Cardioprotective effect of L-glutamate in obese type 2 diabetic Zucker fatty rats.

1. Because diabetic hearts have an increased threshold for cardioprotection by ischaemic preconditioning (IPC), we hypothesized that protection by L-glutamate during reperfusion is restricted in Type 2 diabetic hearts. Previously, we found that L-glutamate-mediated postischaemic cardioprotection mimics IPC. 2. Rat hearts were studied in a Langendorff preparation perfused with Krebs'-Henseleit solution and subjected to 40 min global no-flow ischaemia, followed by 120 min reperfusion. L-Glutamate (0, 15 and 30 mmol/L) was added to the perfusate during reperfusion of hearts from non-diabetic (Wistar-Kyoto) and diabetic (Zucker diabetic fatty (ZDF)) rats, studied at 16 weeks of age. The infarct size (IS)/area-at-risk (AAR) ratio was the primary end-point. Expression of L-glutamate excitatory amino acid transporter (EAAT) 1 (mitochondrial) and EAAT3 (sarcolemmal) was determined by quantitative polymerase chain reaction and immunoblotting. 3. The ISS/AAR ratio did not differ between control hearts from Wistar-Kyoto and ZDF rats (0.52 ± 0.03 and 0.51 ± 0.04, respectively; P = 0.90). L-Glutamate (15 mmol/L) significantly reduced the IS/AAR ratio in non-diabetic hearts, but not in diabetic hearts, compared with their respective controls. The higher concentration of L-glutamate (30 mmol/L) reduced infarct size in diabetic hearts to the same degree as in non-diabetic hearts (IS/AAR 0.35 ± 0.03 (P = 0.002) and 0.34 ± 0.03 (P = 0.004), respectively). The mitochondrial L-glutamate transporter EAAT1 was downregulated in hearts from ZDF rats at both the mRNA and protein levels (P < 0.0005 and P < 0.0001, respectively). However, there was no change in EAAT3 expression at the protein level. Myocardial L-glutamate content was increased by 43% in diabetic hearts (P < 0.0001). 4. Hearts from obese diabetic rats have an elevated threshold for metabolic postischaemic cardioprotection by L-glutamate. These findings may reflect underlying mechanisms of inherent resistance against additional cardioprotection in the diabetic heart.

Serum levels of reproductive steroid hormones in captive sand tiger sharks, Carcharias taurus (Rafinesque), and comments on their relation to sexual conflicts.

Levels of reproductively-related steroids were determined in captive male sand tiger sharks, Carcharias taurus, maintained at two institutions: SeaWorld Adventure Park Orlando and the National Aquarium in Baltimore. Sexual conflicts were absent at the former, but were documented at the latter. Serum titers of 17beta-estradiol, progesterone, testosterone, and 5alpha-dihydrotestosterone were determined via radioimmunoassay in adult male sharks from 1988 to 2000. Sampling overlap between the two institutions occurred for 3 months of the year, but steroid concentrations were compared only for April due to the occurrence of sexual conflicts in the sharks at the National Aquarium in Baltimore in that month. For April, testosterone and dihydrotestosterone were significantly higher in the SeaWorld males, and progesterone was significantly higher in the National Aquarium in Baltimore males, while estradiol was not significantly different. Steroid levels were also determined from serial samples taken monthly over 17 months from three male sharks and one female shark at the National Aquarium in Baltimore in 2001-2002 and were compared with corresponding observed sexual conflicts. The steroid levels obtained showed distinct annual hormonal cycles in the male sharks and corroborated a biennial cycle for the single serially-sampled female shark. Furthermore, the steroid levels for individual males correlated with sexual conflicts as well as their position within the male dominance hierarchy. As this species is depleted in some regions globally, insight into the steroid profile of mature sand tiger sharks is important for a greater understanding of the relationship between their reproductive physiology and behavior, and may aid in captive management and reproduction.

Estrous state influences on investigative, aggressive, and tail flicking behavior in captive female Asian elephants.

