by Pat Rock
The British have a word for it: gobsmacked.
That is what I felt when I read the research paper on influence of position of a fetus in the uterus of litterbearing mammals. As if the new understanding of the changes to the genetic code after conception (epigentics) isn’t muddying the waters of selective breeding enough—now this!
It seems that sex hormones are manufactured by a fetus, and those active hormones diffuse into the amniotic fluid and into the adjoining ones. Scientists studying the effect of male hormones have devised a coding system for each position. A female next to one male (1M); a female in between two males (2M); and a female between two other females, or at the cervical or ovarian end of the uterine horn and next to just one female (0M).
The research so far has been done with rodents.
The influence is strong enough in rats that a physical change is evident in the 1M and 2M females so the determination may be made without a C-section. (Mammals that have litters don’t necessarily alternate between expelling a pup from one horn and then the other, so birth order isn’t predictive). What you can take note of is singleton litters, or litters of all one sex or the other. Litters such as three males and a female provide a strong index of suspicion that the one female you have is either 1M or 2M. What are the effects on the pups? In mice 2M individuals typically exhibit heavier body weights than 0M mice of either sex. 2M male rats have heavier testes than 0M male rats. Intrauterine position has an effect on secondary sex ratios, which over generations influences populations dynamics. Adult 2M female mice produce litters that are approximately 60 percent male and 40 percent female (and of course, this ncreases the likelihood that any females will be 2M). 1M mothers have the usual 1 to 1 ratio. 0M mothers have litters that average 40 percent male and 60 percent female. In other words, 0M mothers are more likely to have 0M daughters, and 2M mothers are more likely to have 2M daughters. Reproductive physiology is also affected; intrauterine position affects age at puberty. 2M females stop producing young earlier than 0M females and produce fewer viable litters. Behavior is also affected. A male rat will use his urine for marking more when in the presence of a 2M female than with a 0M female, indicating awareness of the higher level of testosterone in the 2M female. 2M female mice initiate more fights than 0M females.
Granted, all this research is in rodents, but it made me think of a lot of “what ifs.”
You know the phenomenon of a significant number of top-winning female show dogs who never make it as significant dams? What if the characteristics that make a superior show dog are associated with 2M females? What about the bitches who just don’t seem to want to be bred, even though a mother or sister was enthusiastic in the breeding pen? I’m going to be looking at sex ratios in litters much closer from now on.
Reference: Jay, Hillary. “The intrauterine position effect: A novel source of non-genomic variation in litter bearing mammals.” Barnard College of Columbia University, New York, NY 12/12/14