184
The Biology of Sea Turtles, Vol. II
Animals already subjected to physiological stresses (high or low temperatures,
capture trauma, starvation) are likely to experience increased circulating glucocorticoid levels, which in turn depress immune function and accelerate catabolic processes. Recurrent environmental stressors may reduce survival if they result in
persistent glucocorticoid secretion (Nelson and Demas, 1996); however, the potential
links between environmental stressors, stress hormones, and immune function in sea
turtles have not been investigated.
6.4.2 IMMUNOLOGICAL RESPONSES
It is now commonly accepted that manipulation of neural and endocrine functions
alters vertebrate immune responses, and the antigenic stimulation that generates an
immune response results in changes in neural and endocrine functions; thus the
immune status of an individual also has consequences for behavior (Ader and
Cohen, 1993).
The suppression of the immune response by adrenocortical hormones, especially
the glucocorticoids, is a well-described vertebrate response. Although most work
describing the link between immunosuppression and elevated adrenocortical hormones has been done on mammals (Munck and Naray-Fejes-Toth, 1994), a few
reptile studies have been performed. Saad and el Ridi (1988) observed significant
lymphocytic destruction and the impairment of immune reactivity in the lizard
Chalcides ocellatus, which they associated with sustained high levels of endogenous
corticosteroid levels in the autumn and winter. By contrast, fully developed splenic
lymphoid tissue and immune responses were coincident with low summer corticosteroid levels. The administration of exogenous corticosteroids to “summer” lizards
depleted lymphoid elements and suppressed immune responses, whereas the pharmacological inhibition of corticosteroid synthesis in autumn ameliorated the natural
winter-dependent immune depression (Saad and el Ridi, 1988).
The immune response of reptiles is of course affected by numerous factors.
Steroid sex hormones, for example, also have significant effects on immunological
activity in reptiles and other vertebrates, although again most studies in this area
involve mammals. The reptile immune system is strongly affected by seasonal
changes caused by both temperature changes and changes associated with the breeding cycle. Seasonal changes in thymic mass in turtles were first reported in 1912;
Aime (1912) reported decreased thymic mass during winter estivation, and thymic
regeneration in the spring. Androgens, like the glucocorticoids, appear to have
immunocompromising properties. In poikilotherms, lymphoid mass and immunological activity is greatest in spring and summer, after breeding activities have been
completed and testosterone levels decline. In the turtle Mauremys caspica, lymphocyte proliferation induced by mitogens showed high values in the spring and winter
and decreased responses in summer and fall (Munoz et al., 2000; Munoz and De la
Fuente, 2001), whereas a single injection of testosterone (200 mg/g body weight)
produced thymic involution and intense lymphopenia in the spleen and peripheral
blood compartment (Saad et al., 1991). Female mammals generally have higher
immune activities by several indices than male conspecifics, whereas gonadectomized mice and rats treated with physiological or greater estrogen levels exhibited
1123 book.book Page 184 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
Animals already subjected to physiological stresses (high or low temperatures,
capture trauma, starvation) are likely to experience increased circulating glucocorticoid levels, which in turn depress immune function and accelerate catabolic processes. Recurrent environmental stressors may reduce survival if they result in
persistent glucocorticoid secretion (Nelson and Demas, 1996); however, the potential
links between environmental stressors, stress hormones, and immune function in sea
turtles have not been investigated.
6.4.2 IMMUNOLOGICAL RESPONSES
It is now commonly accepted that manipulation of neural and endocrine functions
alters vertebrate immune responses, and the antigenic stimulation that generates an
immune response results in changes in neural and endocrine functions; thus the
immune status of an individual also has consequences for behavior (Ader and
Cohen, 1993).
The suppression of the immune response by adrenocortical hormones, especially
the glucocorticoids, is a well-described vertebrate response. Although most work
describing the link between immunosuppression and elevated adrenocortical hormones has been done on mammals (Munck and Naray-Fejes-Toth, 1994), a few
reptile studies have been performed. Saad and el Ridi (1988) observed significant
lymphocytic destruction and the impairment of immune reactivity in the lizard
Chalcides ocellatus, which they associated with sustained high levels of endogenous
corticosteroid levels in the autumn and winter. By contrast, fully developed splenic
lymphoid tissue and immune responses were coincident with low summer corticosteroid levels. The administration of exogenous corticosteroids to “summer” lizards
depleted lymphoid elements and suppressed immune responses, whereas the pharmacological inhibition of corticosteroid synthesis in autumn ameliorated the natural
winter-dependent immune depression (Saad and el Ridi, 1988).
The immune response of reptiles is of course affected by numerous factors.
Steroid sex hormones, for example, also have significant effects on immunological
activity in reptiles and other vertebrates, although again most studies in this area
involve mammals. The reptile immune system is strongly affected by seasonal
changes caused by both temperature changes and changes associated with the breeding cycle. Seasonal changes in thymic mass in turtles were first reported in 1912;
Aime (1912) reported decreased thymic mass during winter estivation, and thymic
regeneration in the spring. Androgens, like the glucocorticoids, appear to have
immunocompromising properties. In poikilotherms, lymphoid mass and immunological activity is greatest in spring and summer, after breeding activities have been
completed and testosterone levels decline. In the turtle Mauremys caspica, lymphocyte proliferation induced by mitogens showed high values in the spring and winter
and decreased responses in summer and fall (Munoz et al., 2000; Munoz and De la
Fuente, 2001), whereas a single injection of testosterone (200 mg/g body weight)
produced thymic involution and intense lymphopenia in the spleen and peripheral
blood compartment (Saad et al., 1991). Female mammals generally have higher
immune activities by several indices than male conspecifics, whereas gonadectomized mice and rats treated with physiological or greater estrogen levels exhibited
1123 book.book Page 184 Monday, November 11, 2002 11:11 AM