All mammals are dioecious (sexes are separate) and reproduce sexually by way of internal fertilization.

Protherians or Monotemes lay eggs (ovipary) and incubate their young. They have a cloaca or common opening for their urinary and reproductive tracts. They have true mammary glands but lack nipples.

Metatherians or Marsupials also have a cloaca but give birth to live young (vivipary). Marsupials have a placenta, but it is very much reduced and not efficient. Marsupials have a very short gestation period (period of time from fertilization to partrition) and therefore give birth to altricial young (poorly developed).

Eutherians or Placentals are viviparous and have a highly developed placenta that facilitates respiratory and excretory exchange between maternal and fetal circulatory systems. Placentals have a longer gestation period than do Protherians or Metatherians.

Testis

Sperm production (spermatogenesis)

 

- Scrotum

 

 

 

Penis

Baculum (os penis)

 

 

Shapes (?) 

 

 

Anaterior or posterior to the scrotum

The female reproductive system possesses a pair of ovaries that produce gametes (eggs) and hormones, and oviducts (fallopian tubes) into which the eggs are released.

The ovaries contain follicles and each follicle encloses a single egg that is released into the oviducts when the follicle bursts (Ovulation). The development of follicles in the ovaries and subsequent release is regulated by hormones.

The placenta is a combination of tissue comprised of embryonic and maternal tissues that provide several important functions during gestation.

Anchors the fetus to the uterus

Transports nutrients from the mother to the fetus

Excretes metabolites of the fetus into the maternal compartment

Produces hormones

 

 

Both Placentals and Marsupials have placenta. However there are significant differences between the two.

Marsupials have Choriovitelline Placenta:

Placentals have Chorioallantoic Placenta:

Trophoblast cells may be key to differences in gestation periods between Marsupials and Placentals.

Marsupials do not have trophoblast cells. Early in development the fetus has an egg shell like membrane surrounding it and prevents a maternal immuno-rejection. Later in gestation the eggshell membrane dissolves.

Placental fetuses have a Zona Pellucida (early) and Trophoblast cells (later) that surround the fetus and effectively prevent immuno-rejection. This enables much longer gestation periods in Placentals.

Evolutionary Significance???

Most often seen in bats involves the female retaining motile sperm in the uterus for a long period of time prior to ovulation.

- In many bats copulation occurs in the fall and winter

- Ovulation will occur sometime in the late Winter or Spring at which time the egg is fertilized.

- Copulation occurs and a new blastocyst is conceived

- The blastocyst implants in the uterine lining -BUT DEVELOPMENT IS VERY SLOW

- For one species, delayed development allows young to be born during the end of the dry season when fruit is plentiful.

Delayed implantation is a widespread phenomenon occurring in mammals in which the embryos upon reaching the uterus cease development for a period of weeks or months. During this period the embryos remain as viable, hollow blastocysts unattached to the uterine lining, but no significant development takes place. As a result, the gestation period may be extended to nearly a year in some species. Delayed implantation occurs in a wide variety of mammals including a mole, some shrews, a few bats, most members of the weasel family, most bears, some armadillos, a European deer, most seals, and has recently been discovered in many of the marsupials of Australia.

A Faculitive arrest (delay) may occur in rats and mice. If a female is inseminated shortly after giving birth, implantation of the resulting crop of embryos may be delayed for several days do to lactation (i.e., a hormanal response to suckling prevents implantation). If the first litter is removed from the mother there is no delay, hence the term facultive.

In most other examples of delayed implantation, the arrest of the blastocyst invariably occurs, and is called Obligate arrest. For example, many species in the family Mustelidae display obligate arrest breeding in midsummer, but the blastocyst does not implant in the uterine lining for 5 - 8 (species specific) months and the young are born the following spring.

Faculative delay may serve to prevent the occurrence of two litters of different ages requiring nursing. In obligate arrest the event may serve to time the birth of young at a favorable period for their maturation in winter (or when food resources are at a premium). For some species there appears to be no clear advantage to the system. Because almost no intermediate stages are known it is not at all clear how the phenomenon evolved but it has probably occurred for many millions of years.

• Lactation

Fats, proteins, and lactose (milk sugar)

- transmits passive immunity (Colostrum) - Lysozyme (kills bacteria and fungi) 

- may support intestinal micro-orgs

Variation in milk composition among species and among age group w/in a species

- Whales and seals 40-60% fat, 11-12% protein

- The enormous variation among mammals in size and life-history strategies is associated with great differences in reproductive performance.

* If we eliminate the exceptions - - and there are many, we can relate certain aspects of reproductive performance: litter size, frequency of litters,, numbers of litters in a lifetime, rate of growth of young, and duration of the mother-young bond to body size and metobolic rate.

- In general: small mammals have a high metabolic rate and the rate progressively decreases with an increase in body size.

Thus, for small mammals, then, the best reproductive investment is to have:

- large frequent litters

- rapid development of the young

- short dependence for the young.

In doing this small mammals typically allocate a relatively high percentage of their lifetime energy budget to reproduction.

Large mammals in contrast, (> 10 kg), generally have

- Fewer young per litter (lots of exceptions)

- Gestation is long

- Mother/young bond is long lasting