When too much sex is exhausting

Larger rams may run out of sperm

source: Alison Motluk
New Scientist April 10 2000 p8

Researchers from the University of Stirling, Scotland, have studied Soay sheep on St Kilda island, Scotland, and found that larger rams initially father more lambs, but this changes toward the last two weeks of the breeding season, when smaller rams father as many as bigger rams. Larger rams father more lambs overall, but toward the end of the five-week breeding season, they may become low on sperm.