Reproductive biology

Information on sigmodontine reproduction comes primarily from observations recorded during specimen collections (e.g., external measurements, testes position, presence of vaginal closure membranes, and number of embryos). From these observations, inferences on the part of the year where species are reproductively active, number of estrous cycles per year, and litter size, are made. There are species that seem to breed throughout the whole year; for example Zygodontomys brevicauda reproduce continuously, even in habitats with marked seasonality (e.g., dry and wet season). Litter size ranges from one to 11, averaging 4.5. Similarly, Sigmodon hispidus also breeds, at least in the warmer parts of its distribution, during the whole year. It has one to 15 young per lit-

Sumichrast's harvest mouse (Reithrodontomys sumichrasti) is a natural predator of the monarch butterfly (Danaus plexippus). (Photo by Gregory G. Dimijian, M.D./Photo Researchers, Inc. Reproduced by permission.)

ter. Other sigmodontine species restrict reproduction to one or two specific parts of the year (seasonally active). Reithrodon auritus concentrates breeding in spring and early summer. Litter sizes vary from one to eight, with a mean of 4.5 embryos. Sigmodontines are in general short-lived, and in several species at least one sex reaches sexual maturity in the same season of birth. This is the case of Reithrodon auritus females. Eligmodontia typus rarely live longer than 9 months; it reaches sexual maturity at approximately 45 days (Pearson et al., 1987).

Information on the patterns of sigmodontine copulatory behavior is sparse. Calomys, however, is a relatively well known genus in this regard. There is information on three of the 11 species of the genus. They display intravaginal penile thrusting and multiple ejaculations, in combination with single or multiple pre-ejaculatory intromissions. Locks are either rare or absent. In addition, C. musculinus, a non-monogamous species, undergoes a stereotyped precopulatory behavior composed mostly of agonistic behaviors in which the female and the male play the aggressive and submissive roles, respectively (Laconi and Castro-Vasquez, 1998).

A remarkable aspect of sigmodontine population dynamics is the massive population irruptions or outbreaks recorded in several parts of South America since the sixteenth century. These explosive increases in sigmodontine population abundance or density during a relatively short period have been called ratadas. Starting in 1522, 63 ratadas are well-documented in literature; these have occurred in Argentina, Brazil, Chile, and Peru. The outbreak estimated densities of some sigmodontines are as follows: Oryzomys xantheolus (range = 250-259 individuals per 2.5 acres [1 ha]), Abrothrix olivaceus (52-237), Phyllotisdarwini (30-225), Oligoryzomyslongicaudatus (46-154), and Akodon azarae (115). As ratadas involve one to four species simultaneously, the total combined sigmodontine density may be even higher. Naturalists noted that massive flowering and subsequent massed seeding of bamboos preceded several rodent irruptions, so sigmodontine outbreaks became associated with bamboo blooming. Later, it was shown that several ratadas are associated with rainfall peaks, some of which are caused by El NiƱo Southern Oscillation disturbances. Both types of ratadas occur with approximately equal frequency and appear geographically located. Brazil, southern Chile, and southern Argentina are affected mainly by bamboo-associated rodent outbreaks, while Peru, northern Chile, and northern Argentina are affected by rainfall-associated outbreaks. Functionally, ratadas are the result of a population closely tracking changes in the environment. However, as it was shown in studies of the leaf-eared mouse Phyllotis darwini, a species that undergoes ratadas in semi-arid Chile, intrinsic factors such as density-dependence variables and competitors or predators also play a key role in shaping the demographic dynamics of these species. This evidence underscores the interplay of different phenomena regulating reproduction and demography in natural populations.

Finally, it is interesting to note that in nine species of Akodon (azarae, boliviensis, kofordi, mollis, montensis, puer, sub-fuscus, torques, and varius) there are females with a XY pair of sexual chromosomes (and not XX as is typical in mammals; XY females also occur in the arvicolines Dycrostonix torquatus, Microtus cabrearae, and Myopus schisticolor). The prevalence in wild populations of these heterogametic females ranges from 30 to 60%. These females are fertile. XY sex reversed females are assumed to occur due to a deficient expression of the Y chromosome linked gene Sry, resulting in the developing of ovaries instead of testes.

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