Question

glass frogs are not actually made of glass. but their skin is transparent, just like glass. in fact, the skin on the underside of a glass frog is so clear that you can see inside its body. without using an x - ray machine, you can see the frogs heart and other organs. sometimes you can even observe blood or food moving through the body.
however, these unusual - looking frogs can be difficult to spot in the rain forests of central and south america. the clear skin on their fronts and the light green color of their backs often blend in with the leaves they rest on. also, they are tiny - just about the size of a quarter.
these frogs make up for their small size with an interesting way of catching insects. scientists believe the glass frogs spotted back may be designed to look like the frogs eggs. if a fly settles down for a meal on the frogs back, it will be in for a surprise when the glass frog attacks it.
the glass frogs real eggs are protected by the males. male glass frogs are very protective of their eggs. to keep away unwanted guests, a male glass frog will try to scare away any other male frog that gets too close. at first, the male frog makes loud noises to warn the other male frog to leave the area. if that does not work, then the frogs may wrestle each other until one gives up and leaves. these fights can last for thirty minutes.
based on the text, what does the male glass frog do first if another male frog comes to its area?
it makes a loud warning sound. it fights with the other frog.
it gives the other frog its eggs. it leaves the area.

Explanation:

The text states "At first, the male frog makes loud noises to warn the other male frog to leave the area." So the first - action of a male glass frog when another male comes to its area is making a warning sound.

Answer:

It makes a loud warning sound.