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Which of the following does the author suggest occurs when a mother rat gives birth?
According to the passage, after birth oxytocin in mother rats does not drop but must be suppressed artificially, by introducing antagonists.
As the last sentence of the second paragraph indicates, when the injection of oxytocin antagonists, or blockers, ceased, the mother rats started behaving maternally. So we learn that when oxytocin is suppressed, maternal behavior is also suppressed, to return after oxytocin is no longer suppressed. From this we learn of the direct relationship between oxytocin and maternal behavior.
Since no other interventions (e.g. no oxytocin injections, or other additions of oxytocin) are described, we can infer that oxytocin was already present in the mother rat's bloodstream, but suppressed by the antagonists. Therefore, the mother rat must have produced the hormone herself and released it into her bloodstream.
The passage tells us that scientists had to inject oxytocin antagonists into the blood of mother rats who had just given birth, which suggests that they are not naturally present.
The passage does not mention vasopressin in connection with mother rats. Vasopressin is only mentioned in the third paragraph.
There is no information in the passage that tells us that there are oxytocin antagonists in the bloodstream of a mother rat or in what amount. The passage only states that scientists injected mother rats with such antagonists.