Animals migrate to eat insects that live there year round. Many animals in the tundra hibernate during the long, cold winter months. Animals are most active in the short summer as the snow melts and shallow wetlands form
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the corpus luteum secretes estrogens and progesterone. the latter hormone causes changes in the uterus that make it more suitable for implantation of the fertilized ovum and the nourishment of the embryo. if the egg is not fertilized, the corpus luteum becomes inactive after 10–14 days, and menstruation occurs.

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