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Successful conservation of an endangered species relies on a good understanding of its reproductive biology, but there are large knowledge gaps. For example, many questions remain unanswered with regard to gestation and fetal development in the giant panda. We take advantage of a sample size that is unprecedented for this species (nZ13) to explore patterns in reproductive development across individuals at the China Conservation and Research Center for the Giant Panda. We use ultrasound techniques on multiple giant pandas for the first time to empirically confirm what has long been suspected that pandas exhibit delayed implantation of the embryo. We also show that the duration of postfetal detection period is remarkably similar across individuals (16.85G1.34 days). Detection of fetus by ultrasound was strongly correlated to the peak in urinary progesterone (rZ0.96, tZ8.48, d.f.Z8, PZ0.0001) and swelling in the mammary glands (rZ0.79, tZ3.61, d.f.Z8, PZ0.007) and vulva (rZ0.91, tZ6.40, d.f.Z8, PZ0.0002) of adult females. When controlling for both the duration of the total gestation period and the postfetal detection period, infant birth weight was only significantly predicted by the latter (bZ11.25, S.E.M.Z4.98, tZ2.26, PZ0.05), suggesting that delayed implantation increases flexibility in the timing of birth but is not important in dictating infant growth. This study informs reproductive biology by exploring the little-studied phenomenon of delayed implantation in relationship to physiological changes in pregnant giant panda females.

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