ECO PHYS UNIT 3 (NOT COVERED)

Insulation acclimatization (Seasonal acclimatization)

Changes in pelage or peripheral blood flow can reduce heat loss.  Improved insulation can shift the lower critical temperature downward.

Porcupine study

Porcupines were acclimated to 32°C and 10°C for three weeks. Cold-acclimated porcupines had a lower lower-critical temperature and a lower upper critical temperature, shifting the TNZ. O₂ consumption was higher within the TNZ. Thermal conduction decreased, indicating increased insulation. Energy consumption was higher.

Hare study

Body temperature did not differ between autumn and winter. Metabolic rate within the TNZ was about 20% lower in winter. Lower critical temperature shifted about 10°C lower in winter. Thermal conduction decreased about 20%. Density of downy hairs increased about 130%. Guard hairs also increased in length and density.

Compare the studies

Porcupines: cold acclimation was associated with higher energy consumption. Hares: winter acclimatization was associated with lower MR within the TNZ and increased insulation. Use the contrast to think about how different species can solve the energetic problem of cold in different ways.

Adaptations to Cold

Arctic species generally have lower lower-critical temperatures than tropical species. A lower lower-critical temperature allows an animal to tolerate colder temperatures before substantially increasing metabolic rate. Cold adaptations can improve fitness by reducing heat loss and energetic expenditure.

Regional heterothermy

Some animals allow appendages to become colder while maintaining a warmer core. Appendages often contain tissues with relatively low metabolic demands, such as tendons, ligaments, and bone. Countercurrent heat exchange helps regulate this heat transfer.

Short-term: Torpor

Torpor is controlled hypothermia involving reduced body temperature and metabolic processes. It minimizes energy costs and water loss. The lecture emphasizes that torpor is a controlled physiological response, not simply a passive consequence of environmental temperature.

Long-term: Estivation

Estivation is similar to hibernation but occurs during summer/hot, dry conditions. It helps animals avoid high temperatures and drought. African lungfish aestivate during the dry season. They can sustain long periods of aestivation by reducing metabolic rate, heart rate, and respiration.

Hibernation Physiology

Hibernation involves metabolic downregulation. Periodic episodes of arousal occur during hibernation. Expression of genes involved in cellular respiration is reduced, resulting in fewer enzymes. As body temperature decreases, temperature-dependent cellular processes such as enzyme activity also decrease.

Hibernation and body size

Most hibernating mammals are smaller than 5 kg, although this is not universal. Hibernation may be favored in small mammals because their energetic demands create stronger selection for energy conservation. The lecture also connects this topic to the allometric cost of locomotion/migration.

Hibernation in Bears

Some mammologists argue that bears do not technically hibernate depending on the definition used. A strict definition may require metabolic rate and body temperature to decrease to near ambient temperature, below about 10°C. Arctic ground squirrels can reach about −2 to −3°C. Bear body temperature remains around 33°C. Despite this, bears undergo substantial biochemical metabolic downregulation.  Black bears can reduce metabolic rate to about 25% of BMR and heart rate to about one-fifth of normal. Their weight-specific metabolic rates can be similar to those of other hibernators.

Onset & Duration of Torpor/Hibernation

Changes in AMP/ATP and NAD/NADH ratios are thought to signal changes associated with lipolysis. These signals can be interpreted as reflecting energy availability. Photoperiod can influence the endocrine system and provide predictive seasonal information.

Food availability

A predictive model suggests hummingbirds may enter torpor if they do not accumulate enough energy during the day. Duration of torpor is negatively correlated with food availability in pocket mice. More food → shorter torpor; less food → longer torpor.

Big Takeaway

Torpor and hibernation are closely connected to energy balance. Animals can regulate their metabolic state when the energetic benefits of activity are outweighed by its costs.