Exam 1
Answer key

Short answer (5 points each).

1. Describe the changes in jaw structure and hearing apparatus that accompanied the evolutionary transition from reptiles to mammals.

The answer for this question comes directly from the following table in the lecture outline for Mammalian Origns:

Reptiles	Mammal-like reptiles	Mammals
Mandible composed of many bones, including the dentary	Increase in size of dentary bone, decrease in size of postdentary bones	Mandible composed of a single bone, the dentary
Articulation of mandible with cranium occurs between the articular bone (lower jaw) and the quadrate bone (cranium)	Articulation varies, and sometimes occurs in two places	Articulation of mandible with cranium occurs between the dentary bone and the squamosal
Sound is conducted from the tympanic membrane to the inner ear via a single bone, the stapes	Sound is conducted via the stapes, but the articular and quadrate bones also play a role in transmitting sound	Sound is conducted from the tympanic membrane by a series of three bones: the malleus (formerly the articular bone), the incus (formerly the quadrate) and the stapes. They are collectively known as the ossicles.
Dentition is homodont	Dentition is heterodont	Dentition is heterodont

The information contained in each cell was worth 1/2 point.

2. Describe or diagram the two basic types of mammary glands. What hormonal and sensory factors control the production of milk?

The first type consists of glandular tissue (produces milk), milk ducts (collect the milk) and a nipple (the external projection into which the ducts empty). Infants suck on the nipple to obtain milk. The second type consists of glandular tissue (produces milk), a cistern (large compartment for storage of milk) and a teat (external projection through which the infant obtains the milk in the cistern). I also gave partial credit if you described the mammary gland of the monotremes, which contains no external projection for suckling but which instead has ducts that lead directly to the skin surface. Milk production is stimulated by the hormones Prolactin and Growth Hormone and by the sensory stimulation of infants suckling. NOTE FOR THOSE WHO WROTE THAT ESTROGEN AND PROGESTERONE CONTROL MILK PRODUCTION: While Estrogen and Progesterone contribute to the development of the mammary gland during late pregnancy, it is only after levels of E and P drop after parturition that milk production begins.

The correct description of the mammalry glands (including all three components of each) was worth a total of 3 points. Correct identification of the hormones responsible for milk production was worth 1 point and the sensory stimulus for milk production was worth 1 point.

3. Mammals show many variations on the basic reproductive pattern, including delayed fertilization, delayed implantation and spontaneous abortion. Pick one such variant and describe how it differs from the basic mammalian reproductive pattern. Under what sorts of social/environmental conditions would the variant be advantageous?

• Usual sequence
  copulation -> ovulation-> fertilization -> pregnancy -> birth

• Variants
  • Delayed fertilization
    copulation -> females store sperm -> ovulation -> fertilization
    This variant is found in hibernating bats. Femaes mate in the fall and store sperm through the winter hibernation period. In the spring, they ovulate, the egg is fertilized, and development procedes. This is advantageous for a species with a short growing season before the onset of a severe winter. It allows offspring to be born early in spring and gives them time to put on adequate weight to be able to survive the upcoming winter. If females mated in the spring, the birth of offspring would be greatly delayed because it takes a few weeks for males to reinitiate sperm production following hibernation.
    
  • Delayed implantation
    copulation -> ovulation -> fertilization-> early development -> dormancy -> implants and finishes development
    This variant occurs in many carnivores. It is advantageous when environmental resources are somewhat unpredictable, because it allows females to adjust the length of their gestation (and thus the timing of birth) so that offspring are born at the time of year most conducive to their survival.
    
  • Delayed development
    copulation -> fertilization -> implantation -> dormancy -> resumption of development
    This variant is found in some bats, and is advantageous in the same types of situations as delayed implantation.
    
  • Embryonic diapause
    This variant is basically a form of delayed implantation found in marsupials. Development of the embryo is delayed as long as there is another infant suckling in the mother's pouch. When the infant leaves the pouch (or if it dies before doing so), the embryo that is in diapause starts to develop and is born soon after. This type of reproductive strategy is advantageous because it allows females to reproduce almost continuously and allows for almost immediate replacement of lost young.
  • Spontaneous abortion
    copulation -> fertilization -> abortion of embryos prior to normal end of gestation
    This occurs in mice when a female encounters a strange male (or his scent) and the male who fertilized her is no longer around. The strange male is likely to be infanticidal towards newborn young that he has not sired. By aborting her developing litter, a female can quickly mate with the new male and produce young, rather than wasting time and energy gestating a litter that is likely to be killed.

    You received 2 points for accurately describing the variant, 1 point for correctly pointing out how it differs from the normal pattern and 2 points for your explanation of the specific circumstances in which the variant would be advantageous.

