Per capita sperm metabolism is density-dependent

From bacteria to metazoans, higher-density populations have lower per capita metabolic rates than lower-density populations. This relationship between density and metabolism was actually first proposed 100 years ago and was focused on spermatozoa but contemporary studies of sperm metabolism specifically assume that sperm concentration has no effect on metabolism. We did a systematic review to estimate the relationship between sperm aerobic metabolism and sperm concentration, for 203 estimates spanning 49 species, from protostomes to humans from 89 studies. We found strong evidence that per capita metabolic rates are concentration-dependent: both within- and among-species, sperm have lower metabolisms in dense ejaculates but increase their metabolism when diluted. On average, a 10-fold decrease in sperm concentration increased per capita metabolic rate by ~60%. Metabolic plasticity in sperm may be an adaptive response but this requires further testing.