Females of species that live in matrilineal hierarchies may compete for temporally limited resources, yet maintain social harmony to facilitate cohesion. The relative degree of aggressive and nonaggressive interactions may depend on the reproductive condition of sender and receiver. Individuals can benefit by clearly signaling and detecting reproductive condition. Asian elephants (Elephas maximus) live in social matrilineal herds. Females have long estrous cycles (14-16 weeks) composed of luteal (8-12 weeks) and follicular (4-8 weeks) phases. In this study, we observed the behavior of four captive Asian elephant females during multiple estrous cycles over 2 years. We evaluated whether investigative, aggressive, and tail flicking behaviors were related to reproductive condition. Investigative trunk tip contacts showed no distinct pattern by senders, but were more prevalent toward female elephants that were in their follicular compared with their luteal phase. The genital area was the most frequently contacted region and may release reproductively related chemosignals. Aggression did not differ significantly with estrus; however, rates of aggression were elevated when senders were approaching ovulation and receivers were in the luteal phase. Females in the follicular phase may honestly advertise their condition. Contacts by conspecifics may serve to assess condition and reduce aggression. A behavior termed "tail flicking" was performed mainly during the mid-follicular phase when estrogen and luteinizing hormone levels are known to spike. Tail flicking may disperse chemical signals in urine or mucus as well as act as a tonic signal that could provide a means of anticipating forthcoming ovulation by elephants and also for human observers and caretakers.

Sex, seasonal, and stress-related variations in elasmobranch corticosterone concentrations.

Serum corticosterone was previously studied in numerous elasmobranch fishes (sharks, skates and rays), but the role of this steroid, widespread throughout many taxa, has yet to be defined. The goal of this study was to test whether corticosterone varied in response to acute and chronic capture stress, and across the reproductive cycle in the bonnethead shark, Sphyrna tiburo, and Atlantic stingray, Dasyatis sabina. Serum corticosterone in S. tiburo increased following capture and again 24 h post-capture, possibly caused by interference with 1alpha-hydroxycorticosterone, the primary stress hormone in elasmobranchs. Higher serum concentrations in males compared to females were observed in both species. Variations in corticosterone also occurred during the reproductive cycle in both species. Consistent with other taxa, elevations in male bonnethead sharks and stingrays coincided with peak testicular development and mating. Elevations in female bonnethead sharks occurred from the time of mating through sperm storage into early gestation. In contrast, corticosterone levels in female stingrays were low during their protracted mating season, but elevated through late gestation and parturition. These results indicate that corticosterone has a limited role, if any, in acute and chronic stress associated with capture in S. tiburo, but likely has physiological functions associated with its glucocorticoid properties across the reproductive cycle of both species.

Insect pheromones and precursors in female African elephant urine.

Using automated solid-phase dynamic extraction and gas chromatography-mass spectrometry, our search for urinary chemical signals from ovulatory female African elephants (Loxodonta africana) has revealed the bark beetle aggregation pheromones frontalin, exo-brevicomin, and endo-brevicomin, as well as their precursors and the aphid alarm pheromones (E,E)-alpha-farnesene and (E)-beta-farnesene. Enantiomeric ratios for brevicomins have been determined. Prior discovery of common insect/elephant pheromones in Asian elephants, namely, (Z)-7-dodecenyl acetate and frontalin, suggests that the present findings may yield valuable insights into chemical communication among African elephants.

Brain of the African elephant (Loxodonta africana): neuroanatomy from magnetic resonance images.

We acquired magnetic resonance images of the brain of an adult African elephant, Loxodonta africana, in the axial and parasagittal planes and produced anatomically labeled images. We quantified the volume of the whole brain (3,886.7 cm3) and of the neocortical and cerebellar gray and white matter. The white matter-to-gray matter ratio in the elephant neocortex and cerebellum is in keeping with that expected for a brain of this size. The ratio of neocortical gray matter volume to corpus callosum cross-sectional area is similar in the elephant and human brains (108 and 93.7, respectively), emphasizing the difference between terrestrial mammals and cetaceans, which have a very small corpus callosum relative to the volume of neocortical gray matter (ratio of 181-287 in our sample). Finally, the elephant has an unusually large and convoluted hippocampus compared to primates and especially to cetaceans. This may be related to the extremely long social and chemical memory of elephants.

Elephant albumin: a multipurpose pheromone shuttle.

(Z)-7-dodecenyl acetate (Z7-12:Ac) is present in the urine of female Asian elephants (Elephas maximus) approaching ovulation and functions as a female-to-male sex pheromone. Here we show that a significant fraction of the pheromone in the urine is bound to a protein, elephant serum albumin (ESA), and provide evidence for key physiological functions of urinary ESA. Our biochemical and behavioral experiments suggest a three-fold role of ESA in pheromone signaling: (1) transporting Z7-12:Ac from serum into urine; (2) extending the presence of the pheromone in the environment without hampering detection; and (3) targeting pheromone delivery to chemosensory organs through localized release of the ligand induced by a pH change. The exploitation of albumin in pheromone transport clearly distinguishes the elephant from other mammals studied, and complements the uniqueness of elephant anatomy, physiology, and behavior.

