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Physiological and behavioural correlates of life-history variation: a comparison between tropical and temperate zone House Wrens B. I. TIELEMAN, T. H. DIJKSTRA, J. R. LASKY, R. A. MAUCK, G. H. VISSER and J. B. WILLIAMS
Introduction Balance between parental investment and selfmaintenance. Temperate zone birds tend to have larger clutches but lower survival rates than do tropical species. Increased survival of tropical birds compared with temperate zone equivalents should result from a combination of physiological and ecological factors. Field metabolic rate (FMR) integrates energy expenditure for all activities throughout the daily cycle and thus provides an important assessment of ecological and behavioral components of the life history
Introduction Species with large geographical distributions are uniquely suited for investigations of life-history diversification
Introduction Birds with higher adult survival rates invest less in annual reproduction. This is reflected in fewer feeding trips to provision their young, and lower basal and field metabolic rates. These are related to an expected reduction in intrinsic mortality. In addition, the authors investigated if the large environmental variation during a breeding season in temperate regions has consequences for the amount of energy and time required to produce offspring. Comparisons with birds from the relatively constant environmental conditions in the tropics.
Introduction Why is Parental Investment (i. e. , reproductive costs) greater in temperate than tropical regions? Lower temperatures – particularly at night Higher numbers of nestlings – increased overall costs for parental care
Hypothesis Tropical birds with higher adult survival rates should invest less in annual reproduction than their temperate zone counterparts.
Materials and methods Study occurred during May–August 2003 in Ohio, March–July 2004 in Panama Birds attracted by placing nestboxes. The breeding season in Ohio starts at the end of April and lasts about 3 months In Panama breeding season is year-around, but major activity occurs between Apr. and Oct.
Materials and methods Feeding rates was counted on day 3, 6 and 10 in Ohio; day 4, 7 and 11 in Panama. On day 5 (Ohio) and 6 (Panama) both parents were captured and injected them with doubly labeled water. Parents were recaptured on day 7 (Ohio) and day 8 (Panama) to take a final blood sample. Finally, in Panama, parents were recaptured on day 12, and transported to the laboratory for BMR. At the same time, they also observed and compared feeding frequency between areas and between sexes.
FMR Field metabolic rate (FMR) and water influx rate (WIR) were measured using the doubly labeled water (DLW) technique, in which the rate of decline of H 2 in the body water pool provides a measure of WIR. DLW Methods: took a 80– 100μl sample of blood (initial) from the brachial vein, after about 48 h, recaptured birds, took a second blood sample (final), measured body mass and released them. Also took blood samples of four uninjected individuals to determine background levels of isotopes After calculating the CO 2 Production, converted these values to energy expenditure using RQ for insectivorous (protein) diet.
BMR They measured rates of oxygen consumption for postabsorptive birds during the night by standard flow-through respirometry methods. Birds were placed in 2· 1 -liter stainless steel metabolism chambers. Temperature regulation provided by a Peltier Cooler to control ambient temperature (at 31– 32°C). Flow rate is 518 mil/ min, and used silica gel to remove the water. After let the birds sitting in chamber for 3 h, traces the O 2 consumption for at least 10 min. Then take the birds out and measure their body temperature.
Statistical analysis Two-way ANOVA interaction terms were used in models. Dependent variable = mass-specific FMR Independent variables = location, Julian date Regression of FMR vs. progression of the season. Multiple regression analyzing factors (# nestlings, location, age) possibly affecting feeding frequency. P=0. 05 is the level for significance.
Results Brood size was 40% smaller in Panama. Body mass of House Wrens was 32% larger in Panama than in Ohio Whole organism FMR of wrens was 13% lower in Panama than in Ohio, Mass-specific FMR was 34% lower in Panama. Field water influx rate of wrens was significantly lower in Panama than in Ohio. Parents in Ohio made more feeding trips per hour to the nest than birds in Panama
Results Feeding frequency per nestling did not differ between Panama and Ohio The fractional feeding contribution of males was 0· 53 ± 0· 32 (n = 14) in Ohio and 0· 52 ± 0· 09 (n = 17) in Panama, an insignificant difference. Mass-specific BMR was 47% lower in Panama (1· 38 k. J day− 1 g− 1) than in Ohio (2· 28 k. J day− 1 g− 1), comparable with the difference found for FMR. The ratio FMR/BMR was 2· 1 in Panama and 1· 9 in Ohio.
Results As the breeding season progressed … • Mass-specific FMR declined for birds in Ohio, but not in Panama • Mass-specific WIR increased for birds in Panama. • WEI also increased for birds in Panama – associated with onset of rainy season.
Discussion Some authors have invoked food limitation as the proximate factor explaining the small clutch sizes in tropical birds But the authors for this paper think that tropical House Wrens do not time their breeding to co-occur with peak food availability, indicating that sufficient food may be available throughout the year. They propose that selection acts indirectly on wholeorganism energy expenditure in the tropical wrens by specifically reducing mass-specific levels of metabolic rate, and selecting for reduced reproductive rates in tropical wrens.
Discussion Also the number of daylight-hours per day is lower in the tropics than in the temperate zone during the breeding season. Therefore, on a 24 -h basis, chicks are fed less often in the tropics and thus are expected to grow slower (longer nestling period). The shorter daylight period in the tropics results in longer nights, which results in a relatively greater contribution of night-time MR to FMR, which helps explain the lower FMR in the tropics.
Discussion Three factors change or potentially change in the course of a breeding season: day length, temperature and food availability. FMR decreases over the course of the season in Ohio, which suggests that temperature is the most important environmental factor influencing FMR. Parental FMR suggests that reproduction is more time -constrained in the temperate zone, compressed by the shorter breeding season, and as a result more energetically costly for the parents.
Discussion General hypothesis was supported Tropical wrens did expend less energy on the current reproductive effort than temperate wrens Prediction = Tropical wrens will not compromise investments in self-maintenance in response to experimentally increased costs of reproduction, whereas temperate zone wrens with a lower annual survival probability should be more willing to compromise their survival in order to maximize a current reproductive event.