The sperm of many onychophorans must cross body walls and swim for years before fertilizing an egg. We propose that the cause for such obstacles is the high cost of reproduction in these animals, which leads females to select eggs early in their development; and to invest heavily in their atypically large Golgi apparatus and in the production of glycogen. Velvet worm spermatozoa are subjected to a marathon that selects the most resistant.

Keywords: female and male gametes, high reproductive investment of onychophorans, environmental pressure, female selection