| A | B |
|---|
| Sound of a predator | animal hides or runs away |
| changes in the length of a day | may trigger migration behavior |
| sight of a potential mate | may trigger courtship behaviors |
| changes in the length of a day could be an external stimuli | Alaskan Caribou might think it is time to migrate |
| hunger messages from the brain, could be an internal stimuli | Alaskan Caribou might think it is time to eat more for the migration |
| Circadian rhythm | the daily cycle of activity that occurs over a 24-hour period of time |
| Biological clock | the internal mechanism that controls the activity patterns of the circadian rhythm |
| stimulus | type of information that has the potential to make an organism change its behavior |
| Internal stimuli | tell what is occurring in the body. |
| External stimuli | give information about surroundings. |
