Rhythmic behaviours are cyclic responses to periodic environmental changes. They can be endogenous (driven by an internal biological clock) and are reset by external cues called zeitgebers (e.g. light).
Types of biological rhythm
Rhythm
Cycle
Example / survival value
Circadian
~24 hours
Sleep/wake, leaf movements → active when conditions best (avoid predators/heat)
Circatidal
~12.4 hours
Shore crabs active at high tide → feed safely, avoid stranding
Circalunar
~29.5 days
Coral mass spawning on the full moon → synchronise reproduction
Circannual
~1 year
Hibernation, breeding seasons, migration → time events to favourable seasons
Endogenous vs exogenous
Endogenous: the rhythm continues even without external cues (an internal clock): proves it’s not just a direct response.
Zeitgebers (e.g. light, temperature, tides) entrain (reset) the clock to keep it in step with the environment.
Survival value: lets the organism anticipate predictable change and prepare in advance, rather than just reacting.