Delayed implantation in mustelids, with special emphasis on the spotted skunk.

Three distinct reproductive patterns are exhibited by mustelids. Some species (ferret and least weasel) breed during spring and summer and do not exhibit delayed implantation. Others (mink and striped skunk) exhibit variable gestation periods with brief periods of delayed implantation occurring only if the females are mated early in the season. Most mustelids (western spotted skunk, badgers, marten, wolverine, etc.) always exhibit a prolonged period of delayed implantation lasting several months. In such species, the luteal cells fail to undergo complete differentiation during the prolonged period of embryonic diapause. The process of luteal differentiation, which involves doubling of luteal cell size, extensive production of smooth endoplasmic reticulum, and mitochondrial changes, and which is accompanied by increased progesterone secretion, occurs a few days before implantation and coincides with renewed embryonic development. Attempts to induce implantation in mustelids by injecting progesterone have failed, suggesting that other ovarian hormones hormones are required to induce nidation. Changes in photoperiod will hasten or delay luteal development, progesterone secretion and implantation. Hypophysectomy before the time of increased luteal function prevents implantation and increased progesterone secretion, whereas hysterectomy has no effect on progesterone secretion. This indicates that only pituitary hormones are required to induce the final steps in luteal cell differentiation and progesterone secretion. Attempts to induce increased luteal function and implantation by administration of exogenous gonadotrophic hormones have been inconclusive. Prolactin may be important in regulating luteal function in mink and ferrets but may be less significant in other mustelids such as the spotted skunk and European badger.