Temperature Regulation in Ectotherms

Presented ByNida Sajjad

Physiology of AdaptationMPhil Zoology

University of Sargodha

Contents What are ectotherms? Ectothermy and poikilothermy Temperature independence of ectotherms Adaptations for cold environment Freeze avoidance and tolerance Adaptations in other ectotherms Advantages and disadvantages Conclusion

What are Ectotherms? Any animal whose regulation of body temperature depends on external sources, such as

sunlight or a heated rock surface.

The ectotherms include the fishes, amphibians, reptiles, and invertebrates.

The body temperatures of aquatic ectotherms are usually very close to those of the water.

Ectotherms do not require as much food as warm-blooded animals (endotherms) of the same

size.

They occur in every ecological niche on Earth.

They are unable to maintain their body temperature as endotherms do.

Poikilothermy And Ectothermy Both are the same thing.

Two terms simply emphasize different aspects of one phenomenon.

Poikilothermy emphasizes the variability of body temperature.

Ectothermy emphasizes that outside conditions determine the body

temperature.

How Ectotherms Achieve TemperatureIndependence? It is actually Independence from maintenance of temperature within narrow

range. Behavioral Adjustments Ectotherms cannot control their body temperature physiologically.But they

can regulate their body temperature behaviorally. Such as desert lizards, exploit hour-to-hour changes in solar radiation to keep

their body temperatures relatively constant. Behavioral patterns help to maintain a relatively steady body temperature of

36 to 39 C while the air temperature varies between 29 and 44 C. The desert iguana of the southwestern US prefers a body temperature of 42

C when active and can tolerate a rise to 47 C, a temperature that is lethal to all birds and mammals and most other lizards.

Metabolic Adjustments

Even without the help of the behavioral adjustments, most ectotherms can adjust their metabolic rates to the prevailing temperature such that the intensity of metabolism remains mostly unchanged. This is called temperature compensation.

It involves complex biochemical and cellular adjustments. These adjustments enable a fish or a salamander to benefit from almost the

same level of activity in both warm and cold environments.

ADAPTATIONS TO COPE WITH COLD ENVIRONMENT

Thermal Environments of Cold-hardy Ectotherms

Avoidance often is an animal's primary means for protection from

extreme temperatures.

If migration to warmer climes is not an option, survival may depend

on finding an overwintering site, or hibernaculum, that insulates from

damaging cold.

For example, some toads and terrestrial turtles, being proficient

excavators, descend into the soil column and overwinter below the

reach of frost. Various snakes and woodland salamanders evade frost

by following abandoned rodent burrows.

Servival benefits at overwintering site

To survive in winter cold-hardy ectotherms must seek thermally buffered sites. Ideal hibernaculum conceals its occupant from potential predators, permits gas exchange,

and prevents excessive desiccation. Some species prefer relatively exposed sites from which they can readily detect

environmental cues stimulating spring emergence. For others, such as plant gall-inhabiting insects and the hatchlings of some turtles there is

no choice in the matter: winter is passed in the very place where one hatches. Even locally, temperatures can range from mild to severe, depending on site characteristics

and the physical features of individual hibernacula, and, being subject to the vagaries of the weather, can vary markedly from year to year.

Within hibernacula, prevailing temperatures follow a pronounced seasonal rhythm.They can detect tolerable temprature.

Freeze Avoidance

Ice-nucleating Agent (INAs) Any agent that promotes the formation of ice crystals. An organism must remain free of INAs to carry out physical phenomenon of supercooling in

a freeze-avoidance strategy. Many inorganic particulates, microorganisms, proteins, and organic residues can organize

water molecules into a crystalline arrangement. INAs occur in overwintering sites of many ectotherms and may enter the body through

orifices or ingested with food. Many freeze-avoiding ectotherms prepare for dormancy by eliminating ingested INAs, and

by inhibiting endogenous ice-nucleating proteins. Physical contact with ice can invade the body and initiate freezing. Species that rely on supercooling for winter survival can reduce the risk of such "inoculative

freezing" by selecting hibernacula that limit their exposure to environmental ice. Ectotherms accumulate one or more cryoprotectants in advance of winter to avoid icing.

