Rape: Why Are Humans Perpetrating It While Animals Apparently Don't

As I was walking I suddenly had this thought. Why is it that the institution of rape exists only in human beings and rarely / never in animals? If Man evolved from animals then Man has gotten this unique habit recently. Why does he do it - more importantly why did he start raping only after having become Man? 

As I was discussing with a friend while walking.. We came up few ideas in relation to the question raised above.

1. Animals strictly adhere to the "(Female's) No means NO" rule
2. Perhaps females in the human species have a higher expectation from the males than do females in other lower species with the result that more human males are prevented from mating resulting in "need" for giving an outlet to sexual urge
3. Animal kingdoms have a (alpha) male mating with multiple females and hence fewer unattached females are available for copulation and rape by males. Among humans, all males are treated as equals and no one man can have more than one woman at a time. Note that rape can be eliminated by removing all females from the scene.
4. A strict punishment enforced for transgressing males in animals

My friend and I thought that one or more of the points above could have caused human beings to coerce a woman into sex.

At this point in time I am still very confused. My initial belief that rape was a truly human invention is itself wrong. Of course many of the articles say that the term "rape" is inappropriate when used in the context of animals and that the right term is forced copulation. Whatever... A rape by any other name... is still rape.

Additional reading:

1. http://www.livemint.com/Opinion/jkywrmQMip9SG6QVYDoe0H/Rape-in-the-animal-kingdom.html
2. https://en.wikipedia.org/wiki/Sociobiological_theories_of_rape
3. I was fairly sure that gang rape was unique to humans until I read this article (read under bottlenose dolphins and also in the para on ducks): http://english.mathrubhumi.com/news/columns/faunaforum/rape-happens-in-animal-kingdom-too-maneka-sanjay-gandhi-animals-1.1435307
4. Why rape is supposedly high in Sweden - lovely article: https://en.wikipedia.org/wiki/Rape_in_Sweden
5. Nice series of answers to  a similar question: https://www.quora.com/Is-there-really-any-such-thing-as-rape-All-animals-mate-without-incident-of-rape-except-human-beings-What-makes-rape-real-with-men-but-not-apes
6. Another nice article showing how apes do "rape" but is not as bad as human rapes: http://www.npr.org/sections/13.7/2013/10/03/228809153/why-gorillas-arent-sexist-and-orangutans-dont-rape
7. Rapes by animals: http://www.washingtoncitypaper.com/columns/straight-dope/article/13045860/can-dolphins-rape-humans-when-animals-attack-sexually
8. http://www.goodreads.com/book/show/2896628-when-fathers-rape: This book seems to be interesting, unfortunately there is no online version available as of Nov 5, 2016.