf surviving to the age of one - by about half.Furthermore, the effect continued as the sons grew older, indicating they were a lifelong burden on their mothers, the scientists said.

Raising sons is an exhausting experience that leaves killer whale mothers less likely to produce more offspring, a new study suggests.According to the research, each living son cut a mother's annual likelihood of successful breeding - a calf surviving to the age of one - by about half.Furthermore, the effect continued as the sons grew older, indicating they were a lifelong burden on their mothers, the scientists said.