Reproductive biology

Rodent reproduction can be subdivided into two forms. The families Muridae (mice and rats), Geomyidae (pocket gophers), and most other sciurognaths have short life expectancies, short gestation periods (17 to 45 days), produce multiple litters per year (one to four), and have large numbers of al-tricial (helpless) offspring per litter. Most hystricognath rodents such as the Caviidae (guinea pigs and relatives), Erethizontidae (porcupines), and other caviomorph families, have longer life spans, have long gestation periods (ranging from 60 to 283 days), produce few litters per year (generally one to two), and give birth to smaller numbers of precocial offspring per litter. Although body size is generally a good predictor of the form of reproduction employed, hystricog-nath rodents tend to have longer gestation periods than similar size sciurognath rodents. Hystricognath rodents also produce precocial young that are weaned and reach sexual maturity at an early age. Although species of rodents with large altricial litters tend to gamble in terms of reproduction, many species of rodents in more unpredictable environments tend to synchronize reproduction in an effort to produce offspring under optimal conditions (e.g., during periods of maximum plant productivity). Hibernating species in the families Sciuridae (squirrels) and Zapodidae (jumping mice) generally produce one litter per year, whereas non-hibernating species can be polyestrous, breeding more than one time per year. These hibernators have a narrow window where resources are

A dwarf hamster (Phodopus sungorus) foraging. (Photo by Hans Reinhard/Okapia/Photo Researchers, Inc. Reproduced by permission.)

optimal for reproduction and raising offspring. For instance, the jumping mouse hibernates for nine months and has a three-month window for successful reproduction and weaning of offspring. Even within the same species of small rodent, such as the California meadow vole (Microtus califor-nicus), the old-field mouse (Peromyscus polinotus), the Eastern woodrat (Neotoma floridana), timing of reproduction and litter size can vary with respect to environmental conditions such as rainfall and food abundance.

Chemical communication is important to the reproductive biology of rodents. Female house mice tend to experience increased ovulation in the presence of males. In rodents with short life spans, flexibility in the timing of ovulation to increase the chance of fertilization by a male optimizes reproductive success. The "strange male effect" (or Bruce effect, after the author who described it) in mice occurs when pregnancy is blocked in an inseminated female upon encountering an unknown male. After the new male inseminates the female, reintroduction of the previous male fails to block implantation. It has been proposed that the female presumably has "an olfactory memory" that prevents the female from blocking implantation upon encountering her first mate. Young prairie vole females do not come into heat until they are separated from their family and encounter an unfamiliar male (or his odor).

The mating system in rodents varies, depending upon the species. Many rodents are promiscuous, with offspring from

A black rat (Rattus rattus) threatening with its teeth bared. (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)

a single litter often being sired by more than one male. Recent genetic studies using DNA fingerprinting have confirmed a high incidence of multiple paternity in many promiscuous species. Some species of rodents have a monogamous mating system. For instance, male and female South American mara or Patagonian cavy (Dolichotis patagonum) establish pair bonds that can last for multiple mating seasons, and throughout this period the male and female stay in close proximity to each other as they forage. This same species has a tendency to form communal nurseries where several pairs house their young, visiting each day to provision their own young. Some species of rodents that demonstrate monogamy also show a correlation between male parental care and successful rearing of offspring. For instance, the California mouse (Peromyscus californicus) female is successful at raising a litter if the male is present. Male and female beavers also tend to form long-lasting pair bonds. Many species of caviomorph rodents have a harem based mating system defined by a linear hierarchy of males. Rock cavies (Kerodon ru-pestris) have a resource-based form of polygyny, whereby males defend rocky outcrops that are considered ideal resources, thus attracting females. Capybaras (Hydrochaeris hy-

drochaeris) are semi-aquatic rodents that live in social groups lasting multiple years. Breeding within these groups is harem-based with a dominant male and several females and subordinate males.

The naked mole-rat (Heterocephalus glaber), a species occupying regions of eastern Africa, has an unusual mating system, analogous to social insects, with a single reproductive female and a few reproductive males. Although mole-rat colonies can be quite large, breeding is suppressed in subordinate females, and the entire social system is based on reproductive and non-reproductive individuals. In this system, non-reproductive individuals perform duties related to excavation of the underground burrow system, foraging for food, and tending pups of the reproductive female. In captive populations, the same female can remain reproductive for multiple breeding seasons, and she uses a combination of aggression and possibly chemical communication to suppress reproduction by other adult females. This unusual social system has been considered analogous to the eusocial system seen in social insects that also have overlapping adult generations, recruitment into the natal group, and well defined reproductive and non-reproductive individuals. Apparently, several other species of African mole-rats have similar mating systems. These rodents have stimulated much debate regarding the prerequisites for such complex social behavior, including the possibility that unpredictable arid environments and monogamy may be early precursors that foster the eventual evolution of complex social systems.

0 0

Post a comment