Rapid non-genomic effects of aldosterone on rodent vascular function.

The main role of aldosterone is to maintain body sodium homeostasis by promoting salt reabsorption in the collecting ducts of the kidney. In the cardiovascular system, aldosterone may be harmful in a number of disease states by inducing fibrosis and vascular dysfunction. The present review describes novel results from several laboratories, which show that aldosterone also has beneficial effects in the cardiovascular system by stimulating the production of nitric oxide (NO) from the endothelium. The effect of aldosterone is seen within minutes, and is not inhibited by blockers of gene transcription, thus pointing to a non-genomic mechanism. Furthermore, this potentially beneficial effect is observed at low physiological concentrations of aldosterone (0.1-10 pm). The effect is mediated by the classical mineralocorticoid receptor, and it involves heat shock protein 90, phosphatidylinositol (PI)-3 kinase, protein kinase B, endothelial nitric oxide synthase, and liberation of NO. It is proposed that in healthy individuals with a functioning NO system, the detrimental effects of aldosterone on cardiovascular function are balanced by activation of the potentially beneficial effect of NO. However, in situations with endothelial dysfunction, such as congestive heart failure and hypertension, the negative effects of aldosterone are unopposed and inhibition of aldosterone is warranted.

Maternal serum and yolk hormone concentrations in the placental viviparous bonnethead shark, Sphyrna tiburo.

Among vertebrates, maternal transfer of hormones to offspring has been studied extensively in mammals (placental transfer) and more recently in oviparous birds and reptiles (yolk transfer). The placental viviparous bonnethead shark, Sphyrna tiburo, allows the investigation of both yolk and placental hormone transfers in a single organism. In this species, yolk provides nutrition for the first half of embryonic development and placental transfer provides the second half. As sex determination is complete prior to development of placental connections, it was postulated that yolk hormones would have a prominent role in embryonic regulation. The goal of the current study was to determine serum and yolk hormone concentrations during five reproductive stages, from pre-ovulatory through pre-implantation (pre-placental) stages. Radioimmunoassay was used to determine 17beta-estradiol, progesterone, and testosterone concentrations in both serum and yolk. When yolk and serum concentrations were compared, the yolk had significantly higher concentrations of both estradiol and progesterone during post-ovulation and early pregnancy. Yolk concentrations of testosterone were significantly less than serum at pre-ovulation, but there were no differences after that stage. When yolk concentrations were compared between stages, significantly higher concentrations of estradiol were present in ovulatory, post-ovulatory, and pre-implantation stages, while progesterone was significantly higher in post-ovulatory, early pregnancy, and pre-implantation stages and testosterone was higher in pre-ovulation. Most of these results are consistent with the published findings in birds and reptiles. Further, in the bonnethead shark, they suggest that yolk transfer of hormones is adequate for sexual differentiation in embryonic development and that estradiol probably has a significant developmental role.

A tangerine-scented social odour in a monogamous seabird.

Social odours, conspecific chemical signals, have been demonstrated in every class of vertebrate except birds. The apparent absence is surprising, as every bird examined has a functional olfactory system and many produce odours. The crested auklet (Aethia cristatella), a monogamous seabird, exhibits a distinctive tangerine-like scent closely associated with courtship. Using T-maze experiments, we tested whether auklets preferred conspecific odours and whether they distinguished between different types of scent, two prerequisites of chemical communication. Crested auklets exhibited: (i) an attraction to conspecific feather odour; (ii) a preference for two chemical components of feather scent (cis-4-decenal and octanal), which we identified as seasonally elevated; and (iii) differential responses to odours, as indicated by a preference for auklet odour, an aversion to mammalian musk, but no significant response to banana essence (amyl acetate). Our results suggest that crested auklets detect plumage odour and preferentially orientate towards this stimulus. The striking and well-described courtship display that involves the scented neck region, the 'ruff sniff', provides a conspicuous behavioural mechanism for odour transmission and the potential for scent assessment during sexual selection. Although the importance and full social function of chemical signals are just beginning to be understood in birds, including crested auklets, social odours promise to reveal a largely unexplored and possibly widespread means of avian communication.

