Length of delay to birth of a second litter in dwarf hamsters (Phodopus): evidence for post-implantation embryonic diapause.

In Phodopus, all first litters are born after an 18 (or 19) day gestation. Birth of subsequent litters conceived during a postpartum estrus is routinely, but not always, delayed beyond 18 days. With an interval from first mating to birth of a second litter of as little as 36 days, Phodopus have the most compressed reproductive cycle of any eutherian mammal. Although pups of the Siberian hamster P. sungorus gain thermal independence from maternal care faster than pups of the Djungarian hamster P. campbelli, no species difference in the extent of delay to second litters was found. However, poor growth of first litter pups as a result of unintentional limitation of water bottle access increased delays before birth of a second litter in P. sungorus. Weight of pups in the first litter was a good predictor of the length of delay. In P. campbelli, females with large pups early in lactation were the only ones to deliver a second litter without delay, and short delays enhanced weaning weights for pups in the first litter. Patterns in P. sungorus were similar. The timing of the delay in second litters was investigated in P. campbelli. Delayed implantation during second pregnancies was common. Postimplantation embryonic diapause occurred in 43% of females following unilateral ovariohysterectomy on day 8 of a first pregnancy. As pup growth curves also implicated a postimplantation embryonic diapause, Phodopus may be the only known mammalian genus outside the order Chiroptera in which postimplantation diapause can occur.