The effect of elevated levels of thyroxine on the aerobic capacity of locomotor muscles of the tufted duck,Aythya fuligula

Following extended periods of relative inactivity, or prior to migration, birds are able to increase the aerobic capacity of their locomotory muscles. Thyroid hormones may influence this process. A preliminary study was undertaken to assess the ability of elevated levels of thyroxine to increase the aerobic capacity of the locomotory and cardiac muscles of adult tufted ducks. Administration of thyroxine in the food for 8 weeks had little effect on body mass or on the masses of the pectoralis, semitendinosus and iliofibularis muscles, although there were increases in resting oxygen consumption and in the mass of the cardiac ventricles. The maximum activity of the aerobic enzyme, citrate synthase, was significantly greater in the left ventricle, liver, and iliofibularis muscles (P<0.005) of treated birds. However, while there was clearly no difference in activity in the semimembranosus leg muscle, that of the pectoralis was not quite significant (P=0.078). It is concluded that addition of supra-physiological levels of exogenous thyroxine may induce a differential increase in the maximum activity of citrate synthase in the locomotor muscles of the tufted duck, which is correlated with the fibre type composition of these muscles. These results are consistent with those found in studies on rats, with slow oxidative fibres being the most sensitive, and fast glycolytic fibres the least sensitive, to thyroxine treatment.Abbreviations BM body mass - CS citrate synthase - CYTOX cytochrome c oxidase - FG last glycolytic - FOG fast oxydative glycolytic - VO₂ oxygen consumption - SO slow oxidative - T₄ thyroxine - T₃ triiodothyronine     

Immunocytochemical investigation of the hypothalamo-neurohypophysial system in birds

Summary The results of an immunohistochemical investigation of the hypothalamo-neurohypophysial system in several species of birds have shown that: (1) mesotocin and vasotocin are synthesized in separate neurons; (2) in all species investigated the distribution of mesotocinergic and vasotocinergic perikarya follows a common pattern; (3) the external zone of the avian anterior median eminence contains exclusively vasotocinergic nerve fibers, originating in supraoptic and ventral paraventricular regions; (4) the distribution of immunoreactive elements in the neural lobe shows a definite species-dependent pattern.     

两种鹪莺的种间生态位关系研究

本文研究了褐头鹪莺（Ｐｒｉｎｉａ ｓｕｂｆｌａｖａ）和黄腹鹪莺（Ｐ．ｆｌａｖｉｖｅｎｔｒｉｓ）在取食高度、取食基层和取食方法３个生态维度上的生态位宽度和生态位重叠以及它们与复合种群密度的关系。结果表明,两种鹪莺的现实生态位有较大的重叠,且种群密度变化时,生态位宽度和生态位重叠发生了显著移动,褐头鹪莺与黄腹鹪莺之 间存在着竞争;它们并没有采取生态位宽度来回避竞争,而是相反地各自扩展生态位宽度来减少重     

Bounded hybrid superiority in an avian hybrid zone: effects of mate, diet, and habitat choice

There has been considerable debate in the study of hybrid zones as to whether hybrids may be superior to parental types within the area of contact (bounded hybrid superiority). In birds, naturally occurring hybridization is relatively common, and hybridization within this group always involves mate choice. If hybrids are superior, females choosing heterospecific mates should be expected to show higher fitness under the conditions prevalent in the hybrid zone. Hybrid superiority under these circumstances would reduce reinforcement and thereby help to maintain the hybrid zone. To examine this issue, we studied reproductive performances of hybrids and parental species of gulls (Larus occidentalis and Larus glaucescens) at two colonies within a linear hybrid zone along the west coast of the United States. This hybrid zone contains predominantly gulls of intermediate phenotype. Previous studies indicated that hybrids were superior to one or both parental types, but provided no data on possible mechanisms that underlie this hybrid superiority. Using a hybrid index designed specifically for these species, we identified to phenotype more than 300 individuals associated with nests, including both individual males and females within 73 pairs in the central portion of the hybrid zone and 74 pairs in the northern portion of the hybrid zone. There was little evidence of assortative mating, and what little there was resulted solely because of pairings within intergrades. In the central hybrid zone, females paired with hybrid males produced larger clutches and hatched and fledged more chicks compared with females paired to western gull males. This was a result of heavy predation on eggs in sand habitat, where male western gulls established territories. In contrast, many hybrid males established territories in vegetated cover that was less vulnerable to predation. In the northern part of the hybrid zone, clutch size did not differ among pair categories, however, there were differences in hatching and fledging success, with females paired to hybrid males showing better success compared to females paired to glaucous-winged gull males. Hybrids showed better hatching and fledging success in the north because hybrids are more like western gulls than glaucous-winged gulls in foraging behavior, taking a higher percentage of fish in their diet, which enhances chick growth and survival. This is believed to be the first documentation of bounded hybrid superiority that delineates the mechanisms that underlie hybrid superiority.     

