It's Lorna's! New Scientist has at last given me the information I need - a study at the University of Wisconsin-Madison's National Primate Centre revealed that males gain an extra 10% of their body weight during the pregnancy of their partner. Possibly a store of energy for when the baby is born, the gain is probably triggered by higher levels of prolactin, a hormone produced during the latter stages of pregnancy.
So, see? It's not my fault. Maybe this hormone is also present in biscuits and cheese. I think we should be told.