Frontalin: a chemical message of musth in Asian elephants (Elephas maximus).

Musth is an important male phenomenon affecting many aspects of elephant society including reproduction. During musth, the temporal gland secretions (as well as the urine and breath) of adult male Asian elephants (Elephas maximus) discharge a variety of malodorous compounds together with the bicyclic ketal, frontalin. In contrast, teenage male elephants in musth release a sweet-smelling exudate from their facial temporal gland. We recently demonstrated that the concentration of frontalin becomes increasingly evident as male elephants mature. In the present study, we demonstrate that behaviors exhibited towards frontalin are consistent and dependent on the sex, developmental stage and physiological status of the responding conspecific individual. To examine whether frontalin functions as a chemical signal, perhaps even a pheromone, we bioassayed older and younger adult males, and luteal- and follicular-phase and pregnant females for their chemosensory and behavioral responses to frontalin. Adult males were mostly indifferent to frontalin, whereas subadult males were highly reactive, often exhibiting repulsion or avoidance. Female chemosensory responses to frontalin varied with hormonal state. Females in the luteal phase demonstrated low frequencies of responses, whereas pregnant females responded significantly more frequently, with varied types of responses including those to the palatal pits. Females in the follicular phase were the most responsive and often demonstrated mating-related behaviors subsequent to high chemosensory responses to frontalin. Our evidence strongly suggests that frontalin, a well-studied pheromone in insects, also functions as a pheromone in the Asian elephant: it exhibits all of the determinants that define a pheromone and evidently conveys some of the messages underlying the phenomenon of musth.

Olfactory adventures of elephantine pheromones.

Understanding the linkage between behaviour of mammals in their natural environment and the molecular basis of their sensory modalities presents challenges to biologists. Our olfactory investigations that involve the largest extant land mammal, the elephant, offer some clues of how these events mesh in sequence. Proboscideans have developed a sophisticatedly organized society and they rank with primates and cetaceans with respect to cognitive abilities. Our studies of discrete, quantifiable pheromone-elicited behaviours demonstrate that Asian elephants utilize their olfactory senses during fundamental, life-strategy decisions, including mate choice, female bonding and male hierarchical sorting. How biologically relevant odorants traverse mucous interfaces to interact with cognate odorant receptors remains a basic question in vertebrate olfaction. We have partially tracked the molecular odour reception trail of behaviourally distinct pheromones, ( Z )-7-dodecenyl acetate and frontalin (1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane), using approaches developed for insect studies and taking advantage of the extensive, highly mucoidal olfactory and vomeronasal systems that permit detailed investigations of pheromone-binding proteins. We have combined studies of quantifiable responses and behaviours with biochemical and biophysical investigations of the properties of protein-ligand complexes, their sequential pathways and associated protein-ligand fluxes. In the delineation of these sequential integrations of behavioural, biochemical and molecular events, we have discovered novel spatial and temporal adaptations in both the main olfactory and vomeronasal systems.

Molecular and functional characterization of an odorant binding protein of the Asian elephant, Elephas maximus: implications for the role of lipocalins in mammalian olfaction.

The sex pheromone present in the pre-ovulatory urine of female Asian elephants is the simple lipid (Z)-7-dodecen-1-yl acetate (Z7-12:Ac). Using radiolabeled probes, we have identified a pheromone binding protein that is abundant in the mucus of the trunk; this protein is homologous to a class of lipocalins known as odorant binding proteins (OBPs). To test five previously proposed roles for the OBP in chemosensory perception, we determined the equilibrium dissociation constant of the OBP-pheromone complex, as well as the association and dissociation rates. Using a mathematical model in conjunction with experimental data, we suggest that the binding and release of the pheromone by the OBP are too slow for the OBP to function in transporting the pheromone through the mucus that covers the olfactory sensory epithelium. Our data indicate that the elephant OBP only modestly increases the solubility of the pheromone in the mucus. Our results are most consistent with the notion that elephant OBP functions as a scavenger of the pheromone and possibly other ligands, including odorants. In light of these findings, and published results for other mammalian OBP-ligand complexes, a general model for the role of OBPs in mammalian olfaction is proposed. Moreover, the potential implications of these findings for interaction of Z7-12:Ac with insect antennal proteins are discussed.

Chemosignalling of musth by individual wild African elephants (Loxodonta africana): implications for conservation and management.

