Temperature Relations: Physiological Ecology and Body Temperature Regulation

Temperature Relations

Introduction to Physiological Ecology

Physiological ecology examines the interactions between organisms and their abiotic environment, focusing on how temperature, water, and nutrients constrain adaptation, growth, survival, and reproduction. Temperature is a critical environmental factor influencing the rates of chemical reactions and the ecological performance of organisms.

• Adaptation: The process by which organisms adjust to environmental constraints to grow, survive, and reproduce.

• Constraints: Temperature, water, and nutrients are primary abiotic factors limiting organismal performance.

Microclimates and Temperature Variation

Macroclimate vs. Microclimate

Temperature varies at different spatial and temporal scales. Macroclimate refers to large-scale weather patterns over long periods, while microclimate describes small-scale variations over shorter periods, often influenced by local landscape features.

• Macroclimate: Regional weather patterns (e.g., seasonal temperature trends).

• Microclimate: Localized temperature variations caused by altitude, aspect (direction a slope faces), vegetation, ground color, and the presence of boulders or burrows.

• Example: North-facing slopes (shaded) are cooler than south-facing slopes (sun-exposed).

Aquatic Temperatures

Water has a higher heat capacity than air, meaning it heats and cools more slowly. This property stabilizes aquatic temperatures and influences the thermal environment of aquatic organisms.

• Riparian ecosystems: Areas adjacent to rivers and streams where canopy cover can moderate water temperature.

Temperature and Organism Performance

Thermal Performance Curves

Most species perform optimally within a narrow range of temperatures, largely due to energy limitations and the temperature sensitivity of biochemical processes.

• Principle of Allocation: Energy allocated to one function (e.g., growth) reduces energy available for others (e.g., reproduction). Adaptation to one set of conditions often reduces fitness in others.

Biomolecular Level: Enzymes

Enzymes have optimal temperature ranges for activity. For example, different forms of acetylcholinesterase in rainbow trout function best at temperatures matching winter (2°C) and summer (17°C) conditions.

• Example: Rainbow trout express different enzyme forms seasonally to maintain physiological function.

Plants: Photosynthesis and Temperature

Photosynthesis is sensitive to temperature, with extreme temperatures reducing its rate. Different plant species have evolved to maximize photosynthetic efficiency at different optimal temperatures.

• Photosynthesis equation:

$6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6CO_2 + 6H_2O$

• Example: Boreal mosses photosynthesize best at 15°C, while desert shrubs peak at 44°C.

Microorganisms

Microbial species are adapted to a wide range of temperatures, from cold to extremely hot environments, but each species performs best within a relatively narrow thermal range.

• Example: Some bacteria grow optimally at 4°C, others at 69°C, but no single species thrives across all temperatures.

Body Temperature Regulation

Heat Balance Equation

Organisms regulate body temperature by balancing heat gain and loss through various pathways:

$H_s = H_m \pm H_{cd} \pm H_{cv} \pm H_r - H_e$

• $H_s$: Total heat stored in an organism

• $H_m$: Heat gained via metabolism

• $H_{cd}$: Heat gained/lost via conduction

• $H_{cv}$: Heat gained/lost via convection

• $H_r$: Heat gained/lost via radiation

• $H_e$: Heat lost via evaporation

Thermal Strategies

• Poikilotherms: Body temperature varies with the environment (e.g., most fish, amphibians, reptiles, plants).

• Ectotherms: Rely mainly on external energy sources for temperature regulation.

• Endotherms: Rely heavily on metabolic energy to regulate body temperature.

• Homeotherms: Maintain a relatively constant internal temperature (subset of endotherms).

Temperature Regulation by Plants

Desert Plants

Desert plants must minimize heat gain and avoid overheating. They use morphological and behavioral adaptations rather than evaporative cooling.

$H_s = H_{cd} \pm H_{cv} \pm H_r$

• Decrease heating via conduction ($H_{cd}$)

• Increase convective cooling ($H_{cv}$)

• Reduce radiative heating ($H_r$)

• Example: Highly reflective leaves, open growth forms, and leaf orientation reduce heat gain.

Arctic and Alpine Plants

To stay warm, these plants increase radiative heating and decrease convective cooling. Many species have evolved to do both.

• Example: Compact growth forms and dark pigmentation increase heat absorption.

Temperature Regulation by Animals

Ectothermic Animals

Ectotherms, such as alpine lizards, regulate body temperature through behavioral adaptations like basking, burrowing, and pigmentation changes to increase radiative heating and decrease convective cooling.

• Example: Alpine Liolaemus lizards use sun basking and dark pigmentation to thrive in cold environments.

Endothermic Animals

Endotherms maintain body temperature through metabolic heat production. The thermal neutral zone is the range of environmental temperatures where metabolic rate remains constant.

• Below the thermal neutral zone, metabolic rate increases (shivering, fat metabolism).

• Above the thermal neutral zone, mechanisms like increased blood flow and sweating dissipate heat.

Species	Thermal Neutral Zone Breadth
Tropical species (e.g., sloth, night monkey)	Narrow
Arctic species (e.g., polar bear, arctic fox)	Broad
Humans	Intermediate
Ground squirrel	Broad
Marmot	Intermediate
Eskimo dog	Very broad
Additional info: Table inferred from slide image and typical textbook content.

Endothermic Aquatic Animals

Few aquatic animals are endothermic due to the high heat loss in water. Aquatic birds and mammals maintain body temperature with insulation (fat or fur) and countercurrent heat exchangers in appendages and swimming muscles.

• Example: Marine mammals and fish like tuna use countercurrent heat exchangers to retain heat in swimming muscles.

Insect Flight Muscles

Some insects, such as bumblebees and sphinx moths, can warm their flight muscles through metabolic activity, allowing them to fly in cool conditions.

Thermogenic Plants

Most plants are poikilothermic ectotherms, but some, like those in the Araceae family (e.g., eastern skunk cabbage), can generate metabolic heat to warm their flowers.

Surviving Extreme Temperatures

Behavioral and Physiological Strategies

• Inactivity: Seeking shelter during extreme conditions (e.g., burrowing, nocturnal activity).

• Reducing Metabolic Rate: Entering states such as torpor (short-term), hibernation (winter dormancy), or estivation (summer dormancy) to conserve energy during unfavorable temperatures.

• Example: Hummingbirds enter torpor at night; bears hibernate in winter; turtles estivate in summer.