BCB705 Biodiversity: Chapter2 - Evolution of Biodiversity

Table of Contents

1. THE INFINITE VARIETY: THE BEGINNING OF LIFE
2. BUILDING BODIES: INVERTEBRATES OF THE OCEANS
3. THE FIRST FORESTS
4. THE SWARMING HORDES
5. THE CONQUEST OF THE WATER AND THE BIRTH OF THE VERTEBRATES
6. THE INVASION OF THE LAND
7. A WATER-TIGHT SKIN AND THE SHELLED EGG
8. LORDS OF THE AIR
9. EGGS, POUCHES AND PLACENTAS
   1. The Spiny Echidna
   2. The loss of teeth
   3. Absorbing heat
   4. An endothermic metabolism
   5. Mammals: A polyphyletic origin
   6. The earliest true mammal
   7. The Opossums: The pouch
   8. Caring for the young
   9. Marsupial fossils
   10. Carnivorous marsupials
   11. The splitting world
   12. Evidence supporting continental drifting
   13. An alternative method of nourishing the young
   14. Placental mammals vs Marsupials
   15. Marsupials: Opportunity to evolve
   16. Survival today
   17. Marsupials and placental mammals: A resemblance
   18. Major differences
   19. The rat kangaroo
   20. Kangaroo: Escaping predators
   21. Kangaroo: The teeth
   22. Kangaroo: Reproduction
   23. The effect of drought on reproduction
   24. The placental mammals dominate
   25. The placenta
   26. The mammalian sexual cycle
   27. Immunological rejection
   28. Placental mammals: Success
   29. Test Yourself
   30. Assignments
10. THEME AND VARIATION
11. THE HUNTERS AND THE HUNTED
12. A LIFE IN THE TREES
13. THE COMPULSIVE COMMUNICATORS

The mammalian sexual cycle

All of this makes for great biochemical complications. But there are further ones. The mammalian sexual cycle involves the regular production of a new egg. This causes no problem to the marsupial, for in every species, the neonate emerges before the next egg is due to be produced. In the placental animal the foetus, however, stays in the uterus for a very much longer period. So the placentalfoetus secretes a hormone which suspends the mother's sexual cycle for as long as the placenta is in place so that no more eggs are produced to compete with the foetus in the uterus.