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KRISTAL AND ELEFTHERIOU
(CXBH × CXBG) × CXBG backcross eliminates the possibility that the
progenitor strains both harbor increasors for placentophagia. The explanation
of the 37.1% incidence of placentophagia in the RI-strain backcross is difficult
to explain without further information about why the genotype of the
CXBH females results in 0% placentophagia.
The CXBG RLstrain was the only group tested that exhibited an
unequal response to the two types of placenta used. This unequal and
unreplicated response, since it occurred in only one of the 14 groups tested, is
unexplainable. Although the response of the (CXBH X CXBG) X CXBG
females might be interpreted as being unequal, the difference in response of
this group to the two types of placenta is proportionally small, and is in a
direction opposite to that observed in the CXBG group.
It is difficult to conceptualize the motivational mechanism underlying
placentophagia in nulliparous females, considering that virtually all female
mice eat placenta at parturition. Why would only some of the nulliparous
females with a particular genotype manifest placentophagia? The answer may
lie in the behavior of the nulliparous females that do not eat placenta. Kristal
and Williams (1973) reported that among virgin mice not eating placenta,
many backed into the farthest corner of the cage from the dish of placenta
and emitted tail-rattling. The same phenomenon was seen, but not quantified,
in the present study. Perhaps avoidance (aversion, fear, neophobia) of certain
types of stimuli in the absence of previous experience is actually the direct
result of average dominance of the alleles in question. Placentophagia in
virgins could then be considered to be the consequence of the absence of
some of the alleles responsible for avoidance of the substance, or inhibition of
placentophagia. At parturition, the hormonal milieu and sequence of behav-
ioral events may then override the behavioral responses competing with
ingestion of this particular novel substance, ultimately resulting in the
disinhibition of placentophagia. Subsequent to parturition, the genotype of
the female would then determine the facility with which placenta is removed
from the conceptual category of novel substances as evidenced by the
interaction of strain and parturitional experience observed in the study by
Kristal and Williams.
Although the analysis of the genetic influence on placentophagia in
nonpregnant nulliparous female mice has been neither exhaustive nor con-
clusive, certain general statements can be made. We feel that it is safe to
conclude from the data that there is a genetic component to the response of
female mice to placenta in the absence of previous experience; that the
genetic component is complex and certainly involves more than a single locus,
but that two-loci models are feasible; that alternative genetic models consider-
ing that the. result of average dominance of the alleles either can produce
placentophagia, or can produce no placentophagia by promoting a competing
response, are both conceivable.