Paul Licht

Proximate Constraints on the Evolution of Egg Size, Number, and Total Clutch Mass in Lizards

Proximate constraints on egg size, number, and total clutch mass in side-blotched lizards were examined by experimentally reducing average clutch size from 4.6 eggs to one, two, and three eggs. Eggs from experimentally altered clutches were larger than those from controls, reflecting the trade-off between egg size and number. Moreover, the increased frequency of females with oviducally bound eggs or eggs that burst at oviposition suggests that egg size in clutches with very few eggs are at a functional upper size limit. These proximate constraints may also limit evolution of egg size in another group of lizards (Anolis) that only produces one-egged clutches.

Anaerobic metabolism during activity in lizards

SummaryA new technique developed for the determination of total lactate production in small animals was used to evaluate the role of anaerobiosis during activity at different temperatures in lizards. Measurements on six species of small lizards indicate little interspecific variation or thermal effect in resting lactate levels (0.35 mg lactate/g body weight) or maximal lactate levels achieved at exhaustion (1.4 mg lactate/g). Normally activeAnolis in captivity had a lactate content of 0.5 mg lactate/g. Rates of lactate formation were most rapid during the first 30 sec of activity and had a low thermal dependence (Q10=1.1–1.3 above 20 °C). The lactate formed during activity persists for long periods; e.g., for 30 to 60 min between 20 and 37 °C inAnolis carolinensis (Fig. 1). Recovery rate generally increases with temperature. Muscle lactate concentrations peak at the end of activity, but liver and blood lactate are not maximal until 10 and 30 min, respectively, after activity (Fig. 2). The decrease in the blood lactate is shown to be a poor estimator of total recovery. An estimated 80–90% of the total energy utilized during initial vigorous activity comes from anaerobic sources. Because of its low thermal dependence, anaerobiosis permits high levels of activity in lizards at all body temperatures without requiring high levels of aerobic resting metabolism.

Evolution of gonadotropin structure and function.

Publisher Summary It has long been recognized that the control of gonadal function by pituitary hormones (gonadotropins) is a general feature of vertebrate reproductive physiology. Numerous studies on the hormones of eutherian mammals have established the existence of two chemically distinct types of gonadotropin molecules in the pituitary-luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Recent biochemical studies on these molecules have revealed that each is a glycoprotein consisting of two chemically non-identical subunits, designated α and β. The physiological actions of the two gonadotropins in mammals are still subject to intensive investigation; however, it is agreed that FSH and LH have somewhat different roles in the regulation of gonadal function. This chapter describes a flow diagram of the protocol for the fractionation of pituitary hormones, with particular reference to gonadotropins and growth hormone (GH), from various nonmammalian species. The strong resemblance in chromatographic behavior among the fractions identified as FSH and LH from most nonmammalian species and their mammalian counterparts provided preliminary evidence of chemical similarity among the different species of FSH and of LH. Biochemical analyses on the six highly purified species of LH and FSH (chicken, turkey, alligator, snapping turtle, sea turtle, and bullfrog) yielded additional evidence for the homologies between mammalian and nonmammalian hormones.

The Role of Behavioral Thermoregulation in the Growth Energetics of the Toad, Bufo Boreas

Newly metamorphosed Western toads, Bufo boreas, are diurnally active and thermoregulate by the behavioral exploitation of available microenvironments including exposure to solar radiation. In the laboratory, toads thermoregulate by basking beneath incandescent lamps when placed within a photothermal gradient. Basking depends on feeding, and when food is withheld, individuals abandon the available heat source and assume lower body temperatures. The preferred temperature of feeding individuals is around 26—27 degrees C as determined from animals both in the laboratory and in nature; fasting animals remain between 15—20 degrees C. Growth of young toads with ad libitum food supply was measured at a variety of temperatures. Energy ingestion (appetite), linear growth, weight increase, and gross conversion efficiencies were all maximal at 27 degrees C and were nearly identical to that of toads allowed to thermoregulate in a photothermal gradient. Weight—specific energy ingestion, weight—specific growth, and gross conversion efficiency decreased with age. Survivorship of starved toads in a thermal gradient was prolonged over that of toads maintained at a constant 27 degrees C. Metabolism increased with body temperature over the range 10—33 degrees C. We have concluded that this terrestrial ectotherm has evolved a behavioral thermoregulatory mechanism which maximizes growth and economic utilization of energy. The diurnal behavior patterns of small toads (compared with the more strictly nocturnal adults) may have evolved to maximize the growth rates of younger individuals, thus shortening their time to adult size.

