Male sexual arousal. The penis has started to swell up, the veins are starting to show, and the foreskin has begun to slip back. The testicles have tightened in the scrotum. As the penis continues to swell it will point upwards.
The penis will swell up and become stiff (erect) so it points outwards and upwards. This is the clearest sign of sexual arousal in men.
Animal sexuality is not very well understood today, but scientists are sure that animals do get sexually aroused without having sexual intercourse. Dolphins and Bonobos both use sex as a "social tool to strengthen and maintain bonds."
↑McCarey, Kevin (writer). (1999). Dolphins: The wild side [Documentary]. USA: National Geographic Television. Retrieved on 2007-04-24. "Like humans and some chimpanzees, dolphins use sex for reasons other than procreation. Sex is as frequent as it is casual, a social tool used to strengthen and maintain bonds."'Dolphins: The wild side' at the Internet Movie Database