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It has developed a really thick skin so it can retain water for longer periods than other plants. It has spines to help protect it so that desert animals can not easily get the water from inside the cactus. The water needs of cacti are in general lesser than those of other plants.
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It may be that he has eaten something that has caused him to be constipated. It may also be something more serious such as a blockage somewhere in his digestive tract. I would not let this go on too much longer before taking the animal to the vet.
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First examine his paw and see if there is anything lodged in the paw. Either in one of the pads or in between the pads. If there is nothing in the paw you should probably take him to the vet. It may only be a sprain but it may be a broken bone as well.
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Man and his automobiles. When I lived in Florida the only thing I ever heard about killing the panthers at the time was cars. In previous generations they were hunted by man until laws were passed against this. There are no natural predators of the Florida Panther other than man.