The usual reproductive pattern among eutherian mammals is fertilization within the oviduct with early cleavages occurring while traversing the oviduct. The blastocyst stage is reached at about the time the embryo reaches the uterus and, perhaps after some delay, implantation occurs. In the common pattern, development is continuous from fertilization to birth. However, three other patterns are found in some mammals: delayed fertilization, delayed implantation, and delayed development.
This phenomenon is known from at least two families of bats, including the Vespertilionidae. Possibly all vespertilionids except tropical species undergo delayed fertilization. In the best studied species, the following sequence appears: The testes descend to the scrotum in spring, with the testes beginning to enlarge in the spring and are largest in September. Spermatogenesis occur mostly in late August and September, with the males becoming sexually active in August. Although spermatogenesis ceases before winter, the accessory reproductive organs remain enlarged and motile sperm are retained in the epididymes until February. In the females, a single follicle (in most) enlarges in the fall but remains in the ovary until the end of winter. Insemination occurs during fall and winter, with most females inseminated by the end of November. However, the sperm are retained in the uterus and ovulation doesn't occur until late winter or early spring, long after insemination.
In Artibeus jamaicensis (Jamaican Fruit Bat), a blastocyst conceived in July or August implants normally but then becomes dormant until mid-November. Thus late summer matings are delayed until the young can be born in early spring when there is abundant fruit.
A number of species within the Chiroptera, Xenarthra, Carnivora, and Artiodactyla follow this route. Development is normal up to the blastocyst stage. At that point, development ceases and the blastocyst remains in the uterine cavity for about 12 days to 11 months, depending on the species. The blastocyst remains covered by the zona pellucida during this stage. Eventually dormancy is broken and the blastocyst implants, with normal development following. In some cases, delayed implantation is facultative, meaning that it occurs under some conditions but not under others. This usually is where insemination takes place shortly after the birth of a litter, and in some cases where a large litter requires a large expenditure of energy.
Altricial young are born essentially helpless. In most, there is little or no hair, the eyes are sealed shut, and locomotion is out of the question. Usually some type of shelter is involved. Included in the group are rabbits, carnivores, and many rodents. Precocial young are born fully haired, the eyes are open, and they are able to stand and walk almost immediately after birth. Shelter usually is unnecessary except, in some cases, for concealment. Hares are good examples as are the large grazing animals such as Bison and Horses.
The actual litter size tends to represent the best reproductive investment for the environment. In an environment where survival is likely to be high, litter sizes tend to be small. The less metabolic strain during a reproductive event, the more likely subsequent reproduction will be successful. In environments where survival is less likely, larger litters are necessary to offset the greater mortality. As environments differ from place to place, so may the parental investment in litter size, even within the same species. The general pattern is larger but fewer litters in northern and high elevation climates; smaller but more frequent litters in temperate regions; and many small litters per year in tropical environments. Within some species, litter size varies with local food abundance. Another feature sometimes seen is litter reduction. In eastern packrats (Neotoma floridanus), for example, if food is restricted during lactation, the mother neglects the male members of the litter, resulting in a higher death rate among male littermates than females. Since males of this species are polygynous (males servicing more than one female), not all males get to breed, but probably all females do.
Last Update: 4 Feb 2008
Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso