Mongolian gerbils Meriones unguiculatus a social rodent spec

Mongolian gerbils (Meriones unguiculatus), a social rodent species, are widely distributed in the typical steppe, desert steppe and desert habitats of northern China, Mongolia, and Trans-Baikal region of Russia. In wild, the recorded maximum lifespan (apparent longevity) was 29.5 months for female Mongolian gerbils and 22.2 months for males, and the average longevity of male and female gerbils is 4.6 months (standard deviation [SD] = 3.93) and 5.3 months (SD = 4.95), respectively (Xia et al., 1982; Liu et al., 2009, Liu et al., 2017). Field studies also report that the minimum age at maturity or first breeding of gerbils is 2–3 months. The reproduction and recruitment occur primarily from March to August and the gerbils hoard food with a breeding lull in September and October; however, a minority of gerbils could breed during December to February (Liu et al., 2007, Liu et al., 2017). Accordingly, gerbils could be usually divided into three seasonal cohorts based on the month of their birth. The winter cohort consisted of a mix of winter-born and summer-born individuals; the spring cohort consisted of young born from March to May, which became independent during April and early June; and the summer cohort consisted of gerbils born from June to August, which left the nest between the end of June and breeding season of the year (Liu et al., 2017). The previous studies have reported seasonal cohort-specific life-history patterns in the apparent longevity, age of maturity, potential reproductive lifespan and dispersal in a natural enclosure. For example, the gerbils born in winter and spring displayed an earlier age of sexual maturity compared to those born in summer, while the summer cohort gerbils commonly delayed their reproduction to the following year (Liu et al., 2017). Although seasonal life-history patterns are apparent, the neural basis for seasonal reproduction and food hoarding in wild rodents is still under scrutiny. We hypothesized that the variations in GnRH Heparin sodium and cell proliferation facilitated life-history traits of seasonal breeding and food hoarding. To test this hypothesis, we took seasonal samplings in April, July, and September in natural populations, manipulated food-induced hoarding in the breeding season, and tested the role of photoperiod in inducing seasonal reproduction and brain plasticity in Mongolian gerbils.
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Discussion Small mammals usually breed in the most appropriate seasons to optimize their fitness. Both males and females show better reproductive performance in spring and summer than in autumn (Liu et al., 2007; Smaï-Hamdidouche et al., 2013; Kheddache et al., 2017). Moreover, the females are more likely to suppress their fertility during harsh time due to more reproductive investments than males (Bronson, 1989; Hammond and Diamond, 1992). For instance, the onset of seasonal infertility in Syrian hamsters occurs earlier in females than males (Beery et al., 2007). The seasonal pattern of breeding in Mongolian gerbils is typical both in agricultural croplands and semi-natural condition (Xia et al., 1982; Liu et al., 2007). In the present study, we found that the gerbils captured in field showed seasonal variations in sexual development, with accentuation in April and July and attenuation in September, and such seasonal variations are significant in males, but not in females including the percentage of open vaginas and ovary structure. The female breeding ratio was 5/20 in April, 5/15 in August and 0/18 in September in all subadult and adult gerbils we captured. The females that were in pregnancy or lactation were released to the field and only the non-reproductive females were included in our study. A common idea in rodent behavioral ecology is that young females show delayed sexual maturation or reproductive suppression when reared with conspecific females (Wasser and Barash, 1983; Solomon and French, 1997). Therefore, the female subjects in present study may not totally represent the reproductive status of population due to the defect in samples. There is also another possibility of the individual difference in reproductive development in females. The previous study report that some female gerbils breed during December to February and about 17.1% of new recruitments are born in winter (Liu et al., 2007, Liu et al., 2017), which may offer a perspective on the undetected change in female fertility in gerbils.