Single cell responses from the optic tectum of the zebra finch (Taeniopygia guttata castanotis Gould)

Responses of neurons of the optic tectum, the prominent, highly laminated mesencephalic station of the tectofugal visual pathway in birds, to computer-generated and other visual stimuli were examined in zebra finches. Our study shows that the contralateral retina projects to the tectum in topographic order. The representation of the visual field is tilted against the horizon by 22°. The representation of the contralateral hemifield extends to the ipsilateral side by 15°. Most neurons have receptive fields with excitatory centres of different shapes and inhibitory surround. A new type of neuronal receptive field is described which has an excitatory centre and a surround which is movement sensitive and preferably excited by very small spots. The first type of neurons is mostly located in upper tectal layers, the latter only in deeper layers. Excitatory centre sizes increase with depth, and there is a tendency of smaller receptive fields in the foveal region. The representation of the frontal visual field does not show specializations which could be expected if it were used for fixation of grain during pecking. Our results are in accordance with previous behavioural experiments. Accepted: 30 April 1999     

Temporal genetic variation of mitochondrial DNA and the female effective population size of red drum (Sciaenops ocellatus) in the northern Gulf of Mexico

We studied genetic drift of mitochondrial DNA (mtDNA) haplotype frequencies in a natural population of red drum (Sciaenops ocellatus) from the northern Gulf of Mexico (Gulf). The amount of genetic drift observed across temporally adjacent year classes (1986–89) was used to estimate variance effective (female) population size (N_ef). N_ef was estimated to be 14 308 and the ratio of female effective size to adult female census size was approximately 0.004, which is among the lowest value reported for vertebrate animals. Low effective size relative to census size among red drum in the northern Gulf may result from yearly fluctuations in the number of breeding females, high variance in female reproductive success, or both. Despite low genetic effective size relative to census size, the genetic effective population size of red drum in the northern Gulf appears sufficiently large to preclude potentially deleterious effects of inbreeding.     

MORTALITY OF SEA OTTERS IN PRINCE WILLIAM SOUND FOLLOWING THE EXXON VALDEZ OIL SPILL

Abstract: This paper presents an estimate of the total number of sea otters that died as a direct consequence of the oil spill that occurred when the T/V Exxon Valdez grounded in Prince William Sound, Alaska on 24 March 1989. We compared sea otter counts conducted from small boats throughout the Sound during the summers of 1984 and 1985 to counts made after the spill during the summer of 1989. We used ratio estimators, corrected for sighting probability, to calculate otter densities and population estimates for portions of the Sound affected by the oil spill. We estimated the otter population in the portion of Prince William Sound affected by the oil was 6,546 at the time of the spill and that the post-spill population in the summer of 1989 was 3,898, yielding a loss estimate of approximately 2,650. Bootstrapping techniques were used to approximate confidence limits on the loss estimate of about 500–5,000 otters. The wide confidence limits are a result of the complex scheme required to estimate losses and limitations of the data. Despite the uncertainty of the loss estimate it is clear that a significant fraction of the otters in the spill zone survived. We observed otters persisting in relatively clean embayments throughout the oil spill zone suggesting that the highly convoluted coastline of Prince William Sound produced refuges that allowed some sea otters in the oil spill area to survive.     

