The energetic investment per spermatozoon and in spermatogenesis is central to a male's reproductive strategy. Relatively little, however, is known about environmental influences on variation in male allocation decisions and associated trade-offs. Plasticity in sperm length and testis size in response to variable food and temperature conditions either before or after adult eclosion was investigated in Scathophaga stercoraria, a classic model organism for sperm competition. Both measures showed interesting and clear environmental effects and also a heritable component. Testis length, and thus presumably sperm production, showed a hypoallometric (b < 1), but non-linear increase with body size, indicating that the allometric relationship changed with size. Like body size, testis length decreased with increasing developmental temperatures, but also showed a complex cubic relationship with adult temperatures. In contrast, sperm length increased or showed a negative quadratic relationship with increasing temperatures. The increase of within-male variation in sperm length with increasing developmental temperature and decreasing adult food indicates that some of our treatments were stressful. Nevertheless, there was no evidence of a trade-off between testis size and sperm length. The missing effect of adult or larval food availability on testis and sperm length, despite strong effects of larval food on body size, suggests that investment into reproduction is less sensitive to food restriction than investment into growt
