FUNCTION OF SLEEP: ECOLOGIAL (EXPLANATIONS)
Sleep has an important adaptive value. Sleep conserves energy at times when food is scarce and enables animals to avoid predators.
Explanation 1: ENERGY CONSERVATION (WEBB, 1982)
A01: Energy Costs
- Maintaining body temperature and having a high metablolic rate (chemical processes in body) have a physiological cost; foraging and escaping from predators also have an energy cost.
A02: Supported by...
ALLISON and CICCHETTI (1976), who found that larger animals experience less NREM sleep but not less REM sleep, showing that only NREM sleep is important in energy conservation.
A01: Energy Conserved:
- Sleep is enforced inactivity (like hibernation)
A02: It may be that...
- NREM sleep evolved first (reptiles don't have REM sleep) for energy conservation
- REM sleep evolved later in animals with larger brains - to exercise neural circuits.
A01: Negative Correlation
- The amount of sleep is negatively correlated with body size. This relationship is further modified by foraging needs and predator danger.
A02: However... energy conservation in sleep is minimal (5-10%) and the risks are large. Energy can be conserved by partial inactivity (e.g. unilateral sleep)
Explanation 2: FORAGING REQUIREMENTS
A01: Animals with higher metabolic rates
- Such animals sleep for longer (ZEPELIN and RECHTSCHAFFEN, 1974)
A02: However... despite this finding, some species (e.g. the sloth) contradict this trend
A01: Sleep duration
- This is influence by foraging requirements. Herbivores eat food low in nutrients and sleep little. Carnivores eat food rich in nutrients so sleep longer.
A02: This account offers an explanation for why... there is so much variety in sleep patterns among species - they must adapt to the different pressures of their environments.
Explanation 3: Predator avoidance (MEDDIS, 1975)
A01: Negative correlation: The amount of sleep taken by an animal correlates negatively with the amount of danger typically experienced (ALLISON and CICCHETTI, 1976)
A02: Disadvantages of sleep: Sleep is a costly behaviour as while asleep, animals cannot be vigilant against predators, forage or protect their young.
A01: Prey species sleep less to be vigilant
- But if they have to sleep it is best to do so when they are least vulnerable (night)
- It is best to stay still (i.e. sleep) when there is nothing better t do.
A02: However, it is possible that the greater 'stillness' of sleep renders prey species safer from predators than when simply resting.
A01: Unilateral sleep: Dolphins and some birds display unilateral sleep to maintain predator vigilance whilst conserving energy.
A02: However, BOEREMA et al. (2003) found that sleep-deprived chickens slept with both halves of their brain at the cost of reduced alertness.
FUNCTION OF SLEEP: ECOLOGICAL (STUDIES)
Study 1: Sleep length and energy conservation:
There is a negative correlation betewen the body size of a species and the amount of time spent sleeping. Small animals (e.g. brown bats) spend more hours asleep than larger animals (e.g. elephants) (ZEPELIN and RECHTSCHAFFEN, 1974)
A02: This supporst the claim that... animals with higher metabolic rates need to conserve energy by sleeping for longer periods.
A02: However... ALLISON and CICCHETTI (1976) found that larger animals experience less NREM sleep but not less REM sleep, showing that only NREM sleep is important for energy conservation.
Study 2: Endothermy and NREM Sleep
- Research has provided evidence for the universal presence of NREM sleep in endothermic (warm-blooded) mammals and birds and its absence in reptiles and other ectothermic (cold blooded) species
- NREM sleep is seen to have evolved as a way of conserving the energy needed to maintaining endothermy.
KAVANAU (2004) claims that the loss of muscle tone associated with NREM sleep would have been insufficient to prevent muscle contractions during sleep.
Study 3: Sleep length and predator avoidance
ALLISON and CICCHETTI (1976) found that the more danger typically faced by a species, the less it slept.
A02: This supports the claims that...
- Evolution of sleep is associated with the need to remain safe from predators.
- Large amounts of REM sleep are disadvantageous to prey species.
However... data are correlational, i.e. do not demonstrate a causal relationship.
Also... the inverse relationship between sleep and predator avoidance does not hold true to all animals, e.g. rabbits (high danger) sleep the same as moles (low danger rating)
Study 4: Unilateral sleep
- Birds watching for predators keep one eye open and corresponding hemisphere active. (RATTENBERG et al,1999)
- MUKHAMETOV (1984) found that bottlenose dolphins 'switch off' one hemisphere at a time. During sleep they are still able to come up to the surface to breathe.
A02: This shows that... Marine mammals are able to gain the benefits of sleep while at the same time avoiding dangers in their environment.
However... BOEREMA et al. (2003) found that other birds (e.g. chickens) abandon unilateral sleep when sleep-deprived at the cost of reduced alertness.
Study 5: REM sleep and developmental complexity
- ZEPELIN (1989) has shown that altricial animals (unable to care for themselves) have much larger amounts of REM sleep at birth than precocial mammals, with immaturity at birth the single best predictor of REM sleep time throughout life.
A02: This suggests that... there us a link between the need for REM sleep and greater developmental complexity.