Regulation of the Annual Testis Cycle by Photoperiod and Temperature in the Lizard Anolis Carolinensis

Males of the lizard Anolis carolinensis are potentially continuous breeders but an annual testicular cycle is induced by seasonal climatic changes. Both photoperiod and temperature are involved as proximate factors in the regulation of testes activity but the importance of each varies with the phase of the cycle. The regenerative phase of spermatogenesis between late fall and spring is controlled primarily by temperature, whereas, the maintenance and eventual regression of the testis in late summer depends primarily on photoperiod. The male reproductive system shows a discrete period of photosensitivity; in nature the lizard shows photo—sexual responses only between late June and mid—October. It is the reduction in day length below about 13.5 hours (the critical day length) in August that induces gonadal regression. The onset and rate of regression is slightly retarded by high temperatures (32 degrees C). The critical day length and temperature dependence of these photoperiodic responses gradually change during the 4—month photosensitive period; and some photorefractoriness is evident early in this period. The factors that determine the onset of the photosensitive phase are poorly understood; short day lengths appear to be important for the termination of this phase. Photoperiod may also affect appetite and growth but these photoperiodic responses differ in several respects from the photo—sexual responses. The two types of photoperiodic responses have different seasonalities, critical day lengths and temperature dependencies. Analysis of the photo—thermal relations of testis activity in A. carolinensis in light of phenological and experimental data for testis cycles in other species indicates that considerable interspecific variation exists in saurian physiological adaptations.

Evolutionary biology of prolactins and somatotropins II. Electrophoretic comparison of tetrapod somatotropins

Proteins of the adenohypophyses of representatives of all four classes of tetrapods were separated by polyacrylamide disc gel electrophoresis. These were tested for somatotropic activity in the toad, Bufo boreas, using increases in linear growth as the criterion for somatotropic activity. In all species examined, somatotropic activity was associated with prominent, slow-migrating adenohypophysial proteins. However, some prolactins also exhibit somatotropic activity. Ovine and bovine prolactins (NIH) had somatotropic activity that cannot be accounted for by growth hormone contamination: these prolactins are considerably less potent than mammalian growth hormone. In two turtles, Pseudemys scripta and Chrysemys picta, proteins which previously had been identified as prolactins were as potent as growth hormones with respect to their somatotropic activity in the Bufo. Thus, these chelonians contained two distinct and prominent adenohypophysial proteins with somatotropic activity. Prolactin from a toad (Bufo marinus) may also have somatotropic activity but is less potent than the homologous growth hormone. Our results clearly indicate that in all tetrapod species examined, prolactins and somatotropins are separate molecular entities, the prolactin always having the higher electrophoretic mobility. However, the results with the turtles, toad, and with purified ovine and bovine prolactin indicate that, as in primates, adenohypophysial proteins may have both prolactin and somatotropic activities. Human placental lactogen (=chorionic somatomammotropin) was ineffective in the toad growth test. Estimation of the concentration of prolactin and growth hormone in adenohyphphyses by densitometry revealed that mammals had relatively high growth hormone levels in comparison to prolactin. Avian and reptilian glands showed lower concentrations of both hormones, with no appreciable differences between them. In amphibians, several species had unusually high prolactin levels and relatively low growth hormone concentrations: growth hormone could not be detected in the salamander (Ambystoma tigrinum). Electrophoretic identification of somatotropins of these tetrapods should permit meaningful studies to be undertaken to elucidate the physiology of this hormone in nonmammalian species.

