While we have known for some time that serotonin, or lack of it , is linked to depression, substance abuse and other reckless behaviors, it appears that scientists already know there is a gene-environment interaction that can cause this to happen.
Since serotonin can control reproductive systems, I was wondering if this might explain why only the Alpha female (normally) has the only litter in the wolf pack. Maybe the other females are so environmentally challenged through harassment by the dominant pair, that serotonin is reduced and that affects ovulation; just a thought...

Just another thought: too much serotonin is also linked to depression.

Gene–environmental interaction (G E) between a common functional polymorphism in the promoter region of the serotonin transporter gene (5-HTT) and environmental adversity on the onset of depression in humans has been found in fifteen independent studies.