    4. Each form of mammalian locomotion is associated with numerous anatomical specializations. Contrast the specializations of the limbs and tail typically found in arboreal species with those found in marine species.

    Arboreal species (2 points) are characterized by claws on their digits or prehensility (increased grasping ability) of the ands and/or feet. Prehensile tails are also common. In marine species (3 points) the forelimbs are modified into flippers (webbing between the digits), the hind limbs are not externally visible and the tail is modified into a flattened fluke that provides propulsion.

    5. Describe five characteristics of early mammals and indicate how each of these contributed to the success of mammals relative to their therapsid ancestors.

    There are many possible correct answers, including:

    • Lactation-higher survival of offspring
    • Hair-insulative capacity allowed for endothermy, wich in turn allowed early mammals to be active at night and avoid predation by diurnal dinosaurs
    • Endothermy-could be active at night and avoid predation by diurnal dinosaurs
    • Nocturnal activity pattern-active when primary predators (dinosaurs) were inactive, decreases the risk of predation
    • Small size-easier to hide from dinosaur predators
    • Insectivorous diet-generally not competing with reptiles for food; diet is compatible with nocturnal habits
    • 3 ossicles-increased sensory acuity to find food and escape predation
    • Dentary-squamosal jaw joint -more efficient food gathering and processing
    • 2 occipital condyles-increased agility to be able to find food and escape predation
    • Loss of cervical and lumbar ribs-increased agility to be able to find food and escape predation
    • Big brain-better sensory acuity and foraging abilities

    The characteristic itself was worth 1/2 point and describing how the characteristic contributed to the success of mammals relative to therapsids was worth another 1/2 point.

    NOTE: Heterodont dentition, secondary palate and other characteristics that were found in both early mammals and therapsids only received partial credit, since these characteristics could not be responsible for the success of mammals over therapsids.

    6. In 1693 John Ray proposed that mammals included those animals that (1) had blood, (2) breathed with lungs, (3) had 2 ventricles in their heart and (4) were viviparous. Evaluate this definition based on your knowledge of mammalian characteristics. Is this definition effective in differentiating mammals from other vertebrates? Which (if any) parts of Ray's chosen characteristics are diagnostic of mammals? Which current mammalian orders would be excluded from his definition?

    Although each component of the definition considered alone is not effective in differentiating mammals from other vertebrates, taken as a whole the definition successfully excludes all non-mammals. Thus, none of the components of the definition is diagnostic. The definition excludes the order Monotremata (1 point, partial credit if you put Prototheria).

    7. Mammals have numerous adaptations of the circulatory, respiratory and digestive systems that allow them to maintain a high metabolic rate. Describe five such structures and indicate the role of each in maintaining a high metabolic rate.

    Again, many potential correct answers. Describing each structure was worth 1/2 point and its role in maintaining a high MR was worth 1/2 point. As per the question, each feature had to be a structure. That is, "homeothermy" and other such characteristics could not be correct because they are not structures. Some potential correct answers:

    • Enucleate erythrocytes-red blood cells lacking nuclei, which gives them more room to carry oxygen to the tissues to support a high metabolic rate
    • Heterodont dentition-ability to gather and process food more efficiently; allows for the greater quantities of nutrients required to support an increased metabolism
    • Fur-insulation against heat loss to the environment
    • 4-chambered heart-complete separation of oxygenated and deoxygenated blood, for more efficient oxygen delivery to metabolically active tissues
    • Muscular diaphram-more efficient respiration
    • Secondary palate-effective separation of oral and nasal cavities allowing mammals to breathe and chew at the same time and as a result greatly increase their rate of food intake
    • Single bone (dentary) comprising the mandible-allows for a stronger jaw and more efficient food procesing

    When I wrote this question I was thinking in terms of universal mammalian characteristics, but I also gave credit for other specializations (e.g., mulit-chambered stomach) if you made clear its role in maintaining a high MR. Points were lost for answers that were vague (e.g. "specialized digestive systems" was not a sufficient description of a structure).

    8. Describe the composition of hair (including the factors accounting for differences in coloration). What muscular and glandular structures are associated with the hair follicle and what are their functions?

    Hair is composed of keratin (a protein)(1 point). The color of hair is determined by the presence of pigments eumelanin (resulting in brown or black color, depending on its concentration)(1 point) and pheomelanin (resulting in yellow to red coloration, depending on concentration)(1 point). The typical mammalian brown color results from alternating stripes of eumelanin and pheomelanin along each hair shaft. White coloration derives from the absence of either pigment. Each follicle is associated with a muscle (the erector pili) that upon contraction makes the hair stand up(1 point). This results in an increase in the insulative capacity of the hair. There are also sebaceous glands associated with the hair follicle that secrete oil. This oil serves to lubricate waterproof the hairs, and reduce the amount of breakage that the hairs experience (1 point).