Exam 2 key

III. Short answer. Your answers will be graded based upon whether they are factually correct and complete. (worth 5 points each)

1. Describe, compare and contrast the alternative reproductive strategies of delayed fertilization and delayed implantation. In what kinds of environments would you expect to find each type of strategy?

   Delayed fertilization occurs when females mate in the fall and store sperm over the winter while they hibernate. In the spring, they ovulate, the egg is fertilized and development proceeds. This is a good strategy in highly seasonal environments for species that hibernate. Individuals born in the spring need to put on as much weight (fats stores) as possible to survive the winter. This means that the earlier in the spring they are born, the higher their likelihood of survival. If females store sperm, they can have the embryo start its development much earlier than if they waited for males to crank up sperm production in the spring, which would mean getting pregnant and giving birth much later. (2.5 points with partial credit taken off for portions of this that were missing from your answer)

   Delayed implantation is when the embryo stops developing when it reaches 100-200 cell stage (a bubble of cells). The period of dormancy can last days to weeks and factors that reinitiate development are not known. When development resumes, the embryo implants into the uterus and is deeply imbedded in the uterine lining. This reproductive variant occurs in eutherians when resources are somewhat unpredictable (so that birth can be time to coincide with favorable resources) or when there is a mismatch between the length of gestation and the time between mating and giving birth (e.g. seals). (2.5 points, with partial credit taken off for portions of this that were missing from your answer)

2. Considering the various phylogenies for eutherian mammals that have been proposed in the last 10 years are so, what are the major points of disagreement and what may account for these discrepancies? You need not mention specific studies in your answer.

   Major points of contention include:

   • the basal spit (Afrotheria and everything else, or xenarthra and everything else, etc.)
   • the monophyletic vs. polyphyletic nature of the insectivora and edentata/xenarthra
   • the placement of pholidota (what are their sister taxa?)
   • the placement of bats

   (2.5 points)

   The discrepancies in the trees may be due to the different data that was used to construct each of the trees. Some studies used molecular data only, others used morphological or a combination. Even the studies that used molecular data donŐt always agree, because some used a single gene, others used multiple genes, and the genes could be either mitochondrial, nuclear or both. (2.5 points)

3. What role do the hormones Estrogen, Progesterone, Growth Hormone and Prolactin play in lactation? Compare the levels of these hormones in the following three situations: (1) during pregnancy, (2) after birth, when offspring survive and (3) after birth when no offspring survive.

   (2 points) During pregnancy, E and P are produced in high concentrations by the ovary. They stimulate the mammary gland to become larger and stimulate the pituitary to release GH, which also makes the mammary gland grow.

   (2 points) After birth, the ovary stops secreting E and P, which allows the pituitary to secrete Pr. Pr and GH act on the mammary gland to induce it to produce milk. Suckling by offspring creates positive feedback to the pituitary (through the hormone oxytocin) that stimulates more release of GH and Pr to maintain milk production.

   (1 point) After birth, if no offspring survive, there is no suckling stimulus to provide positive feedback to the pituitary, so GH and Pr secretion go down and milk stop being produced. 4.

4. Some members of two orders (Monotremata and Soricimorpha) are venomous. How do they differ in the location of the venom glands, how the venom is delivered and the function of the venom?

   (2 points) Shrews make venom in submaxillary glands (modified salivary glands) at the base of their lower incisors. It is delivered through a groove in the lower incisors when the shrew bites its prey.

   (2 points) The platypus makes venom in a gland located on their thigh. (NOT in the spur!). It flows out along a groove in the spur when the platypus scratches another animal.

   (1/2 point) The venom of shrews is used to immobilize prey that are bigger in body size than the shrew.

   (1/2 point) The venom of the platypus is used in male-male fighting.

5. Compare and contrast the anatomy of the carnivore families Otariidae and Phocidae. How do these differences relate to differences in locomotion?

   Otaraiidae

   • have enlarged front limbs that can support weight of body
   • Enlarged vertebral spines on cervical and thoracic vert. for attachment of powerful muscles
   • Relatively small hind limbs that can fold underneath themselves
   • Pull themselves through the water with front limbs
   • So they are not as good swimmers, but they are more mobile on land.

   (2.5 points)

   Phocidae

   • have smaller, weaker front limbs that can't support weight of the body
   • have enlarged hind limbs and process on the lumbar vertebrae for attachment of big muscles (powered from behind)
   • hind limbs can't rotate under body, so movement on land is hindered, but they are better adapted for swimming.

   (2.5 points)

6. How do choriovitelline and chorioallantoic placentas differ? How do these differences affect nutrient transfer and gas exchange, and the degree of infant development at birth?

   (2 points) Choriovitelline placentas have fusion and vascularization of the chorionic and vitelline membranes. The surface of the membranes are convoluted for greater surface area, but there is limited diffusion of nutrients and gases across the placental membranes. The yolk sac is very large and contains most of the nutrients needed for the growth and development of the fetus.

   (2 points) Chorioallantoic placentas have fusion af the chorionic and allantoic membranes. The allantois is highly vascularized and the fused membranes have villi (small projections that work their way deep into the uterine lining). This allows closer proximity between the maternal and fetal circulatory systems, so there is more diffusion of nutrients and gases across the placenta. The yolk sac is still present during early development, but most of the nutrients needed for growth and development come from the maternal circulation.

   (1 point) Basically the choriovitelline placenta is not as efficient as the chorioallantoic placenta for gas and nutrient exchange.