How does Jacobson's organ differ among amphibians, reptiles, birds, and mammals?

The Jacobson’s organ, or vomeronasal organ, is a specialized chemosensory structure that detects moisture-borne odor particles, such as pheromones, and is distinct from the primary olfactory system. While it arises in the embryo stages of all tetrapods, its presence and function vary significantly across groups:12

• Amphibians: Many possess the organ, often using specialized mechanisms for chemical transport. For example, lungless salamanders use snout contact and capillary action via grooves, while caecilians utilize tentacles to transport chemicals to the organ.2
• Reptiles: In squamates (lizards and snakes), the organ is completely isolated from the nasal cavity and opens into the buccal cavity. These animals rely on the tongue to collect and deliver chemical particles to the organ. Conversely, the organ is absent or underdeveloped in crocodilians and turtles.12
• Birds: The Jacobson’s organ is absent in birds.12
• Mammals: While present in many mammals, it is absent or underdeveloped in cetaceans and many advanced primates. Some mammals exhibit the flehmen response, curling the lip to facilitate chemical exposure.12