Patterns of frugivore species richness and abundance in forest islands and in agricultural habitats at Los Tuxtlas,Mexico

Destruction and fragmentation of tropical rain forest result in a loss of species and of generating capacity of the ecosystem via animal vectors such as seed dispersal agents. To gather quantitative data regarding this ecological problem, birds and mammals were censused in 30 forest fragments, 15 agricultural islands representing five types of vegetation (coffee, cacao, citrus, pepper and mixed-crops) and in three pastures in Los Tuxtlas, southern Veracruz, Mexico. More than 6000 animals of 257 species were detected thus suggesting the existence of a rich species pool in the fragmented landscape. Frugivores accounted for 60% of species, for 72% of individuals censured and for 85% of the total animal biomass recorded. Clusters of small forest fragments (<100 ha) were richer in species and individuals than clusters of large area (>100 ha) forest islands. Pastures were especially poor in forest birds and mammals. While the agricultural islands studied contributed to only 1% of the total area of vegetation sampled, they contained 58% of all species detected and 34% of all individual birds and mammals censured. Recaptures indicated inter-island movements of forest birds and mammals. Forty percent of the species were detected in forest habitats only, the rest were detected in forest and in agricultural habitats. Seeds of forest interior plants dispersed by birds and bats were detected in the agricultural habitats. The value of agricultural islands as landscape features providing some degree of biotic connectivity among fragmented animal populations is discussed.     

Steady-state sodium absorption and chloride secretion of colon and coprodeum,and plasma levels of osmoregulatory hormones in hens in relation to sodium intake

Summary The plasma levels of four osmoregulatory hormones and their target ion-transport systems in the lower intestines of the domestic fowl were determined in order to elucidate their interrelationship and their setpoints in relation to NaCl intake. White Plymouth Rock hens were adapted to six intake levels of NaCl (0.20±0.02–24.7±1.9 mmoles Na⁺·kg bw^–1·day^–1) for 6 weeks. The Na⁺ absorption and the Cl^– secretion of colon and coprodeum were characterized in vitro by the effects of hexoses, amino acids, amiloride, and theophylline on the short-circuit current (SCC) and electrical potential difference (PD). The NaCl-conserving system of the adult chicken is set at low intake levels of NaCl as the 80% range (quantitized by non-linear, logistic regression analyses) of the change in the plasma [ALDO], the amiloride-inhibitable Na⁺ absorption of coprodeum and colon ( SCC), occurred from 0.18 to 2.3, from 0.9 to 4.3, and from 1.2 to 7.3 mmoles Na⁺·kg bw^–1·day^–1, respectively. These results demonstrate that the amiloride-inhibitable Na⁺ absorption of coprodcum is more closely linked to plasma [ALDO] than that of colon. The aminoacid-Na⁺ coabsorption of colon increased over exactly the same range of Na⁺ intake as the colonic amiloride-inhibitable Na⁺ absorption decreased, whereas the hexose-Na⁺ coabsorption increased at higher levels of Na⁺ intake, from 2 to 11 mmoles Na⁺·kg bw^–1·day^–1. Both these Na⁺ absorption types had reached their maximums at 24.7 mmoles Na⁺·kg bw^–1·day^–1, whereas the plasma [AVT] and plasma [PRL], although significantly increased, apparently had not; their 80% range of change occurred from 9.9 to 99 mmoles Na⁺·kg bw^–1·day^–1, and the main changes in plasma osmolity were predicted to occur from 5.4 to 107 mmoles Na⁺·kg bw^–1·day^–1. These results suggest that these colonic and hormonal variables conserve osmotically-free water and operate at high NaCl intake. The theophylline-induced colonic Cl^– secretion did not change with NaCl intake, whereas the stimulation of SCC in coprodeum decreased with increasing NaCl intake: The main change occurred between 0 and 3.2 mmoles Na⁺·kg bw^–1·day^–1. Thus, all ion-transport capacity disappears in coprodeum with increased dietary NaCl intake, whereas colon maintains its ion-transport capacity (although the nature of the Na⁺ transport changes). It is suggested that hormones defending the extracellular volume and composition are regulated close to zero input and output of both NaCl and water, regardless of whether they are NaCl conserving or free-water conserving. Therefore, changes in their stable plasma concentrations occur at the extremes of tolerable range of NaCl intake.Abbreviation AA aminoacids - ALDO aldosterone - AMI amiloride - AVT arginine vasotocin - bw body weight - CS corticosterone - HEX hexoses - INDO indomethacin - PD potential difference - PRL prolactin - R resistance - SCC short-circuit current - SD standard deviation - SEM standard error of mean - THEO theophylline