Freezing, Supercooling, and Ectotherms

• Ectothermy is the the study of animals that regulate body temperature through external means.
• Chapter 15 is about Ectotherms

Ectotherms

• Ectotherm derives from the Greek word "ektos," meaning "outside," and "therm," meaning "heat."
• They regulate body temperature using external means and behaviors.
• Ectotherms depend on environmental heat sources
• An ectotherm's body temperature changes with the environment

Benefits

• Ectotherms typically have lower metabolic rates.
• They do not create their own heat.
• Require less food to survive
• Snakes may eat as infrequently as every 1-2 weeks.
• They have increased survival rates with low food supply.

Disadvantages

• Ectotherms may become sluggish at cold temperatures.
• They may not be able to escape predators as easily.
• When very warm, metabolic rate of ectotherms increases requiring ectotherms to eat more.
• Ectotherms mitigate this by resting in a cool location.

Deserts and Dryness

• Deserts usually have less than 250 millimeters (10 inches) of rain per year.
• The change from day to night temperature is drastic because the air is dry and loses heat quickly at night.
• Daytime temperatures average 38°C while in deserts temps can hit -4°C at night

Behavioral Modifications for the Desert - Desert Iguanas

• Desert iguanas exploit hour-to-hour changes in solar radiation to maintain a relatively constant body temperature
• Air varied between 29 - 46°C
• They maintain a body temperature of 36 - 39°C
• Desert Iguanas Prefer a body temperature of 42°C when active, but can tolerate a body temperature of up to 47°C
• Temperatures are lethal to birds and mammals and most other lizards
• Desert Iguanas do not drink but instead get water from diet and metabolic water
• They rely on nasal gland to eliminate excess salts

Behavioral Modifications for Life in the Desert - Desert Tortoises

• Desert tortoises Emerge from their hibernation burrows in March
• Tortoises are Active for ~3h every 4 days, in their burrows during inactivity
• March – May, tortoises eat annual plants that sprout from the winter rains
• They gain salt and lost energy during this period
• From May - July, tortoises shortened their activity periods to ~1h every 6 days.
• Tortoises gain energy and lose water.
• When it rains in July, tortoises drink large quantities of water and void their bladders
• After it rains, they eat every 2-3 days and spend more time out of their burrows
• In August which is dry, tortoises eat grasses
• In September, there is more rain; tortoises are able to drink and excrete salts
• October and November, tortoises eat vegetation; low temperatures reduce their activity
• In November, tortoises enter hibernation again

Surviving Cold Environments

• Ectotherms can survive in cold environments

Nucleating Agents

• If a small quantity of pure water is cooled, it does not freeze at 0 °C and the temperature may fall to as low as -39°C before ice crystals form
• Supercooling is when water remains liquid at temperatures below its freezing point
• When freezing occurs, the temperature is known as the temperature of crystallization
• For freezing to occur, water molecules need to form an ice crystal, resulting in the freezing of the total water
• Nucleation refers to the process where substances (an ice crystal or some other sort of particle) cause the water to freeze; these substances are called ice nucleators
• Substances other than ice, like a dust particle can act as ice nucleators and are usually responsible for initiating freezing in natural situations
• Supercooled bodies of water are rare in nature since there are usually enough nucleating agents (soil, sediment and dust) to start ice formation preventing supercooling
• Small organisms that contain minimal water can prevent nucleation enabling it to supercool a substantial amount
• This enables it to survive low temperatures by avoiding freezing

Antifreeze in Fish

• Polar fishes that live in coastal waters cannot avoid contact with ice

• The question of how these fish can survive in waters that are colder than their blood's melting point is addressed

• There are 274 species of fish found south of the Antarctic Polar Front

• Nearly 95% of fish caught in Antarctic coastal waters are from the superfamily Notothenioidea, the Antarctic perches

• The melting point of the blood of Notothenioids is -0.8°C, but it does not completely until -2.0 °C; this is thermal hysteresis

• The molecules the plasma of the blood of Notothenioids dissolves can be separated small and large components through a membrane allowing the passage of the small but not large ones

• A resulting solution containing small molecular weight components, mainly salts, freezes and melts at -0.8 °C, the melting point of the whole blood

• Thermal hysteresis is due to large molecules in the blood; glycoproteins (a protein linked to a carbohydrate)

• Antifreeze glycoproteins are 4% of weight of the blood

• Antifreeze proteins attach to the surface of ice crystals, preventing further water molecules from joining ice crystal lattice

• Antifreeze proteins are in the fluids of body cavity, heart, liver, and muscles

• Most Antarctic teleosts contain antifreeze glycoproteins

• Antifreeze proteins have evolved in many different groups of fish

• The antifreeze proteins found in many Arctic fish develop only in the winter

• Production is triggered by decreasing temperatures and shorter day lengths is under hormonal control

Frozen Ectotherms

• Some ectotherms can temporarily freeze

The Importance of Body Size

• Smallest ectotherms are smaller than smallest endotherms
• Endothermy consumes a large amount of energy at smaller sizes.
• Surface area to volume ratio is factor
• Ectotherms burn 1/10 the energy as endotherms, so they can be smaller.

Importance of Body Shape

• Surface area to volume ratio impacts endotherms
• Endotherms cannot have elongated shapes
• Ectotherms often have elongated shapes (eels, barracudas, caecilians, salamanders, lizards, snakes, etc.)
• There are no flat endotherms

Efficiency of Biomass Conversion

• Of energy an endotherm intakes; 90+% goes to maintain body temperature
• Of energy an an endotherm intakes; Very little goes to growth (~2%)
• Ectotherms devote ~50% of their energy intake to growth

Explore the roles of ice nucleators, supercooling in large water bodies, and adaptations of polar fishes. Understand how organisms prevent freezing and the advantages/disadvantages of ectothermy in various environments, including desert adaptations in iguanas and tortoises.