In mammals during times of scarcity when nutrient intake is low, high levels of ghrelin secretion triggers receptors in the hypothalamus to increase body weight and fat mass: this stored fat can then be used as fuel for the body at a later time, making the signalling process critical when available food sources were often few and far between. However, it was more recently discovered that the garden warbler, a type of migratory bird, relies on ghrelin as an indicator for migration. As the bird consumes forest mast in preparation for the journey, ghrelin levels in the blood increase until they reach what is thought to be a threshold, triggering the initiation of migration; as ghrelin levels decrease they signal the warbler to continue their migratory path until they reach their final destination.Ghrelin: The Hormone Of Hunger Paper
The presence and cascading reactions of ghrelin and its receptors are integral to the lifestyle of organisms from all periods and radiations of life. Current and future research into the broad reach of ghrelin‘s effects have generated exciting results that may prove useful in fully understanding the feeling of hunger and its side effects, including obesity. Lack of ghrelin in mutant mice has been linked to increased anxiety levels when exposed to a variety of stressors, and mice with synthetically higher levels of ghrelin provided antidepressant-like responses when exposed to high-stress conditions, meaning that ghrelin may act as an acute, natural adaptation to counter the effects of stress in an environment.
Ghrelin has also gained notoriety more recently for being a key hormone affecting those with eating disorders: ghrelin levels are found to be higher in individuals with anorexia nervosa and lower in cases of obesity, leading some to believe that ghrelin injections may be a solution towards obesity and genetically-linked metabolic disorders. Although researches regarding the medical possibilities with ghrelin are relatively recent, its significance in the development and survival of vertebrates is without a doubt pertinent in any discussion about the evolution of survival hormones in vertebrates.Ghrelin: The Hormone Of Hunger Paper