Hormones affect seasonal activity of plants

Both plants and animals undergo a number of changes as the daylight hours grow shorter and winter is on the horizon. In the case of animals there are changes that occur in the body usually brought about by changes in the hormones produced, along with behavioral changes related to the length of daylight to nighttime. A name given to the ratio of day length to nighttime is called photoperiodism. When the daylight hours begin to diminish in the fall many animals production of hormones shift from those that are intended for mating and active lifestyles to those that are more useful for accumulation of body fat and growth of fur, feathers etc. In the case of birds the fall triggers the formation of downy feathers, a dulling of plumage, the accumulation of fat, and a slowing of metabolism. This hormonal shift may also be a triggering mechanism to initiate migration for those that over winter in warmer climates.

By the same token many mammals develop thicker coats, accumulate fat, and see a depressed metabolism which can result in a deep sleep or true hibernation. These changes have survival value since they help ensure that young are born when conditions are favorable for survival and that the animal is ready for the harsh conditions and lack of food that normally occurs during winter. Besides length of daylight other changes can be brought about by decreasing temperature and periods of rain or drought. However, many studies confirm that most seasonal changes in animals and plants are triggered by photoperiod and this is the most reliable indicator of change since the earth’s rotation around the sun is one of the constants of life as we know it.

Photoperiodism is much more prevalent in animals that live in higher latitudes such as the Artic or Antarctic than animals that live near the in areas that do not see a wide swing in daylight to darkness of temperature extremes. In addition, smaller animals are more heavily influenced by photoperiod those larger animals since they have are more prone to influences caused by temperature extremes. Examples of this are the artic fox that sees a complete molting of brown or black fur in the fall changing to a winter coat that is white in color. Human’s are not thought to be affected greatly by photoperiodism however there is a condition called Seasonal Affective Disorder (SAD) that is well documented in the human population. People generally develop SAD symptoms in the winter and those affected tend to feel tired, sleep a lot and experience signs of depression.

These symptoms usually disappear once day length increases in the spring. Researchers estimate that about 1% of the residents of Florida have symptoms relating to SAD but about 10% of the residents of New Hampshire have SAD related symptoms. The Nordic population of Europe is very prone to SAD but this disorder is not very prevalent in Iceland. Scientists believe that the Icelanders consume a lot of fish and this may help prevent symptoms of Seasonal Affective Disorder. So if you find yourself becoming a couch potato on these cold wintry nights and are putting on a few extra pounds, blame it on photoperiodism.

Nature Notes for 3/10/2013