Reproductive system of myomorphs
Male
l'he adult male of this group has two testes in an external scrotum. In many species the inguinal canal remains open, allowing the testes to shrink and return to the abdomen during the non-breeding season.
The Iestesof the Chinese hamster. Cricetulus griseus, remain internal. The reproductive tract is complex and comprises several large accessory glands. An os penis lies within the tissue of the penis.Female
Females in this group have a Iiieornuate uterus, with a pair of long uterine horns and little or no uterine body This arrangement has evolved to contain large numbers of developing embryos. The stages of the rodent oestrous cycle can be ascertained by microscopic examination of vaginal smears Iexfoliative vaginal cytology). The female hamster may produce copious amounts of vaginal discharge, particularly around day 2 of the oestrous cycle. Mating may be Conlirmed by the presence of a 'Copulatory plug*. This is a creamy gelatinous plug formed from the secretions of the male accessory glands: it remains in the vagina or may fall on to the cage Iloor.
All females of these species of rodent are polι∣oestroιιs and spontaneous o∖,ulators. Specific details are mentioned in Table 14.2 ∣p. 189). Myomorphs are sexually mature at a young age. e.g. the mouse reaches sexual maturity at 3-4 weeks, and produce large litters of young. Both of these features contribute to the success of rodents in creating ‘population explosions’ over a short period.'Γhe neonates are altricial. i.e. they are totally dependent on the mother for several weeks and are born hairless, blind and deaf. At this stage they are very vulnerable and are kept hidden in some form of nest. If disturbed, the mother may practise cannibalism.
Fig.14.8 General method of se?mg rodents.The a∏oge∏∣ta∣ distance ∣s larger ∣n males compared to females.
Sexual differentiation of myomorphs
As a general rule, the anogenital distance, i.e. the distance between the opening of the anus and the opening of the genital tract Ipenis or vulva), is longer in the male than in the female (Fig. 14.8). There are no teats in the male. There are also some Spccies-SpeciRc indicators of sex:
1. Adult male hamsters have large external testes which are retained in the scrotum by a pad of fat in the inguinal canal. This makes the rear outline of the male conical when viewed from above. The female has no scrotum so the rear outline is more rounded.
2. Newly weaned male gerbils have a pigmented scrotum.
3. Sexually mature male gerbils have a prominent ventral abdominal scent gland.