Science - April 29, 2010

More sons after sex in the rain

Quality of buffalo sperm is better in rainy periods.

African buffalos
African buffalos have more sons than daughters during rainy periods. Pim van Hooft, who works for the Resource Ecology Group, discovered differences in the ratio between the sexes for buffalos in different categories of year and season. You would expect there to be roughly as many males as females born every year and in every season. However, Van Hooft found relatively more males in years and seasons in which a lot of rain fell. His results are being published this week in BMC Evolutionary Biology.
'The obvious explanation is that this excess of males in rainy periods has to do with variations in physical condition', explains Van Hooft. 'Their physical condition improves during rainy periods and the quality of their sperm is also better then than in dry periods.' According to Van Hooft, the buffalos' physical condition affects the activity of the so-called sex-ratio genes on the Y chromosome.