BIOLOGICAL RHYTHMS

Question 1

Biological rhythms occur when?

Answer 1

An animals physiological activities and behaviour patterns can be directly related to environmental features that occur with regular frequency

Question 2

Types of rhythm?

Answer 2

Circadian (daily), Circannual (yearly), circalunar (29.5 days, important in some nocturnal species such as fruit bats), tidal rhythms (12.5 hrs, intertidal animals e.g. shore crabs)

Question 3

How do animals synchronise their behaviour and physiology to cyclic changes in the
external environment?

Answer 3

Direct response to changes in external environment
* Biological clock (endogenous rhythm) synchronised to changes in external environment
* Combination of both
In most cases, it appears to be a combination of both. An endogenous biological clock
provides the basic rhythm, but it is reset or entrained by environmental cues (zeitgebers),
particularly light.

Question 4

Evidence for the existence of biological clocks

Answer 4

1. Rhythm persists in constant conditions of light, temperature, etc
2. Animals reared in isolation exhibit rhythms
   e.g. Lizards hatched from eggs reared under different light regimes all had the same rhythm
   period (just under 24h). Rearing conditions for different groups were: 18 h day (9L:9D);
   24 h day (12L:12D), or 36 h day (18L:18D) (Light: Dark). Activity period of all groups was
   23.4-24.9 h
3. Mutants with altered rhythm have been produced e.g. mutant mice with a free-running
   period of 25 h (normal <24 h)

Question 5

Characteristics of endogenous rhythms (rhythms controlled by biological clocks)

Answer 5

• Persist in absence of environmental cues
• Period is not exactly that of the environmental cycle
• Rhythm is entrained by environmental cues (zeitgebers), especially light
• Temperature compensation:
  rhythm persists despite variation in environmental temperature