GENERAL
AND
COMPARATIVE
Reproductive II. Plasma
ENDOCRINOLOGY
85, 241-241 (1992)
Endocrinology of Macaroni (Eudyptes chrysolophus) and Gentoo (Pygoscelis papa) Penguins
Levels of Gonadal
Steroids and LH in Immature Deferred Sexual Maturity
Birds in Relation
to
T. D. WILLIAMS’ British
Antarctic
Survey,
Natural Environment Research Council, High Cambridge, CB3 OET, United Kingdom
Cross,
Madingley
Road,
Accepted April 25, 1991 Plasma levels of LH, testosterone, estradiol, and progesterone were measured, during the breeding season, in adult and immature macaroni (Eudyptes chrysolophus) and gentoo (Pygoscelis Papua) penguins, at Bird Island, South Georgia (54’S, 38”W), in relation to deferred sexual maturity. Macaroni penguins do not breed until 6-8 years of age whereas gentoo penguins first breed at 2 years of age. Plasma LH was elevated in 3- to 5-year-old macaroni penguins, but not in l- to 2-year-old birds nor l-year-old gentoos. However, 1 year olds of both species responded to an injection of exogenous LH-RH by increasing LH secretion. Some individual 3-, 4-, and 5-year-old macaroni penguins had circulating testosterone levels within the range of peak values in adults, suggesting that in males, gonads were physiologically mature several years in advance of the age of first breeding. One-year-old macaroni penguins had elevated progesterone levels and basal estradiol levels, but in 2-4 year olds plasma levels of both progesterone and estradiol were low. This does not support the hypothesis that high progesterone levels “block” ovarian maturation in immature penguins. Immature gentoo penguins showed a different pattern of hormone secretion. Plasma testosterone was low in all breeding periods, but estradiol levels increased to a peak during the incubation period, 4 weeks later than peak values occurred in adults. In both species, therefore, deferred sexual maturity was associated with delayed and/or reduced secretion of LH and gonadal steroids. B 1992Academic PKSS. h.
Many short-lived bird species (e.g., most passerines) first breed in the year following hatching, but in other species onset of breeding is delayed for 1 or more years. This “deferred sexual maturity” reaches an extreme in long-lived seabird species and some albatrosses, fulmars (Procellariformes), and penguins (Sphenisciformes) do not start breeding until 6-12 years of age (Newton, 1989). Although there have been few studies on species showing longdeferred sexual maturity, various physiological, behavioral, and ecological factors
t Present address: Department of Biology, Queen’s University, Kingston, Ontario, Canada K7L 3N6.
may influence this process (Ryder, 1980; Hector et al., 1986a, 1990; Burger, 1988). The many studies of the reproductive endocrinology of free-living birds have mostly addressed breeding adults. There have been few studies of the endocrinology of free-living immature birds and the physiological bases of deferred sexual maturity are unknown. The only previous study of a species showing long-deferred sexual maturity is that on the wandering albatross (Diomedea exulans, which first breeds at 7-9 years of age; Croxall et al., 1990) by Hector et al. (1986a). This study showed that immature (prebreeding) females had a qualitatively different annual cycle of reproductive hormones:high plasma levels of 241 0016~6480/92 $1.50 Copyright All tights
6 1992 by Academic Press, Inc. of reproduction in any form reserved.
242
T. D.
WILLIAMS
progesterone but low circulating estradiol levels, compared to adult (breeding) birds. Recently, Hector ef al. (1990) suggested that a critical factor in the development of sexual maturity in this species may be whether the gonads respond to gonadotropin stimulation by secreting progesterone or estradiol. If the former, then the bird does not breed; if the latter, then ovarian maturation is stimulated. This mechanism would therefore prevent, possibly costly, energetic demands of yolk production in immature birds (Hector et al., 1990). This paper describes plasma levels of gonadal steroids and luteinizing hormone (LH) in immature macaroni penguins (Eudyptes chrysolophus) to test the hypothesis that deferred sexual maturity is associated with elevated progesterone:low estradiol levels. Macaroni penguins first breed between 6 and 8 years of age (Croxall, 1984). Comparative data are also presented for immatures in the sympatric gentoo penguin (Pygoscelis papua) which delays breeding for only 1 year. The reproductive endocrinology of adults in these two species is described in Williams (1992). MATERIALS
AND METHODS
Fieldwork was carried out between October 1986 and March 1989, at the Fairy Point macaroni penguin colony (ca. RlOObreeding pairs) and the Johnson Cove gentoo penguin colony (ca. 1500 pairs) on Bird Island, South Georgia (54%0’S, 38%2’W). Details of the breeding biology of the two study species are given in Williams (1992; see also Williams, 1990; Williams and Croxall, 1991). Blood samples from prebreeding, immature birds were obtained during periods equivalent to the stages of peak copulation, incubation, and chick-brooding in adult (breeding) birds (see Williams, 1992). One-yearold (immature) gentoo penguins were identified by plumage characteristics (Williams, 1988). One- to 5-year-old (immature) macaroni penguins had been flipper banded as chicks and were therefore of known age. Immature birds could not be sexed because their bills are not fully grown until the age of sexual maturity. Blood was drawn into a preheparinised syringe from the brachial vein in the flipper. The sample was centrifuged within 2-3 hr and the plasma stored at - 20” until assayed in the United Kingdom. In addition Study
area
and field
methods.
