bitterly - adv. to an extreme or forceful degree.

pet - n. an animal (such as a dog, cat, bird, or fish) that people keep mainly for pleasure

mammal - n. a type of animal that feeds milk to its young and that usually has hair or fur covering most of its skin

behavior - n. the way a person or animal acts or behaves

neuroscientist - n. a person who studies the functions of the nervous system and brain