Bilateral distribution of implantation sites in small mammals of 22 North American species

Abstract

Summary. The distribution of activity between the left and right sides of the reproductive tract, as measured by numbers of CL, embryos and placental scars, was studied in small mammals of 22 species. Shrews ovulate from the two ovaries in a distribution that does not differ from the binomial. Implantation of blastocysts in the two uterine horns is more nearly even ('balanced') than would be predicted from the binomial distribution. Balance in this group apparently is achieved by transuterine migration of blastocysts, perhaps in conjunction with some spacing mechanism within the uterus. Some cricetid rodents show little or no balance, but in others the distribution of activity sites (embryos, CL and placental scars) departs significantly from the binomial distribution. Reproductive activity sites of heteromyid and geomyid rodents (Geomyoidea) are highly balanced; uterine balance apparently is achieved by means of ovarian rather than uterine control.We know of no previous reports of ovarian balance and suggest that physiological mechanisms controlling numbers of ovulations in the species exhibiting this characteristic may differ from those in species exhibiting a random distribution of ovulation sites. Hypotheses regarding evolutionary aspects of balance are considered in phylogenetic and ecological terms, generating several testable research questions for physiologists, anatomists, and evolutionary ecologists.

Sex Ratio of Bovine Embryos and Calves Originating from the Left and Right Ovaries1

“…The literature provides multiple examples of lateral asymmetry of both mammalian ovaries and uterine horns [4][5][6]. In rats, the left uterine horn contains fewer implantation sites than the right horn [7].…”

Reproductive strategies and natural history of the arboreal Neotropical vesper mouse, Nyctomys sumichrasti

“…Several factors could influence these results. The majority of cricetid rodents (seven of nine) tested for embryo balance by Baird and Birney (1985) did not exhibit embryo balance, which may indicate a phylogenetic pattern; however, the species included in their analysis are not arboreal or saltatory. Our study was unable to reject the null hypothesis that embryos are randomly distributed between the left and right uterine horns; however, with only 15 pregnant females with two or more embryos and appropriate field notes, the power of the analysis was low.…”

“…We evaluated embryo balance in the uterine horns following the protocol outlined by Baird and Birney (1985). All pregnant females with two or more embryos, regardless of collection locality, were included in this analysis.…”

“…All pregnant females with two or more embryos, regardless of collection locality, were included in this analysis. Baird and Birney (1985) classified the embryo distribution for each female as balanced or unbalanced. To be considered balanced, an even-numbered litter would have to be split evenly between the left and right uterine horns; however, an odd numbered litter is considered balanced if one horn had only one more embryo than the other (e.g., a litter size of three was considered balanced if the left horn had one embryo and the right had two embryos or vice versa).…”

“…The χ 2 values for all litter sizes were summed and compared with a χ 2 distribution to test for significance. See Baird and Birney (1985) for a more detailed description of the method.…”