L14: Energy for hibernation and torpor

Food energy is required to sustain an animal’s metabolism during...

• maintenance

• growth

• reproduction

• physical activity

What happens when these food items are scarce or absent?

• switch to a less desirable food source

• utilize body fat stores

• move to where food is more plentiful → migration

• ↓ body temperature (T b ) to ↓ energy requirements; seasonal hibernation and daily torpor

How do homeotherms thermoregulate

maintains a relatively constant body temperature (regardless of the environmental temperature)

• Endotherms: T_b determined by metabolic activity

Examples of T_b of endotherms:

• birds: 40°C

• placental mammals: 37- 38°C

• marsupials: 34°C

• monotremes: 30°C

When is T_b not maintained

hibernation and torpor

Examples of lowest T_b in endoterhms

• birds: 6°C (up to 4 days)

• placental mammals: -3°C (up to 33 days)

• marsupials: 1°C (up to 23 days)

• monotremes: 4°C (up to 16 days)

How to distinguish between hibernation (voluntary) and torpor (involuntary:

1. Duration of T b reduction:

a)<24 hours: torpor (short-term, daily episode)

b) >4 days: hibernation (long-term, seasonal episode)

2. Minimum T b maintained:

a) <6°C: torpor (moderately above freezing)

b) >15°C: hibernation (moderately below normal Tb

What do larger mammal undergo to conserve energy

Larger mammals undergo an extended dormancy (‘winter sleep’)

• American black bear: T_b of ~31°C (up to 225 days)

• Brown bear: T_b of ~32°C (up to 200 days)

• Eurasian badger: T_b of ~28°C (up to 45 days)

Extended dormancy characteristics

1. Duration of T_b reduction: >45 days

2. Minimum T_b maintained: >28°C (barely below normal T_b)

3. Average body weight: >10kg (<9kg for torpor, <4kg for hibernation)

Reducing energy requirements during hibernation and torpor include:

1. Decrease Tb

2. Decrease metabolic activity

3. Decreased heart rate

4. Decrease respiratory rate

Decrease T_b

• thermoregulatory control is not abandoned

• a new, lower Tb set-point is established

• rewarming achieved using only endogenous heat

Decrease metabolic activity

• a minimum metabolic rate is established

• on average, ~15% of BMR (including arousal periods)

Decrease heart rate

• HR may drop by upwards of 85%

• in some cases, beats may be missed entirely

Decrease respiratory rate

• RR will drop comparably to HR

• periods of apnea may occur

Energy savings relative to remaining active:

Hibernation: ~90%

Torpor: ~65%

Characteristics of animals in hibernation

• Animals hibernate in a curled position (except bats)

• Animals usually hibernate alone (except the hazel dormouse and bats)

• Animals will seek a safe, sheltered area (or insulated nest)

Arousal period

• No animal will remain dormant through an entire hibernation

• Hibernators will experience periodic, spontaneous arousals

• Arousal occurs using only endogenous heat

• Increased Tb , HR, RR (O₂ consumption)

Stages of arousal period

Stage 1 = non-shivering thermogenesis

Stage 2 = visible shivering

Arousal times vary depending on..

abundance of brown adipose tissue (BAT)

Purpose of arousal periods

To eat

1. Food storers

• store food in the hibernaculum

• will eat during arousal periods

2. Fat storers

• deposit fat prior to hibernation

• They don’t eat but their metabolism increases and uses up the fat nutrients

To drink

• depends on water conservation aka urine concentration

To urinate

• need to remove toxins from the body