Elephants have extraordinary olfactory receptive equipment, yet this sensory system has been only minimally investigated in wild elephants. We present an in-depth study of urinary chemical signals emitted by individual, behaviourally characterized, wild male African elephants, investigating whether these compounds were the same, accentuated, or diminished in comparison with captive individuals. Remarkably, most emitted chemicals were similar in captive and wild elephants with an exception traced to drought-induced dietary cyanates among wild males. We observed developmental changes predominated by the transition from acids and esters emitted by young males to alcohols and ketones released by older males. We determined that the ketones (2-butanone, acetone and 2-pentanone, and 2-nonanone) were considerably elevated during early musth, musth and late musth, respectively, suggesting that males communicate their condition via these compounds. The similarity to compounds released during musth by Asian male elephants that evoke conspecific bioresponses suggests the existence of species-free 'musth' signals. Our innovative techniques, which allow the recognition of precise sexual and musth states of individual elephants, can be helpful to managers of both wild and captive elephants. Such sampling may allow the more accurate categorization of the social and reproductive status of individual male elephants.

Mellifluous matures to malodorous in musth.

Male Asian elephants in musth--an annual period of heightened sexual activity and intensified aggression--broadcast odoriferous, behaviourally influential messages from secretions of the temporal gland. From our observations in the wild, together with instantaneous chemical sampling and captive-elephant playback experiments, we have discovered that young, socially immature males in musth signal their naivety by releasing honey-like odours to avoid conflict with adult males, whereas older musth males broadcast malodorous combinations to deter young males, facilitating the smooth functioning of male society. As elephant--human conflicts can upset this equilibrium, chemically modulating male behaviour may be one way to help the conservation of wild elephants.

Source and cyclic release pattern of (Z)-7-dodecenyl acetate, the pre-ovulatory pheromone of the female Asian elephant.

Female Asian elephants (Elephas maximus) release a pre-ovulatory urinary pheromone, (Z)-7-dodecenyl acetate (Z7--12:Ac), to signal males of their readiness to mate. Z7--12:Ac is quantitatively elevated during the follicular stage of estrus, reaching maximum concentrations just prior to ovulation, as demonstrated by two complementary headspace techniques: (i) evacuated canister capture followed by cryogenic trapping; (ii) solid phase microextraction (SPME), used prior to gas chromatography/ mass spectrometry (GC/MS). These patterns were coincident with observed male behaviors and were consistent with biochemical and binding properties of the active ligand, including optimal binding pH. To release maximum amounts of Z7--12:Ac for quantitation, serum and urine samples from three mature female Asian elephants in their luteal and follicular stages of several estrous cycles were subjected to heat and pH changes and were then treated with protease prior to SPME-GC/MS analyses. When the post-luteal serum progesterone concentrations declined to baseline levels, Z7--12:Ac became detectable in the female urine. Throughout the follicular stage pheromone concentrations increased linearly with no apparent relationship to the two serum luteinizing hormone peaks. Pre-ovulatory urine also contained related compounds, including (Z)-7--12-dodecenol. The relative amount of this alcohol increased relative to acetate during long-term storage, with a proportional reduction in bioactivity. Z7--12:Ac was not detected in mucus samples from the urogenital tract. A potential precursor of Z7--12:Ac was identified in liver homogenates from female elephants in the follicular stage.

Characterization of interdigital glands in the Asian elephant (Elephas maximus).

In the Asian elephant, wetness akin to perspiration is commonly observed on the cuticles and interdigital areas of the feet; this observation has lead to speculation regarding the existence of an interdigital gland. Our goal was to search for interdigital glands and characterise them morphologically, histochemically, and immunohistochemically. Necropsy samples of interdigital areas from two Asian elephants were obtained. Multiple sections were fixed and processed routinely, then stained with hematoxylin/eosin and differential mucin stains. Immunohistochemistry was also performed for cytokeratins 8 and 10. Interdigital glands resembling human eccrine glands were detected deep within the reticular dermis. Histochemical staining indicated neutral mucopolysaccharides and nonsulphated acid mucopolysaccharides in glandular secretions, and the glandular epithelium also showed immunoreactivity to cytokeratins 8 and 10. Both the histochemical and immunohistochemical staining patterns are analogous to human eccrine structures. This study shows with certainty that Asian elephants possess sweat glands as they are defined histologically.

Annual cycles of steroid hormone production, gonad development, and reproductive behavior in the Atlantic stingray.

