Characteristics USED to group animals:
* Physical characteristics: This is a primary way to group animals.
* Body structure: Vertebrates (animals with backbones) vs. invertebrates (animals without backbones).
* Number of legs: Insects (6 legs) vs. mammals (4 legs).
* Body covering: Birds (feathers), reptiles (scales), amphibians (smooth skin).
* Behavioral characteristics: How animals interact with their environment and each other.
* Social behavior: Solitary (living alone) vs. social (living in groups).
* Feeding habits: Herbivores (plant eaters), carnivores (meat eaters), omnivores (both).
* Reproduction: Laying eggs, giving birth to live young.
* Genetic characteristics: This involves analyzing DNA and is increasingly used for accurate classification.
Characteristic NOT used to group animals:
* Size: While size can be a descriptive feature, it's not a reliable way to group animals.
* Example: A mouse and an elephant are both mammals, but their sizes vary greatly.
Important Note: Animal classification is constantly evolving as scientists learn more about relationships between species.