Complex sentence: The sperm of large helpers were characteristically similar to those of breeders, but helpers had smaller testes; the small size of the helpers' testes coupled with the physiological equivalence of their sperm compared to breeder sperm imply that the helpers were reproductively suppressed.
Split into simpler sentences: The sperm of large helpers were characteristically similar to those of breeders, but helpers had smaller testes. The small size of the helpers' testes coupled with the physiological equivalence of their sperm compared to breeder sperm imply that the helpers were reproductively suppressed.