Behavioral Adaptations

Torpor

• Physiological state in which energy expenditure is greatly reduced by decreasing body temperature and the rate of metabolism

• Occurs when environmental conditions exceed an individual’s capacity for homeostasis (e.g., harsh climate or food/water shortage)

Ectotherms in Torpor

• regular dormancy cycles, especially in extreme environments in amphibians and reptiles

• do not regulate body temperature during torpor and cannot awake (arouse)

Endotherms in Torpor

• daily or seasonal dormancy in many small to medium-sized mammals and some birds

• regulate body temperature during torpor (albeit at low levels) and can awake (arouse)

Types of Torpor

• Daily torpor

– occurs on a daily basis

• Seasonal torpor

– Hibernation (winter)

– Estivation (summer)

Daily Torpor

• Saves energy during the inactive period of the day

• Lasts only a few hours

• Found in small mammals e.g., bats, pocket mice, shrews) and small birds (e.g., hummingbirds, swifts, caprimulgids)

• Body temp falls 5-15°C below normal

• O 2 consumption < 1/3 - 1/40 normal

• High metabolic cost of arousa

Why Daily Torpor?

• Fast use of energy / poor reserves

• Variable food (insects, nectar)

Seasonal Torpor – Hibernation

• Body temperature (Tb) can be reduced to within 1°C or less of ambient temperature (Ta)

• O 2 consumption markedly reduced

• Heart rate markedly reduced

• Prolonged periods of breathing apnea

• Body slows down much more than in sleep

Avian Hibernator

Common Poorwill

Mammalian Hibernators

• Carnivores (bears, badgers)

• Rodents (many – e.g., woodchucks, chipmunks, ground

squirrels, dormouse, hamsters)

• Insectivores (e.g., African shrews, golden moles)

• NA bats: some species (7+) migrate;

others (9+) hibernate

• Smaller marsupials

Inducing Hibernation (Factors)

• Cold weather

• Low food supply

• Increased fat

• Increased serum magnesium levels

• Change in respiratory control center sensitivity to CO 2

Entrance into Hibernation

1. Decrease in heart rate & breathing rate

2. Drop in oxygen consumption

3. Drop of body temperature

Temperature Control and State of Hibernation

• Resetting thermostat; not turning it off

(controlled physiological state)

• Sudden drop in temperature results in wakening

(Arctic ground squirrel: at -15 °C, either awoke or frozen)

• Not a continuously torpid state for months

• Periodic arousals

Arousal from Hibernation

• Time needed for arousal differs among species (larger species require longer time) e.g., 30 minutes in bats to hours in marmots / hedgehogs

• Active process involving high rate of heat production

• Brown fat tissue generates heat during the early stages

—Energy cost is great (equals 10 days of hibernation)

Why arouse?

– Initiate immune response

– Restore water lost by evaporation

– Fat storage depletion

Arousal from Hibernation (3)

• Increase in heart rate

• Increase metabolism

• Reset temperature regulating mechanism

Ectotherms and Torpor

• Ectotherms usually spend winter where Ta will not dip below freezing

• Bottom of lakes usually 4° C (mud & water)

• Terrestrial hibernators usually find sites below frostline

Freezing Temperatures

• Many ectotherms can withstand brief periods of super- cooling (-1 to -2° C), but freezing is lethal to most!

• Ice crystals physically damage cells & tissue

– Intracellularly - dehydrates cells, raising osmolality

– Extracellularly - blocks O 2 and nutrient flow

Freeze Tolerant Herps

• A few species of herps are freeze tolerant

(a few frogs, turtles, lizards, snakes)

• Hibernate in shallow waters or near the soil surface where temperatures drop below their freezing point

• Antifreeze inside cells = cryoprotectant

• Heart stops, metabolism stops, no brain activity detected for up to 2 weeks in frozen wood frogs...

• Vital functions return within 1-2 hours of thawing!!!

Seasonal Torpor – Estivation

• Less severe depression of core temperature and metabolic rate

• Generally in response to drought or extreme heat (dehydration would occur quickly)

• Some species produce epidermal cocoons or retreat to burrows

• Amphibians in desert face long periods of low humidity and no rain

• Lungfish in S. America & Africa estivate in mud cocoons

• Some desert rodents estivate (e.g., cactus mouse, mohave ground squirrel)

Benefits and Costs of Torpor

• Benefits: balance the energy budget in extreme environmental conditions

• Costs: animal is helpless, little or no growth, reduced reproductive period

• So torpor is a last resource when animals cannot deal with environment in a better way (insulation, migration, etc.)