<p>Sex steroids are critical for the post-natal development of the female reproductive system, and are involved in ovulatory cycling and pregnancy. In mice, <em>Mus musculus</em>, female development, cycling, and pregnancy can be affected by the urine of conspecifics, which is known to contain active steroids. Specifically, puberty can be accelerated (the Vandenbergh effect), estrous cycling can be prolonged (the Lee-Boot effect) or synchronized (the Whitten effect), and blastocyst implantation can be disrupted (the Bruce effect). Since steroids alone can affect females in ways that are indistinguishable from these social reproductive effects, I hypothesized that urinary steroids of conspecifics may be absorbed by females, arrive in the reproductive system, and thereby affect females through known mechanisms. First I showed that tritium-labelled 17β-estradiol (³H-E₂) injected into males is excreted in their urine, and that application of urine from these males to the nose of an inseminated female results in detectable levels in her uterus. When I paired inseminated females with non-sire males injected with ³H-E₂, radioactivity was detected in the brain and reproductive tissues of the females. This was the first demonstration of steroids from one animal directly entering the body of another. Similar results were found when I exposed juvenile females to adult males injected with ³H-E₂, and when I exposed nulliparous adult females to same-strain ³H‑E₂- or ³H-progesterone- (³H-P₄) treated adult males or females. Taken with the existing literature, these results suggest that steroid transfer may underlie various social reproductive phenomena in mice, with potential implications for many other species.</p> / Doctor of Philosophy (PhD)