The mating season of the Atlantic stingray (Dasyatis sabina), which begins in August and continues through April, is the longest documented for any elasmobranch fish. Despite this protracted mating period, female stingrays ovulate synchronously at the end of the mating season and there is no evidence for sperm storage by females. Thus, the proximate causal factors and ultimate function of this extended preovulatory mating are unknown. Annual cycles of the gonadal steroids testosterone (T), dihydrotestosterone (DHT), 17beta-estradiol (E2), and progesterone (P4) were measured for 26 months in a wild estuarine population of Atlantic stingrays to test for associations with their reproductive biology, gametogenesis, and sexual behavior. Serum androgen levels in males showed four phases within an annual cycle: (1) androgen suppression between reproductive seasons (April-July), (2) primary androgen increase during the onset of spermatocyte development (August-October), (3) androgen decrease following maximum testis growth and spermatocyte development (November-December), and (4) secondary androgen increase during the peak of sperm maturation (January-March). Increases in male E2 and P4 were correlated with spermatocyte/spermatocyst formation, maximum testis weight, and the primary (but not secondary) androgen surge. We propose that the production of male androgens across the full seven-month preovulatory mating period promotes their aggressive reproductive behavior and drives the protracted mating season of this species. In females, serum T and DHT showed relatively brief increases near ovulation, whereas E2 and P4 showed brief increases near both ovulation and parturition. The increase in female androgens near ovulation may increase female aggression when they are impregnable by courting males and enhance their choice of mates. This estuary sample population shows higher absolute steroid levels and distinct differences in temporal cycles compared to another Florida fresh water lake population, but the cause and significance of these differences are unknown. Experiments are needed to confirm that the aggressive and protracted mating behavior is the result of prolonged male androgen production and to determine whether the sustained preovulatory mating serves some function related to female reproduction.

Physiological correlates of musth: lipid metabolites and chemical composition of exudates.

Physiological changes related to lipid metabolism, behavior, and chemicals released in body exudates were studied during musth in the Asian elephant, Elephas maximus, as a case study. During musth, changes in serum testosterone and triglyceride concentrations followed similar patterns, with the former increasing sooner than the latter. Deviant behavior increased during changing androgen levels. The observed high concentrations of testosterone were positively and significantly correlated with increased triglycerides. Lipase activity elevated significantly immediately before and after musth. Blood pH increased significantly in alkalinity. Urine and temporal gland secretions released variable amounts of compounds, some of which may be chemical signals. During musth, temporal gland and urinary exudates demonstrated increased acetone and other ketones indicative of lipid metabolic alterations. Large quantities of nonmethane hydrocarbons, especially 2-butanone, were released from the seemingly dry orifice of the temporal gland before the start of overt musth and before maximum blood elevations were observed; isoprene release was similar. However, maximal acetone levels occurred simultaneously in blood, temporal gland secretions, and urine. Metabolically, musth is a series of interwoven, changing stages of increasing and decreasing hormones and lipid-related constituents. Released chemicals can be quantitatively related to these internal physiological events; some observed behaviors appear to result from altered chemical signals.

Serum T(4) and serum T(3) concentrations in immature captive whitetip reef sharks, Triaenodon obesus.

Serum T(3) (3,5,3' triiodothyronine) and serum T(4) (thyroxine) concentrations were repetitively assayed by radioimmunoassay over a three-year period in two male and two female immature captive whitetip reef sharks, Triaenodon obesus. These sharks were maintained at the Waikiki Aquarium, Honolulu, Hawaii, in an open system holding pool receiving 568 liters per minute of water from a saltwater well with an iodide concentration of 0.076 mg/liter. No significant male-female difference was observed for either serum T(3) or serum T(4). No seasonal pattern of serum T(3) was detected (P = 0.07). Serum T(3) concentrations ranged (mean +/- SEM) from 0. 52 to 0.83 ng/mL (0.67 +/- 0.01; n = 64). A significant seasonal difference was observed for serum T(4) (P < 0.001). Serum T(4) concentration was higher in winter (October-January) with a mean (range +/- SEM) of 6.58 ng/mL (1.48-8.77 +/- 0.35; n = 24) and lower in summer (May-August) with a mean of 3.62 ng/mL (1.34-5.71 +/- 0. 22; n = 24). The thyroid hormone T(4) has a seasonal rhythm even in immature sharks and may have an important role in physiology. J. Exp. Zool. 284:500-504, 1999.