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Marine Mammal Physiology: Requisites for Ocean Living
of fat mobilization might require higher thyroid hormone levels (Ortiz et al. 2001). During
extended fasting, reductions occur in levels of both T4 and T3, the former assisted by the
conversion of T4 to the biologically inactive reverse T3 (rT3).
Ghrelin antagonizes the action of leptin and is known as the hunger hormone. Ghrelin
is produced by specialized cells that line the stomach and the pancreas when the stomach is empty, while secretion stops when the stomach is stretched. It acts on hypothalamic brain cells, and its neural receptors are found on the same cells in the brain as the
receptors for leptin. It serves to both increase hunger and to increase gastric acid secretion and gastrointestinal motility to prepare the body for food intake. It increases both
appetite and fat mass by triggering receptors that stimulate production of neuropeptide Y. Ghrelin also functions as a growth hormone-releasing peptide. While leptin and
ghrelin usually work in opposite directions in feeding animals, ghrelin levels should be
lower in naturally fasting animals as a means of suppressing appetite (including suppression of neuropeptide Y).
However, as will become apparent, the results of experimental studies so far indicate
that we have only begun to understand the hormonal control of physiological processes
during fasting in marine mammals and, in fact, a new theoretical framework may be
required given how poorly the experimental data to date match our current expectations.
8.1.4.1 Pinnipeds
As with many aspects of fasting physiology, most studies of hormonal changes have
been conducted on northern elephant seal pups. In this species, many of the hormonal
changes that support fasting occur earlier than expected, indicating that preparatory
actions occur prior to weaning. Cortisol levels have been observed to increase between
early and late nursing, and then again during early fasting, with the largest increase
(more than doubling) seen between early and late fasting (Ortiz et al. 2003). All thyroid
hormones decrease from early to late nursing, and then do not increase during fasting
(except for a small increase in total T4). No significant changes have been observed in
leptin levels during the fast (Ortiz et al. 2001), suggesting that leptin does not have an
expected role in regulating body fat in fasting (or nursing) northern elephant seal pups
(Ortiz et al. 2003).
Studies on the pups of other phocid species also yield unexpected results. Plasma
cortisol levels in harp seal pups remained stable throughout their fast, and at levels equal
to older, feeding pups (Nordøy et al. 1993). Neither was there any observed decrease in
thyroid hormones. Similarly, plasma cortisol levels remained relatively low and stable in
gray seals, except for a rise in some animals toward the end of their 52-day fasts (Nordøy
et al. 1990).
In juvenile elephant seals, cortisol concentrations do not change during their seasonal
fast, and neither do T3. However, both T4 and growth hormone concentrations decrease
dramatically during the fast (Kelso et al. 2012).
Some of the hormone changes observed in adult male northern elephant seals during the breeding fast are closer to those predicted by fasting theory (Crocker et al. 2012).
For example, there was a 43% decrease in GH, with no matching decrease in IGF-1.
This reduction in GH should function to reduce lipolysis and increase hepatic glucose
production, which would seem maladaptive in a fasting animal. However, it has been
proposed that reductions in GH may be required to suppress more serious anabolic
actions given that some level of protein catabolism is required for gluconeogenesis.
While leptin concentrations decreased 11% (although they did not follow changes in
fat mass), ghrelin concentrations did not change. This suggests that the high levels of
Marine Mammal Physiology: Requisites for Ocean Living
of fat mobilization might require higher thyroid hormone levels (Ortiz et al. 2001). During
extended fasting, reductions occur in levels of both T4 and T3, the former assisted by the
conversion of T4 to the biologically inactive reverse T3 (rT3).
Ghrelin antagonizes the action of leptin and is known as the hunger hormone. Ghrelin
is produced by specialized cells that line the stomach and the pancreas when the stomach is empty, while secretion stops when the stomach is stretched. It acts on hypothalamic brain cells, and its neural receptors are found on the same cells in the brain as the
receptors for leptin. It serves to both increase hunger and to increase gastric acid secretion and gastrointestinal motility to prepare the body for food intake. It increases both
appetite and fat mass by triggering receptors that stimulate production of neuropeptide Y. Ghrelin also functions as a growth hormone-releasing peptide. While leptin and
ghrelin usually work in opposite directions in feeding animals, ghrelin levels should be
lower in naturally fasting animals as a means of suppressing appetite (including suppression of neuropeptide Y).
However, as will become apparent, the results of experimental studies so far indicate
that we have only begun to understand the hormonal control of physiological processes
during fasting in marine mammals and, in fact, a new theoretical framework may be
required given how poorly the experimental data to date match our current expectations.
8.1.4.1 Pinnipeds
As with many aspects of fasting physiology, most studies of hormonal changes have
been conducted on northern elephant seal pups. In this species, many of the hormonal
changes that support fasting occur earlier than expected, indicating that preparatory
actions occur prior to weaning. Cortisol levels have been observed to increase between
early and late nursing, and then again during early fasting, with the largest increase
(more than doubling) seen between early and late fasting (Ortiz et al. 2003). All thyroid
hormones decrease from early to late nursing, and then do not increase during fasting
(except for a small increase in total T4). No significant changes have been observed in
leptin levels during the fast (Ortiz et al. 2001), suggesting that leptin does not have an
expected role in regulating body fat in fasting (or nursing) northern elephant seal pups
(Ortiz et al. 2003).
Studies on the pups of other phocid species also yield unexpected results. Plasma
cortisol levels in harp seal pups remained stable throughout their fast, and at levels equal
to older, feeding pups (Nordøy et al. 1993). Neither was there any observed decrease in
thyroid hormones. Similarly, plasma cortisol levels remained relatively low and stable in
gray seals, except for a rise in some animals toward the end of their 52-day fasts (Nordøy
et al. 1990).
In juvenile elephant seals, cortisol concentrations do not change during their seasonal
fast, and neither do T3. However, both T4 and growth hormone concentrations decrease
dramatically during the fast (Kelso et al. 2012).
Some of the hormone changes observed in adult male northern elephant seals during the breeding fast are closer to those predicted by fasting theory (Crocker et al. 2012).
For example, there was a 43% decrease in GH, with no matching decrease in IGF-1.
This reduction in GH should function to reduce lipolysis and increase hepatic glucose
production, which would seem maladaptive in a fasting animal. However, it has been
proposed that reductions in GH may be required to suppress more serious anabolic
actions given that some level of protein catabolism is required for gluconeogenesis.
While leptin concentrations decreased 11% (although they did not follow changes in
fat mass), ghrelin concentrations did not change. This suggests that the high levels of