to blood sampling of nonmanipulated birds, some immature (l-year-old) birds received an injection of synthetic ovine LH-RH (Sigma Chemical Co., Ltd., Dorset, England) intramuscularly at a dose rate of 20 pg . kg-’ body wt (following Hector et al.. 1990). Control birds received an injection of the same volume of saline. Blood samples were then taken 10 min after injection and plasma was assayed for LH. Hormone assays. Samples were assayed for LH using a homologous chicken radioimmunoassay (Follett et al.. 1972) which has previously been validated for penguin plasma (Groscolas et al., 1986). Plasma steroids were extracted in diethyl ether and measured by radioimmunoassay (Dawson, 1983; Hector et al., 1986b). Preliminary tests showed that celite chromatography following extraction was not necessary (Williams, 1992; see also Groscolas et al., 1986). Each assay was validated by including two plasma pools with known amounts of exogenous steroid. Details of the characteristics of each array and cross-reactivity of the antisera with other hormones are given in Williams (1992). As immature birds could not be reliably sexed, all plasmas from immatures were assayed for all four hormones. Statistical analysis. Mean maximum hormone concentrations in immature birds were compared with peak (highest) and basal (lowest) levels in adult (breeding) birds (from Williams, 1992) using one-way analysis of variance with Newman-Keuls test (Zar, 1974) for multiple comparisons. Student’s t test was used for single comparisons. Statistical tests were carried out using Minitab (Ryan et al., 1985). All concentrations are given as means f SE.
RESULTS Macaroni penguins. Peak plasma levels of LH were significantly higher (P < 0.05) in 3-, 4-, and S-year-old macaroni penguins than basal levels (2.3 Fg * liter-‘) in adults (Fig. la), although LH in Syear-old birds (4.5 + 0.6 p.g . liter-‘, during incubation stage) was lower than peak levels in postcopulation adults (7.8 + 1.0 pg * liter-‘, t,, = 2.93, P < 0.025). LH in 1 and 2 year olds was not significantly different from basal levels in adults and was significantly lower then peak values in 3-5 year olds (P < 0.05 in all cases). However, 1 year olds injected with LH-RH had mean LH levels significantly higher than control birds (t = 6.89, P
AND
COMPARATIVE
Reproductive II. Plasma
ENDOCRINOLOGY
85, 241-241 (1992)
Endocrinology of Macaroni (Eudyptes chrysolophus) and Gentoo (Pygoscelis papa) Penguins
Levels of Gonadal
Steroids and LH in Immature Deferred Sexual Maturity
Birds in Relation
to
T. D. WILLIAMS’ British
Antarctic
Survey,
Natural Environment Research Council, High Cambridge, CB3 OET, United Kingdom
Cross,
Madingley
Road,
Accepted April 25, 1991 Plasma levels of LH, testosterone, estradiol, and progesterone were measured, during the breeding season, in adult and immature macaroni (Eudyptes chrysolophus) and gentoo (Pygoscelis Papua) penguins, at Bird Island, South Georgia (54’S, 38”W), in relation to deferred sexual maturity. Macaroni penguins do not breed until 6-8 years of age whereas gentoo penguins first breed at 2 years of age. Plasma LH was elevated in 3- to 5-year-old macaroni penguins, but not in l- to 2-year-old birds nor l-year-old gentoos. However, 1 year olds of both species responded to an injection of exogenous LH-RH by increasing LH secretion. Some individual 3-, 4-, and 5-year-old macaroni penguins had circulating testosterone levels within the range of peak values in adults, suggesting that in males, gonads were physiologically mature several years in advance of the age of first breeding. One-year-old macaroni penguins had elevated progesterone levels and basal estradiol levels, but in 2-4 year olds plasma levels of both progesterone and estradiol were low. This does not support the hypothesis that high progesterone levels “block” ovarian maturation in immature penguins. Immature gentoo penguins showed a different pattern of hormone secretion. Plasma testosterone was low in all breeding periods, but estradiol levels increased to a peak during the incubation period, 4 weeks later than peak values occurred in adults. In both species, therefore, deferred sexual maturity was associated with delayed and/or reduced secretion of LH and gonadal steroids. B 1992Academic PKSS. h.
