Rat spermatozoa are two to four times larger than that of other animal species and are easily damaged by changes in pH, osmotic pressure, and temperature. Because these animals are very frequently used in medical research, a cryopreservation method was developed nearly 20 years ago. However, rat spermatozoa motility after thawing is extremely poor, and unless artificial insemination is performed at night (10:00-11:00 pm) no offspring will be produced. Furthermore, the number of offspring produced after successful artificial insemination is often lower than normal so the cryopreservation of rat sperm is not typically considered practical.
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Source: Phys.org