The role of Glucocorticoid hormones is considered advantageous when stress in a species habitat occurs. Although corticosterone is primarily considered beneficial, a study on marine iguanas in the Galapagos, before and after an El Nino occurrence, proved that too long of duration of high hormone levels can actually have adverse effects on survival. The study was recently presented in the Proceedings of the Royal Society B: Biological Sciences, and was titled “Stress physiology as a predictor of survival in Galapagos marine iguanas”.
The stress hormone studied is called Corticosterone. Similar to Cortisol in humans,
corticosterone is secreted by most amphibians, reptiles, rodents and birds to aid in stress response. Specifically, corticosterone helps to increase metabolism, and in turn, energy level, during a crisis situation. In other words, the hormone helps to activate the sympathetic responses in the nervous system.
A team led by Professor L. Michael Romero, of Tufts University, travelled to Santé Fe Island in the Galapagos between 2002 and 2008.
In breeding season of 2002 (early December), they captured 98 male marine iguanas (Amblyrhynchus cristatus). Once captured, blood corticosterone titres were taken at various stages of stress response. This included baseline corticosterone concentrations, corticosterone responses to acute stressors (presented by capture and handling), maximal capacity to secrete corticosterone (via adrenocorticotropin injection), and ability to terminate corticosterone responses (negative feedback). The titre variances were to be taken into account with later information, which is to say, the mortality rates following the island experiencing El Nino.
El Nino occurs in late December of every three to seven years. It is a catastrophic event in the eastern tropical Pacific, and has traumatizing effects on marine ecosystems. During El Nino, temperatures of surface ocean waters increase. This causes a decrease in upwelling near shorelines, which causes a subsequent decrease in primary production, as nutrient-depleted surface waters are not replaced from nutrient-rich waters from the deep. This leads to a corresponding decrease in those which feed on primary sources of food, including the marine iguana, whose main food intake consists of red and green algae.
In other words, the marine iguanas experienced a significant amount of environmental stress. The Galapagos iguanas provide an accurate model for analysis as they live in stable conditions, with the only exception being the occurrence of El Nino. They have only one source of nutrition and experience no other threat/stress besides the shortage of this food supply.
The team returned to the island in July of 2003. Only 77% of the iguanas previously captured had survived, meaning 23 iguanas had perished due to stress caused by El Nino. Interestingly, the only distinct correlation between mortality rates and corticosterone titres occurred in the last stage of stress, during negative feedback. The results showed that iguanas who had a better ability to terminate corticosterone secretion had much higher survival rates. The survival advantage of negative feedback during starvation is due to corticosterone’s role in metabolism. Corticosterone helps cause “fight or flight” responses in an organism’s body, in particular the activation of protein stores, during stage three of starvation. In stage one, carbohydrates are broken down, followed by stage two, lipid metabolism and stage three, protein metabolism. Increased levels of corticosterone were only found during stage three, meaning that protein metabolism is the difference between survival and death.
Iguanas who survived were shown to have an effective negative feedback response after corticosterone levels had raised above a certain threshold. A more efficient feedback system helped the iguanas to diminish the hormone to normal levels, ceasing protein metabolism, and therefore conserving energy. During the long term stress of starvation, this stress response does not remain beneficial as it cannot help to sustain energy levels for a long period of time. So, contrary to previous impression, levels of the stress hormone corticosterone can have an adverse relationship to survival.
Inference of this study can ascertain that a more effective negative feedback reaction (greater number of corticosterone receptors in the hypothalamus) could become a factor in natural selection. This means that over time, a larger frequency of the iguana population may develop an effective negative feedback loop and allow an overall increase in survival rate of the species during El Nino. This study can also be transferred to predict survival rates of other species in which corticosterone plays a similar role, including many species of reptiles, amphibians, rodents, and birds. It will provide contribution to the overall understanding of stress mechanisms and the effects they have on survival.
