Utero-muscular twisting and sperm storage in viperids

suggested that females store sperm after mating and prior to ovulation/fertilization by “utero-muscular twisting” (UMT).Basically, the posterior region of the uterus becomes convoluted and contracted. Previous workers have indicated that thismechanism also occurs in temperate taxa, including Agkistrodon piscivorus, and others have suggested that UMT is an ancestralreproductive strategy of viperids. The work of these authors ignores earlier histological studies that found infundibular spermstorage tubules (SSTs) in old world viperids Cerastes cerastes and Viperus aspis. The SSTs in these two viperids were similar tothose of harmless snakes, including some more basal (Leptotyphlophidae and Typhlopidae) and others more derived(Colubridae) than the Viperidae. In addition, our studies on seasonal variation in sperm storage in Agkistrodon piscivorus showsperm in infundibular SSTs in all reproductively active females and even those with fetuses in utero, with no indication ofUMT. Our histological examination of the crotalines Sisturus miliarius and Crotalus durissus indicates that SSTs occur in theseforms as well. The evidence for UMT as a sperm storage strategy needs anatomical and physiological confirmation.