Why would an organism want to call attention?
what purpose does the rattlesnake’s rattle serve?
what purpose does sweet scent serve for a plant?
what plant or animal has features or behaviors other than color and pattern to attract attention?
how is an attention mechanism a stimulus/response interaction?
what is communication?
what kinds of stimuli can initiate communication?
what kinds of things do animals need to communicate?

1. Calling for attention could mean asking for a mate, or simply marking territory.
2. The rattlesnake's rattle serves as a warning to all its prey and predators. 
3. A sweet scent for a plant attracts insects to pollinate their flowers.
4. Some may have a unique way of calling out, or weird movements. Take tropical birds for example.
5. I think it's because that the females are attracted to a certain call or movement.
6. communication is talking to each other.
7. Stimuli can induce the hypothalamus in an organism's brain to react accordingly.
8. Animals need a voice, or a call, or a maovement, or anything to get their point across. 

That was a lot of typing, but i'm done. :)