All are of low molecular mass and benign in high concentrations

Class Examples Known From

Carbohydratespolyhydric alcohols (glycerol, sorbitol, ethylene glycol); sugars (glucose, trehalose)

bacteria, marine and terrestrial invertebrates, amphibians, reptiles

Amino acids & derivativestaurine, glycine, proline, alanine, asparagine, glutamic acid, lysine

bacteria, marine and terrestrial invertebrates

Methylaminesglycine betaine, glycerophosphorylcholine, trimethylamine oxide

bacteria, marine invertebrates, beetles

Urea   Terrestrial gastropods, amphibians, reptiles

Table 1: Cryoprotectants used in animal freeze-avoidance and freeze-tolerance.

Freeze Tolerance

Freeze tolerance is an adaptation for the survival of tissue freezing under ecologically-relevant thermal and temporal conditions.Found in arthropods, molluscs, nematodes, annelids, amphibians, and reptiles.

It is molecular and physiological responses that limit injury to cells and tissues.

Control of the freezing process is key to survival in ectotherms.

Many invertebrates, which are prone to supercool owing to their small size, freezing is initiated by INAs in the hemolymph or other tissues.

Ectotherms vary markedly in their limits of freeze tolerance.

No ectotherm can withstand the freezing of more than 50-80% of their body water.

Taxon Examples Lower Lethal Temperature (°C)

Marine invertebrates (intertidal)

barnacles, bivalves, gastropods

-20

Terrestrial invertebrates

free-living nematodes, centipedes, flies, beetles, butterflies and moths, wasps

-80

Amphibians salamander, frogs -40 (frogs, -6)

Reptiles turtles, lizard, snake -4

Table 2: Thermal limits of freeze tolerance in animals

More Adaptations in some other ectotherms

BEHAVIORAL In cold weather, honey bees huddle together to retain heat and some vibrate their

flight muscles to generate heat. Butterflies and moths may orient their wings to maximize exposure to solar radiation in order to build up heat before take-off.

Gregarious caterpillars, such as the Forest Tent caterpillar, benefit from basking in large groups for thermoregulation.

PHYSIOLOGICAL Diving reptiles conserve heat by heat exchange mechanisms, whereby cold blood

from the skin picks up heat from blood moving outward from the body core, re-using and thereby conserving some of the heat that otherwise would have been wasted.

The skin of bullfrogs secretes more mucus when it is hot, allowing more cooling by evaporation.

Junonia lemonias is basking under the sun. A six-foot-long black snake basking in the Inverness, Florida sunshine on a cool morning.

Pseudemys turtles basking for warmth are ectothermic.

Advantages and disadvantages

They depend on ambient conditions to reach operational body temperatures.

But endotherms maintain nearly constant high operational body temperatures by internal heat produced by metabolically active organs (liver, kidney, heart, brain, muscle) or by specialized heat producing organs like Brown Adipose Tissue (BAT).

Ectotherms have lower metabolic rates than endotherms because endotherms rely on higher food consumption, and on food of higher energy content. Such requirements may limit the carrying capacity of a given environment for endotherms as compared to ectotherms.

As ectotherms depend on environmental conditions for thermoregulation, that’s why they are more sluggish at night & in early mornings.

Diurnal ectotherms need to heat up in the early sunlight before they can begin their daily activities. In cool weather the foraging activity of such species is restricted to the day time in most vertebrate ectotherms, and in cold climates most cannot survive at all.

In lizards, most nocturnal species are geckos specialising in "sit and

wait" foraging strategies. Such strategies do not require as much energy as

active foraging, and do not require hunting activity of the same intensity.

From another point of view, sit-and-wait predation may require very long

periods of unproductive waiting.

Endotherms cannot in general afford such long periods without food, but

suitably adapted ectotherms can wait without expending much energy.

Endothermic vertebrate species are therefore less dependent on the

environmental conditions and have developed a higher variability (both

within and between species) in their daily patterns of activity.