Metabolism and Scaling
Hydration
- Maintained homeostatically within a safety zone by compensating water loss with input from metabolic, food and drinking water
- Reptiles are greatly impacted by water availability
- Cwl and rwl largely driven by body size (ie. Surface area to volume ratio)
- Skin resistance to evaporation (rs)
- Tewl seemingly scales with the 0.6 to 0.7 regression in body mass
- Rwl generally smaller than cwl in dmall sized species and increases faster with body mass (3/4th allometric scaling)
- Tewl no clear altitudinal cline
Scaling
- Metabolic
o Mst: a collection of ideas that connects the physiological processes of individual organisms with the larger patterns or ecology
o Power law relationship: relationship between MR and body size is a power law with an exponent less than 1 (mouse has higher MR than elephant relative to its mass)
o Orgasnims size, temp, and mr affect MST
o Compensation hypothesis: (allocation hypothesis) fitness advantage of lower bmr or smr as a result of lower maintence cost = more allocation to reproductive costs
§ Higher bmr = larger organs = larger reproductive tield (increase intake hypothesis)
o Important to measure oerformance as fitness and how it covaries with mr through ontogeny
o Quantitative genetics as a way to understand inheritance and evolution of traits including responses to selection
§ Population level phenotypic variation of quantitative traits into heritable and nonheritable components an links them to fitness via measures of selection
§ Breeders equation: predicts amount of change in a single trait from one gen to the next R=H2s (heritability and selection)
§ Quantitative trait: trait the may be influenced by multiple genes, continuous variation in a population
§ Provides framework to understand how natural selection can change phenotypic traits over time
§ Predict population level response to univariate and multivariate selection
· Random trait pre sewlection, look at direction of selection with fitness on y and trait on x, determine where the majority of population will be post selection
· Multivariate is a quadratic formula and makes the trait appear linear (look at p3 in understanding variation in mr)
· Types of selective pressures
o Stabilizing, disruptive, directional, correlational(?) (form of nonlinear multivariate selection where a combo of two or more traits interact non-additaively to affect fitness)
o
§ Usually use fitness proxies instead of fitness but this can be misleading
· But fitness proxies are dependent on species life history
§ Total phenotypic variance of a trait = sum of variances attributable to genetic and environmental influences
· Subdivided into three categorie:
o Additive: deviations from mean phenotype attributed to additive contribution of particular alleles
o Dominance- non additive:: quantifies interactions between allèles (dominant)
o Interaction – non-additive: quantifies interactions between allèles (epistasis : genetic phenomenon that occurs when effect of a genen mutation is dependent on other gene mutation)
- Heritability studies of MR suggest that it is free to evolve under selection (particularly in endotherms)
o i.e. selection of bmr in lab mice suited for particular environment
- consider multiple traits and how they effect mr to get a better understanding of evolutionary shift in mr (running speed may increase but heretibility decreases = cancel eachother out)
- mr measured as rate of heat production by direct calorimeyty or by co2 or o2 consumption / production
- bmr associated with internal organs
- Msum with muscles
o These two traits are somewhat capable of evolving independently
- Metabolic theory of ecology: assumes body size and temp independent multiplicative effects on mR (3/4th)
- Many other factors affect mr so this scaling theory does not hold true
- Metabolic Rate = R = aM^b
o A = scaling coefficient
o M = mass
o B = scaling exponenet
- Metabolic-level boundaries hypothesis: how etabolic rate relateds to body mass depends on intensity of R or its metabolic level which is a measure of the elevation of metabolic scaling relationship
o Metabolic level depends on intensity of various sources (i.e. reproduction, thermoregulation)
o When metabolic level of RMR is high, sa-related metabolic waste removal predominates and makes b approach 2/3rds
o When not high b approaches 1
o Overall predicts concave upwards relationship in endotherms
- Contextual multimodal theory : sa related fluxes of resources (heat ), physical constraints of internal resource transport on resource supply, body comp, and various processes effecting demand
o Cell size (during ontogeny should be around 2/3rds)
o Cell number (near 1)
o If both = between 2/3 and 1
o Genome size?
o May change with life stage and shifts in relative tissue masses and organ masses
- Extrinsic factors:
o Temperature
§ e-E/kT ; E = activation energy, k is boltsmanns constant, T is temp in K
o salinity, pH, latitude, altitude, o2 supply food supply, environmental toxins , organismal plasticity, stressors
- Interactions:
o Non moving ectotherms:
§ B may decrease with increasing T (bc of high SA related mr processes)
o Moving ectotherms:
§ B may increase with increasing T
§ B increases with cold exposure (opposite in endothermic birds)
o Cooler lat and alt = higher b values
o Cellular changes due to temp = interaction between intrinsic and extrinsic factors
o SA/V ratio (xeric = higher b)
o Life history stage
- Aquatic ectotherms evolved adaptations that enable them to avoid oxygen shortages as size and temperature increase
o i.e. reduction in growth rate
- how does temp and locomotor activity effect metabolic scaling (b) in air vs water breathers
o 1) b increases more steeply with activity level in airbreathers due to v related musculature properties (water breathers countered be pressures to reduce oxygen consumption
o 2) b increases more steeply with warming in water breathers
o The find that b decreases with increased temp in only water breathers and b overall increases with activity in airbreathers
- Ghost of oxygen limitation past: warming contributes to b decrease in water breathers as expected from an evolved avoidance of oxygen limitation at large sizes
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