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  • 1
    Electronic Resource
    Electronic Resource
    Energy metabolism, testosterone and corticosterone in white-crowned sparrows (1999)
    Springer
    Journal of comparative physiology 185 (1999), S. 463-470 
    Details
    ISSN: 1432-1351
    Keywords: Key words Metabolism ; White-crowned sparrows ; Testosterone ; Corticosterone ; Activity ; AbbreviationsBMR basal metabolic rate ; D duration of subjective night ; L duration of subjective day ; RMR resting metabolic rate ; T testosterone
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Abstract The influence of the steroid hormones testosterone and corticosterone on energy metabolism and activity of birds is largely enigmatic. We measured resting metabolic rate during night and day in 12 long-term castrated and 12 intact male white-crowned sparrows (Zonotrichia leucophrys gambelii) under short-day (8:16 SD), long-day (20:4 LD), LD+testosterone implant and LD−testosterone implant conditions. Each male was sequentially measured under all four conditions. Photostimulation increased testosterone, resting metabolic rate, food intake, hopping activity and body mass in castrates and intact males. Surprisingly, testosterone levels and metabolic rates did not differ between intact and castrated males. Testosterone implantation increased activity and food intake, but decreased body mass and resting metabolic rate in both groups. Removing testosterone implants reversed the effects on resting metabolic rate, activity and food intake. Corticosterone levels, measured immediately at the end of metabolism measurements, showed birds were not stressed. Corticosterone had no apparent relationship with resting metabolic rate and there was no interaction between corticosterone and testosterone. Overall, positive changes in testosterone levels resulted in a decrease of resting metabolic rate. We speculate that testosterone increases activity, and birds compensate for increased activity metabolism by reducing resting metabolic rate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003590050407
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  • 2
    Electronic Resource
    Electronic Resource
    Niche expansion, body size, and survival in Galápagos marine iguanas (2000)
    Springer
    Oecologia 124 (2000), S. 107-115 
    Details
    ISSN: 1432-1939
    Keywords: Key words Niche expansion ; Body size ; Reptiles ; Herbivory ; Survival
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract  Foraging theory predicts that dietary niche breadth should expand as resource availability decreases. However, Galápagos marine iguanas often die during algae shortages (El Niños) although land plants abound where they rest and reproduce. On Seymour Norte island, a subpopulation of iguanas exhibited unique foraging behavior: they consistently included the succulent beach plant B. maritima in their diet. We investigated the consequences of land-plant feeding for body size and survival. Batis-eaters supplemented their algae diet both before and after intertidal zone foraging, and more Batis was eaten during tides unfavorable for intertidal zone foraging (dawn and dusk). Larger, energy-constrained iguanas fed more on land than did smaller animals. Compared to intertidal zone algae, Batis was 39% lower in caloric content (1.6 vs. 2.6 kcal g–1 dry mass), 56% lower in protein (8.3 vs. 18.9% dry mass) and 57% lower in nitrogen (1.3 vs. 3.0% dry mass). In spite of its lower nutrient value, iguanas that supplemented their diet with this plant were able to attain nearly twice the body size of other iguanas on the island. Age estimates indicate that many Batis-eaters survived repeated El Niño episodes during which animals of their relative size-class experienced high mortality on other islands. The larger animals were, however, completely dependent upon this supplementary source of food to maintain condition, and all perished in the 1997–1998 El Niño when high tides inundated and killed Batis on Seymour Norte Island. We hypothesize that Batis feeding developed as a local foraging tradition, and that dietary conservatism and strong foraging site fidelity explain why the inclusion of land plants in the diet has been observed in only a single population. Ultimately, a unique algae-adapted hindgut morphology and physiology may limit a switch from marine to terrestrial diet.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004420050030
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  • 3
    Electronic Resource
    Electronic Resource
    Ontogenetic changes in food intake and digestion rate of the herbivorous marine iguana (Amblyrhynchus cristatus, Bell) (1993)
    Springer
    Oecologia 94 (1993), S. 373-379 
    Details
    ISSN: 1432-1939
    Keywords: Ontogeny ; Herbivory ; Food intake ; Digestion ; Marine iguana
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Young reptiles have higher relative energy demands than adults, but the proposed ontogenetic changes in diet to fulfil these demands were not found in the algae-eating Galápagos marine iguanas on Santa Fé. Feeding and digestion rates were investigated to analyse how young achieve higher energy intake. Daily food intake of free ranging marine iguana hatchlings (6–11 months old) was about one third that of adults, but relative intake (g dry mass · g−1 wet mass · day−1) was four times higher in the hatchlings. During feeding experiments, relative daily food intake of hatchling marine iguanas was approximately three times higher than that of adults (0.042 vs 0.013 g dry mass · g−0.8 wet mass · day−1), and mean gut passage time was two times shorter (5 vs 10 days). The hatchlings also maintained high body temperatures (36.7° C) even under relatively cool day-time air temperatures of 32° C. Apparent digestibility of algal food measured both during feeding trials and by Mn2+ AAS (atomic absorption spectrometry) for free-ranging iguanas was 70%, independent of body size and temperature. The red algae prevailing in the diet were high in protein (30% dry mass) and energy (12.1 kJ/g dry mass). Diving iguanas had higher rates of energy intake than intertidal foragers, but daily intake was less. Maintenance of high body temperature enabled hatchlings to achieve high digestion rates and, combined with high relative intake, thus achieve sufficient energy intake for rapid growth despite higher mass specific metabolic rates. Estimates of biomass of marine iguanas and their algal food are given for a section of coastline on Santa Fé.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00317112
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    Paper (German National Licenses)
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