Environmental physiology of reptilian breeding cycles: Role of temperature

Phenological data indicate that most reptiles, even in tropical regions, exhibit some seasonality in reproductive activity; in most temperate species, a single, relatively brief breeding period alternates with a period of sexual quiescence. In lizards, the most studied of the Reptilia, it appears that exogenous, climatic factors act to synchronize these cycles, although some species may also exhibit endogenous (circannual) rhythmicity. Rainfall may influence egg laying in some tropical lizards. Photoperiodism probably also affects certain aspects of the gonadal cycle in a few species, but many reports of photoperiodism have been questioned on the basis of inadequate experimental design. Of all the environmental factors, temperature appears to be the single most important and widespread of the timing cues for saurian reproduction. Available data on the basic reproductive endocrinology in reptiles are reviewed and it is suggested that reptiles may possess only a single (FSH-like) gonadotropin (GTH). Studies on the mechanisms by which changes in temperature generate seasonal reproductive cycles indicate that several loci may be involved. They may act at the level of the central nervous system in relation to GTH-releasing factors and, likewise, may modify the response of the CNS to photoperiodic changes. The physiological responsiveness of the pituitary gonadotropes per se may be altered by temperature but this hypothesis is untested. The possibility that the peripheral metabolism (e.g., half-life) of GTH may also vary with temperature has also not been tested. In both sexes, the peripheral target tissues (gonads) are highly temperature sensitive. More importantly, gametogenesis and steroidogenesis may be differentially affected by changes in temperature; their relative temperature sensitivities vary among species. These two processes also show different sensitivities to the level of circulating GTH.

Serum gonadotropins and steroids associated with breeding activities in the green sea turtle Chelonia mydas. I. Captive animals.

Abstract Circulating levels of gonadotropins (FSH and LH) and several sex steroids were studied in a captive colony of green sea turtles, Chelonia mydas , in association with the annual reproductive cycles in 3 years. Limited data for prebreeding and copulating males indicated that gonadotropins remained low and that androgen levels were lower during copulation than in the prebreeding seson. Females showed consistent hormonal profiles from year to year. All steroids were reatively low and increased gradually between the prebreeding and mating season. Estrogen dropped while testosterone peaked in the season of maximal sexual receptivity and both steroids then remained relatively low througout the remainder of the nesting season. In contrast, progesterone (PRO) continued to rise progressively during the prebreeding and mating season and up to the time of nesting. Both gonadotropins were low during the prebreeding season, but only LH rose during the mating season. FSH showed a pronounced but relatively brief “surge” at the time of nesting (oviposition), while LH and PRO were only slightly elevated and variable at this time. FSH levels fell rapidly (with a half-life of 12–21 min) immediately after the completion of laying and remained at baseline levels until the next nesting (in about 11 days). In contrast, simultaneous surges in LH and PRO began between 6 and 12 hr after nesting and lasted for about 24 hr; these hormones then returned to low baseline levels until the next nesting-ovulatory cycle. These internesting peaks in LH and PRO, which were also occasionally observed in mating animals shortly before the start of nesting, were highly correlated with the timing of ovulation. These data are compared with information on the hormonal profiles of the avian ovulatory cycle and seasonal breeding cycles of other turtles. The implications of the relationships among endogenous hormones in the green turtle are discussed in relation to the hypothalamic control of the pituitary and the physiological actions of the gonadotropins and steroids. In particular, data suggest that the secretion of FSH and LH may be regulated independently; and, contrary to existing physiological data, these two gonadotropins may have distinctive roles in ovarian regulation that differ from those suggested by hormone therapy studies.

