Exam 2
Answer key

Short answer (5 points each).

1. The ancestral form consisted of a zygomatic arch that was basically solid (it had a very tiny hole in the front portion throgh which no muscle could pass). The masseter muscle ran from the lower jaw to the underside of the zygomatic arch only, leaving the first cheek teeth uncovered. (1 point) In the sciuromorph rodents, the zygomatic arch is also solid, but the masseter muscle now attaches along the zygomatic arch and as well as forward onto the rostrum. In side view, the cheek teeth are no longer visible. (1 point) In myomorph rodents, there are small holes in the arch through which a portion of the masseter muscle passes and attaches to the rostrum. Other portions of the masseter muscle attach to the length of the zygomatic arch. In addition, there may be flanges (projections) from the forward edge of the zygomatic arch to which portions of the masseter muscle attach. (1 point). In hystricomorphs, there is a very large hole in the front of the zygomatic arch through which a large portion of the masseter muscle passes before attaching to the rostrum. (1 point) In all three cases, the modifications result in increased chewing power and the ability to move the jaw forward more efficiently for gnawing (1 point).

2. Structural diversity of forelimbs is limited because the forelimbs need to be well developed for grasping at the time of birth, so that the infant can make its way (unassisted by the mother) to the pouch and/or teat to complete development. (NOTE: Although gestation is very short, that is NOT the reason structural diversity is limited. In fact, structural diversity of hind limbs is fairly great. Your answer needed to refer specifically to restrictions on diversity of forelimbs.) Once well developed for one purpose, it is not possible to "redesign" the forelimbs for another type of locomotion. (3 points)
Examples of types of forelimb specializations that are not found in marsupials (1 point each for specialization and type of locomotion):
flippers (aquatic or marine)
wings (flying)
hooves (cursorial)
syndactyly (arboreal)
IMPORTANT NOTE: On several of the tests I made a mistake and marked "syndactyly of the forelimbs" in association with arboreal locomotion wrong. This should have received credit. If you have this answer and I took off points for it, please bring it up to me after class and I will correct it.

3. Adaptations permitting prolonged submersion (1 point each, 5 needed for full credit)

• Extra capillaries in the lungs (more efficient exchange of respiratory gases)
• Use 3 times as much of the oxygen from a breath of air as do terrestrial mammals (can extract more oxygen, and therefore can supply oxygen to tissues for a longer period of time)
• Can force almost all air out of the lungs when exhaling (increases the amount of oxygen in the lungs for next breath, which in turn enables them to extract more oxygen from that breath)
• Twice as high a concentration of red blood cells (can carry more oxygen to body tissues)
• 2-9 times as much myoglobin in muscle tissue (muscles are more efficient at extracting oxygen from circulatory system)
• Heart rate drops to half its normal rate during long dives (oxygen use is conserved)
• During dives, blood is redistributed (also conserves oxygen, so it lasts longer)
• High tolerance to lactic acid (prevents muscle cramping and allows the animal to go longer between breaths)
• High tolerance to carbon dioxide (prevents spontaneous breathing underwater when metabolic wastes start to build up)

4. Possible answers include:

• Pentadactyly (better grasping ability)
• Nails instead of claws (better grasping ability)
• Prehensility of hands and feet (better grasping ability)
• Traction ridges on tips of digits (better grasping ability)
• Tendency towards erectness of posture (more efficient locomotion)
• More reliance on vision (e.g., forward facing eyes, binocular, stereoscopic, most have color vision) (better depth perception for moving quickly in an arboreal environment, and ability to locate ripe fruits through color vision)
• Reduction of snout (to allow for improved vision)
• Big, complex brains (allows learning and tracking of arboreal food sources through space and time)

I accepted other answers (for example, more specific adaptations) if they were well justified. (1/2 point for each characteristics and 1/2 point for its function or significance)

5. A fetus needs protection from the mother's immune system because the immune system is well developed to recognize "self" from "not-self". (1 point) The fetus is "not-self" because one half its genome is paternal in origin. (1 point) Monotremes lay eggs, so there is complete separation of the fetus from the mother and the mother's immune system is not activated. (1 point) Marsupials achieve protection through the shell membrane, which provides a physcial barrier between the fetus and the mother's immune system for the first 2/3 of pregnancy. At this point, the shell membrane is shed and pregnance comes to an abrupt end. (1 point) Placental mammals acheive protection through a physical barrier (the trophoblast, which is part of the placenta). (1/2 point) The trophoblast is also the source of hormones (primarily chorionic gonadotropin) that suppress the mother's immune response. (1/2 point)

6. I gave 1/2 point each for the following:

• large ears
• tragus (flap of skin inside ear)
• facial specializations (baffles, slits)
• nose leaves
• infalted maxillary bones
• indentations in the cranium
• "bent" rostrum (upward deflection)
• muscle specializations of the middle ear that allow dissaciation of the ossicles)

In addition, everybody got a free point on this one.

7.

Similarities (1 point): reduced external appendages, fore and hind limbs modified as flippers, limbs enclosed within body up to knees and elbows, blubber, large body size

Differences (3 points)

Otariidae	Phocidae
Can support the weight of upper body on front flippers	Can't
Can rotate hind limbs underneath body for terrestrial locomotion	Can't (hind limbs drag behind body when on land)
Have well developed forelimb musculature and enlarged cervical and thoracic vertebrae	Have well developed hindlimb musculature and enlarged lumbar vertebrae

The Phocidae are more completely adapted for aquatic locomotion and are less able to move about on land. (1 point)