Title

Resource allocation to growth and thermoregulation during early development in altricial nestlings

Date of Award

2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Keywords

Altricial birds, Brooding, Resource allocation, Thermoregulation, Nestlings

Subject Categories

Ecology and Evolutionary Biology | Life Sciences | Physiology

Abstract

Patterns of limited resource allocation to the mutually exclusive processes of growth and maintenance are products of natural selection and presumably confer fitness benefits. Hypothesized universal scaling of metabolic rate, growth, and resource allocation may not apply for altricial birds. Altricial birds grow rapidly by delaying thermal maintenance costs and preferentially allocating energy to growth. Endothermic nestling thermoregulation is aided by intermittent parental brooding. A general growth model was used as a null-model to test if deviations of altricial growth from universal patterns suggested energetic and fitness advantages. Differences between altricial and universal growth were not explained by changes in proportional water content of tissue or observed systematic increases in body temperature during ontogeny. Nestling body temperatures are maintained below 37ºC for the first few days. Mass- and temperature-normalization of nestling metabolic rates suggest the rapid development of metabolic capacity may explain observed growth patterns, and thus deviations from universal patterns. The hypothesis that the altricial thermoregulatory strategy has a cost savings relative to the endothermic-at-hatch (precocial) alternative was tested for young house wren ( Troglodytes aedon ) nestlings. Nestling and parental thermoregulatory contributions were estimated from rates of wren broods cooling and heating under natural conditions, and costs for brood-level endothermy were modeled. Intermittent brooding was energetically cheaper than endothermy, ∼250 kJ savings or ∼168% over the first 5 days of development at moderate environmental temperatures, ∼21ºC. Relative cost savings for brooding increased with lower environmental temperatures. Projected metabolic capacities necessary for endothermy are unrealistically high, up to 14-fold, for young nestlings in broods. The shift in the brooding strategy cost effectiveness coincides with the age when relative tissue water decreases and growth rate is maximum, suggesting a shift in preferential resource allocation from growth to maintenance. The hypothesis of temperature dependent growth was tested in wrens by experimentally increasing body temperatures ∼2ºC for the first 7 days. No change in growth rate was observed suggesting that nestling metabolic and growth rates may be maximized. Because there is no growth rate advantage with higher temperatures, maintaining lower body temperatures for young nestlings is another cost savings of the altricial strategy.

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