References:
1. L. M. Romero, M. Wikelski. Stress physiology as a predictor of survival in Galapagos marine iguanas. Proceedings of the Royal Society B: Biological Sciences, 2010. 277 (1697): 3157. http://rspb.royalsocietypublishing.org/content/277/1697/3157.full
Jamie Prime 0718393
The stress hormone studied is called Corticosterone. Similar to Cortisol in humans,
corticosterone is secreted by most amphibians, reptiles, rodents and birds to aid in stress response. Specifically, corticosterone helps to increase metabolism, and in turn, energy level, during a crisis situation. In other words, the hormone helps to activate the sympathetic responses in the nervous system.A team led by Professor L. Michael Romero, of Tufts University, travelled to Santé Fe Island in the Galapagos between 2002 and 2008.
In breeding season of 2002 (early December), they captured 98 male marine iguanas (Amblyrhynchus cristatus). Once captured, blood corticosterone titres were taken at various stages of stress response. This included baseline corticosterone concentrations, corticosterone responses to acute stressors (presented by capture and handling), maximal capacity to secrete corticosterone (via adrenocorticotropin injection), and ability to terminate corticosterone responses (negative feedback). The titre variances were to be taken into account with later information, which is to say, the mortality rates following the island experiencing El Nino.
El Nino occurs in late December of every three to seven years. It is a catastrophic event in the eastern tropical Pacific, and has traumatizing effects on marine ecosystems. During El Nino, temperatures of surface ocean waters increase. This causes a decrease in upwelling near shorelines, which causes a subsequent decrease in primary production, as nutrient-depleted surface waters are not replaced from nutrient-rich waters from the deep. This leads to a corresponding decrease in those which feed on primary sources of food, including the marine iguana, whose main food intake consists of red and green algae.
In other words, the marine iguanas experienced a significant amount of environmental stress. The Galapagos iguanas provide an accurate model for analysis as they live in stable conditions, with the only exception being the occurrence of El Nino. They have only one source of nutrition and experience no other threat/stress besides the shortage of this food supply.
The team returned to the island in July of 2003. Only 77% of the iguanas previously captured had survived, meaning 23 iguanas had perished due to stress caused by El Nino. Interestingly, the only distinct correlation between mortality rates and corticosterone titres occurred in the last stage of stress, during negative feedback. The results showed that iguanas who had a better ability to terminate corticosterone secretion had much higher survival rates. The survival advantage of negative feedback during starvation is due to corticosterone’s role in metabolism. Corticosterone helps cause “fight or flight” responses in an organism’s body, in particular the activation of protein stores, during stage three of starvation. In stage one, carbohydrates are broken down, followed by stage two, lipid metabolism and stage three, protein metabolism. Increased levels of corticosterone were only found during stage three, meaning that protein metabolism is the difference between survival and death.
Iguanas who survived were shown to have an effective negative feedback response after corticosterone levels had raised above a certain threshold. A more efficient feedback system helped the iguanas to diminish the hormone to normal levels, ceasing protein metabolism, and therefore conserving energy. During the long term stress of starvation, this stress response does not remain beneficial as it cannot help to sustain energy levels for a long period of time. So, contrary to previous impression, levels of the stress hormone corticosterone can have an adverse relationship to survival.
Inference of this study can ascertain that a more effective negative feedback reaction (greater number of corticosterone receptors in the hypothalamus) could become a factor in natural selection. This means that over time, a larger frequency of the iguana population may develop an effective negative feedback loop and allow an overall increase in survival rate of the species during El Nino. This study can also be transferred to predict survival rates of other species in which corticosterone plays a similar role, including many species of reptiles, amphibians, rodents, and birds. It will provide contribution to the overall understanding of stress mechanisms and the effects they have on survival.
References:
1. L. M. Romero, M. Wikelski. Stress physiology as a predictor of survival in Galapagos marine iguanas. Proceedings of the Royal Society B: Biological Sciences, 2010. 277 (1697): 3157. http://rspb.royalsocietypublishing.org/content/277/1697/3157.full
Jamie Prime 0718393
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