Many short-lived bird species (e.g., most passerines) first breed in the year following hatching, but in other species onset of breeding is delayed for 1 or more years. This “deferred sexual maturity” reaches an extreme in long-lived seabird species and some albatrosses, fulmars (Procellariformes), and penguins (Sphenisciformes) do not start breeding until 6-12 years of age (Newton, 1989). Although there have been few studies on species showing longdeferred sexual maturity, various physiological, behavioral, and ecological factors
t Present address: Department of Biology, Queen’s University, Kingston, Ontario, Canada K7L 3N6.
may influence this process (Ryder, 1980; Hector et al., 1986a, 1990; Burger, 1988). The many studies of the reproductive endocrinology of free-living birds have mostly addressed breeding adults. There have been few studies of the endocrinology of free-living immature birds and the physiological bases of deferred sexual maturity are unknown. The only previous study of a species showing long-deferred sexual maturity is that on the wandering albatross (Diomedea exulans, which first breeds at 7-9 years of age; Croxall et al., 1990) by Hector et al. (1986a). This study showed that immature (prebreeding) females had a qualitatively different annual cycle of reproductive hormones:high plasma levels of 241 0016~6480/92 $1.50 Copyright All tights
6 1992 by Academic Press, Inc. of reproduction in any form reserved.
242
T. D.
WILLIAMS
progesterone but low circulating estradiol levels, compared to adult (breeding) birds. Recently, Hector ef al. (1990) suggested that a critical factor in the development of sexual maturity in this species may be whether the gonads respond to gonadotropin stimulation by secreting progesterone or estradiol. If the former, then the bird does not breed; if the latter, then ovarian maturation is stimulated. This mechanism would therefore prevent, possibly costly, energetic demands of yolk production in immature birds (Hector et al., 1990). This paper describes plasma levels of gonadal steroids and luteinizing hormone (LH) in immature macaroni penguins (Eudyptes chrysolophus) to test the hypothesis that deferred sexual maturity is associated with elevated progesterone:low estradiol levels. Macaroni penguins first breed between 6 and 8 years of age (Croxall, 1984). Comparative data are also presented for immatures in the sympatric gentoo penguin (Pygoscelis papua) which delays breeding for only 1 year. The reproductive endocrinology of adults in these two species is described in Williams (1992). MATERIALS
AND METHODS
Fieldwork was carried out between October 1986 and March 1989, at the Fairy Point macaroni penguin colony (ca. RlOObreeding pairs) and the Johnson Cove gentoo penguin colony (ca. 1500 pairs) on Bird Island, South Georgia (54%0’S, 38%2’W). Details of the breeding biology of the two study species are given in Williams (1992; see also Williams, 1990; Williams and Croxall, 1991). Blood samples from prebreeding, immature birds were obtained during periods equivalent to the stages of peak copulation, incubation, and chick-brooding in adult (breeding) birds (see Williams, 1992). One-yearold (immature) gentoo penguins were identified by plumage characteristics (Williams, 1988). One- to 5-year-old (immature) macaroni penguins had been flipper banded as chicks and were therefore of known age. Immature birds could not be sexed because their bills are not fully grown until the age of sexual maturity. Blood was drawn into a preheparinised syringe from the brachial vein in the flipper. The sample was centrifuged within 2-3 hr and the plasma stored at - 20” until assayed in the United Kingdom. In addition Study
area
and field
methods.
to blood sampling of nonmanipulated birds, some immature (l-year-old) birds received an injection of synthetic ovine LH-RH (Sigma Chemical Co., Ltd., Dorset, England) intramuscularly at a dose rate of 20 pg . kg-’ body wt (following Hector et al.. 1990). Control birds received an injection of the same volume of saline. Blood samples were then taken 10 min after injection and plasma was assayed for LH. Hormone assays. Samples were assayed for LH using a homologous chicken radioimmunoassay (Follett et al.. 1972) which has previously been validated for penguin plasma (Groscolas et al., 1986). Plasma steroids were extracted in diethyl ether and measured by radioimmunoassay (Dawson, 1983; Hector et al., 1986b). Preliminary tests showed that celite chromatography following extraction was not necessary (Williams, 1992; see also Groscolas et al., 1986). Each assay was validated by including two plasma pools with known amounts of exogenous steroid. Details of the characteristics of each array and cross-reactivity of the antisera with other hormones are given in Williams (1992). As immature birds could not be reliably sexed, all plasmas from immatures were assayed for all four hormones. Statistical analysis. Mean maximum hormone concentrations in immature birds were compared with peak (highest) and basal (lowest) levels in adult (breeding) birds (from Williams, 1992) using one-way analysis of variance with Newman-Keuls test (Zar, 1974) for multiple comparisons. Student’s t test was used for single comparisons. Statistical tests were carried out using Minitab (Ryan et al., 1985). All concentrations are given as means f SE.
RESULTS Macaroni penguins. Peak plasma levels of LH were significantly higher (P < 0.05) in 3-, 4-, and S-year-old macaroni penguins than basal levels (2.3 Fg * liter-‘) in adults (Fig. la), although LH in Syear-old birds (4.5 + 0.6 p.g . liter-‘, during incubation stage) was lower than peak levels in postcopulation adults (7.8 + 1.0 pg * liter-‘, t,, = 2.93, P < 0.025). LH in 1 and 2 year olds was not significantly different from basal levels in adults and was significantly lower then peak values in 3-5 year olds (P < 0.05 in all cases). However, 1 year olds injected with LH-RH had mean LH levels significantly higher than control birds (t = 6.89, P
