Topic 6. Biological Rhythms – Study guideAny rhythmic activity or behavior is going to be the result of both an endogenous pacemaker AND an external or environmental synchronizer.Rhythmic activities can be animal behaviors, but virtually every living thing on this planet shows some kind of biological rhythm or rhythms and a lot of our insight into these aspects of living organisms has come from non-animal research. The molecular mechanisms of biological rhythms are highly conserved from an evolutionary perspective.Biological rhythms are also a great way to study both proximate AND ultimate explanations of behavior.Any and every repeated phenomenon is going to have 3 variables: frequency (or period), amplitude (or intensity), and phase.The rhythmic activities of animals are generally categorized by frequency:1. ultradian – less than 24 hours2. tidal – occurring in tidal zones and can show “nested” rhythmic activity3. lunar – occurs approximately every 28 days4. circadian – over a 24 hour period. These are the most well-documented and studied. Sub-categories are diurnal, nocturnal, and crepuscular. Humans showclear circadian rhythms.5. circannual – rhythms that occur over a year’s period of time. Best examples are hibernation and migration. SAD is clearly established as a human circannual cycle.Rhythmic activities, such as circadian rhythms, can change over the lifetime of an animal, or as the temperate climate changes.To examine the rhythmic activity of the endogenous pacemaker, it is necessary to remove any Zeitgeber that might initiate or synchronize the initiation or cessation of an activity period. The most common chronobiological experiment is to place an animal in complete darkness (since photoperiod is the most common Zeitgeber),and monitor its activity using an actograph.These experiments are usually run over at least a 2 week time period, usually beginning with a demonstration of entrainment to establish the hypothesized Zeitgeber. The Zeitgeber is then removed and the animal enters the free-running phase of the experiment.Because the 24 hour day is a human construct, without a Zeitgeber animals will drift away from a perfect match to the 24 hour clock. In complete darkness, naturally diurnal animals will show a slightly LONGER than 24 hour active/inactive rhythm, whereas a naturally nocturnal animal will show a slightly SHORTER than 24 hour rhythm. This is known as Aschoff’s Rule.Although photoperiod is the most common (and most commonly studied) Zeitgeber because it is both predictable and reliable, other aspects of the environment can serve to initiate or synchronize rhythmic animal behavior (rainfall, temperature changes, etc.).Structures in the nervous system or other body parts (e.g. eyes) receive informationfrom the environment to synchronize behaviors. For example, in the mammalian and avian brains, the suprachiasmatic nucleus (SCN) has endogenous pacemaker functions and receives information from the retina. This information, in turn, is used to either stimulate or inhibit behaviors. The inhibitory connection between the SCN and the pineal gland in mammals is essential to initiate wakefulness or sleep. The pineal gland produces melatonin which is associated with sleep patterns.Molecular mechanisms of the pacemaker function (protein synthesis and degradation) have been studied extensively in Drosophila and show a high degree of evolutionary conservation when compared with other animals (and even non-animals) as mentioned earlier.Rhythmic activities show clear fitness advantages in preparing animals for changesin the environment. These are among the ultimate explanations for the existence ofendogenous pacemakers that affect behaviors in animals. Preparation for torpor or hibernation during extreme cold conditions in the winter can involve several physiological and behavioral changes depending on the animal species.One interesting aspect of hibernation is that despite being insulated from the environment, animals can physiologically prepare for reproduction (gametogenesis) almost immediately after emerging from the