Changes in hormone levels in relation to breeding behavior in male bullfrogs (Rana catesbeiana) at the individual and population levels

Fluctuations in plasma androgen (testosterone and 5 alpha-dihydrotestosterone), corticosterone (B), and luteinizing hormone (LH) of male bullfrogs in central California were measured during the spring mating season. Androgen and LH levels generally increased in the population prior to the initiation of chorusing and establishment of territories, whereas plasma B peaked in a 2-week period at the start of heavy chorusing; this coincided with a transitory, 1 week decline in androgen. Individual males showed fluctuations in plasma androgen and LH levels throughout the breeding season, often within 1 day, but there was no clear correlation between changes in the two hormones. No time of day effect was apparent on any of the hormones. B and androgen levels were significantly but weakly correlated, (r = 0.35) but LH and androgen were not. Although the seasonal trend of increasing androgen corresponded with the start of intense chorusing (and presumably sexual activity) by the population, behavior of individuals and their circulating androgen levels did not correlate. After acquiring territories, males showed no overall trend of increased plasma androgen. Moreover, males that showed no vocal or territorial behavior had significantly higher androgen and lower B levels than calling males. Increased B levels suggest that territorial behavior and especially direct agonistic encounters represent stresses that could have an inhibitory effect on androgen secretion.

Annual cycles in levels of pituitary and plasma gonadotropin, gonadal steroids, and thyroid activity in the Chinese cobra (Naja naja)

Abstract Adult Chinese cobras, Naja naja , were purchased monthly from a snake dealer in Hong Kong (6–8 animals per month for each sex, total of 290 snakes) and sacrificed after 1 day of acclimation in the laboratory. Pituitary and plasma gonadotropin (Gn), plasma thyroxine (T 4 ), sex steroids in plasma and testes, and ovarian Gn binding were examined in relation to the annual reproductive cycle. Changes in plasma Gn did not always correspond to pituitary Gn content, and a marked annual cycle in plasma T 4 was evident, despite a lack of change in thyroid weight or epithelial cell height. Although Gn levels were generally elevated during the reproductive season, gonadal activity did not always show the expected relationship to plasma Gn levels. In males, a winter peak in plasma Gn occurred despite low pituitary content; this increase in the plasma coincided with the initiation of testis growth. However, in summer, the testes showed full regression in weight, spermatogenesis, and testosterone (T) secretion, almost 2 months before a decline in plasma Gn was evident. Increased plasma T 4 might be implicated in this apparent dissociation between testis function and circulating Gn levels. Both plasma and testicular T increased only at the time of maximum testicular weight; they did not show the expected increase at the time when spermatogenesis was initiated. In females, plasma Gn showed a bimodal annual cycle but ovarian activity was compressed into a single relatively brief period of about 2 months in spring. A pronounced winter rise in plasma Gn, despite low pituitary Gn levels, was not reflected in detectable ovarian changes. Increases in plasma estradiol (E) and T occurred during the vitellogenic phase; plasma E dropped rapidly after ovulation, but T remained partially elevated until after oviposition. In contrast, plasma progesterone (PRO) did not rise until after ovulation; it then remained fully elevated until after oviposition. Thus, both T and PRO levels along with high Gn binding indicated that the corpora lutea remain functional until oviposition occurs. Steroid levels were reduced to baseline values in animals with atretic follicles. Plasma and pituitary Gn rose during the vitellogenic period and remained elevated until oviposition. Unlike the male, plasma T 4 levels were maximal during the active reproductive period (vitellogenesis and ovulation). The ability of ovarian tissues to bind radiolabeled gonadotropin was lowest in the previtellogenic stages, especially when plasma Gn was highest. Corpora atretica also had significantly reduced binding. We postulate that the ovarian cycle of this snake represents a progressive differentiation in the functional state of the ovary which is induced by relatively constant high levels of a single gonadotropin; thus, functional transitions in ovarian tissue rather than alterations in the nature or concentration of gonadotropins in the circulation are responsible for the changing pattern of ovarian steroid secretion.