User:BarredOwl.00/Ectotherm

An ectotherm (from the Greek ἐκτός (ektós) "outside" and θερμός (thermós) "hot") is an organism in which internal physiological sources of heat are of relatively small importance in controlling body temperature. Such organisms (for example, frogs) rely on environmental heat sources which permit them to operate at optimal metabolic rates.

Some ectotherms live in environments where temperatures are practically constant, such as regions of the abyssal zone of an ocean, and can be regarded as homeothermic ectotherms. However, in environments where temperature varies so widely as to limit physiological activities, many species habitually seek external sources of heat or, in contrast, shelter to cool down. For example, many reptiles regulate their body temperature by basking in the sun or seeking shade when necessary. In addition, most ectotherms regulate internal temperatures through behavioural thermoregulation mechanisms.

In contrast to ectotherms, endotherms rely largely, even predominantly, on heat from internal metabolic processes, and mesotherms use an intermediate strategy.

Pseudemys turtles (shown here basking for warmth) are ectothermic.

In ectotherms, fluctuating ambient temperatures may affect the body temperature. Such variation in body temperature is called poikilothermy, though the concept is not widely satisfactory, and the use of the term is declining. In small aquatic creatures such as Rotifera, the poikilothermy is practically absolute, but other creatures (like crabs) have wider physiological options at their disposal. These creatures can move to preferred temperatures to avoid ambient temperature changes or moderate their effects. Some ectotherms have demonstrated sensitivities to temperatures during developmental stages, seen as variations to their growth rates and adult body size. Ectotherms can also display the features of homeothermy, especially within aquatic organisms. Normally, their range of ambient environmental temperatures is relatively constant, and few attempt to maintain a higher internal temperature due to the high associated costs.

Temperature regulation strategies

Lizards are ectotherms and use behavioural adaptations to control their internal temperature.  The red line represents air temperature, the purple line represents the body temperature of the lizard, and the green line represents the base temperature of the burrow. If they need to raise their internal temperatures, they will seek out the sun at the top of their burrow and return to their burrow once internal temperatures become too high.

Various patterns of behaviour enable some ectotherms to regulate internal body temperature quite successfully. In cold weather, honey bees huddle together to retain heat. Butterflies and moths may orient their wings to maximize body surface exposure to solar radiation to build up heat before take-off. Gregarious caterpillars, such as the forest tent caterpillar and fall webworm, benefit from basking in large groups for thermoregulation. Many flying insects, such as honey bees and bumblebees, raise their internal temperatures endothermically before taking flight. They do so by vibrating their flight muscles (see insect thermoregulation). Such endothermal activity is an example of the difficulty of consistent terms such as poikilothermy and homeothermy.

In addition to behavioural adaptations, physiological adaptations help ectotherms regulate internal temperature. Diving reptiles conserve heat by heat exchange mechanisms. 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. Another example of physiological thermoregulation is how bullfrogs secrete mucus from the skin when their internal temperature is too hot, thereby allowing more cooling by evaporation.

Overwintering species

During periods of cold, some ectotherms enter a state of torpor. Their metabolism slows or, in some cases, such as the wood frog, effectively stops. The torpor might last overnight, for a season, or even for years, depending on the species and circumstances.

Ectotherm animals have adapted two main strategies to survive below-freezing winter seasons or cooler than average temperatures. Some frogs, toads and salamanders can survive months of freezing temperatures without a heartbeat or functioning organs.The two main strategies include freeze tolerance and freeze avoidance, also known as supercooling.

Freeze tolerance

Freeze tolerance includes the production of ice nucleators and cryoprotectants within the body. These properties protect cells from damage and dehydration while a portion of the body’s water is converted to ice. Freeze tolerance is a strategy best used when species can endure colder temperatures for longer durations. This is more true with invertebrate ectotherms than vertebrates, possibly due to the ability of invertebrates, such as insects, to excrete built-up wastes more readily. Some frogs can survive months of freezing temperatures without a heartbeat or functioning organs. The wood frog (Rana sylvatica) uses natural antifreeze to protect its tissues from ice build-up and survives winters in Alaska with below-freezing temperatures. When temperatures drop below 0 degrees Celsius, frost will form on the frog, initiating it to pull water from its extremities to its core, where it will later freeze completely. Over time, the ice will cover the entire amphibian. The heart eventually stops beating, and the organs stop functioning. Once ice covers the whole frog, as much as 65-70% of the water in its body is completely frozen. As soon as warmer weather returns, overwintering ectotherms unthaw and return to their regular functioning activities. Some species use this strategy to survive over winter, though some use it to endure long, cooler seasons. For example, the eastern box turtle (Terrapene Carolina) uses freeze tolerance to maintain activity through cooler months.

The Eastern box turtle uses a freeze tolerance strategy to survive months of cooler than average temperatures.

Freeze avoidance

The second strategy is freeze avoidance, or supercooling. This strategy allows body fluid to remain liquid in a broader range of freezing temperatures. It is accompanied by the formation of cryoprotectants and ice nucleators and physiological responses that protect the body from freeze stress. Freeze avoidance has the disadvantage of potentially undergoing spontaneous freezing, as the body liquid is supercooled. For example, hatchling turtles may survive supercooling at -8 to -18 degrees Celsius before spontaneously freezing.

A few exothermic species can use either strategy, such as the lizard, Lacerta vivipara, in which individuals at the same overwintering area have been observed using one strategy or another. A beetle species, Dendroides Canadensis, has been observed alternating between these two strategies on a yearly basis, depending on the annual conditions.

Aquatic species

A look into thermoregulatory strategies revealed that salamander larvae are thermally passive in their native habitats providing a rare reported case of thermoconformity in temperate aquatic amphibians. Thermoregulation is also vital to fish and can be seen through thermoregulatory behaviour such as heat gradient navigation to reach their optimal body temperature. Finally, there is less effective thermoregulation in aquatic than in terrestrial taxa.

Thermal sensitivities of development

Ambient temperature can alter growth rates, reproduction maturity and body size of ectothermic animals. Not all ectotherms are affected by ambient temperature during development, but variation in some juvenile growth and mortality rates can occur as a result of subsequent high or low-temperature exposure.

Junonia lemonias is basking under the sun.

During high ambient temperatures, some juvenile development is accompanied by increased growth rates and early maturation. The resulting adults will be smaller in size (as seen with the butterfly, Lycaena hippothoe). The effect of the smaller body size of the adult is unlikely to be detrimental to the survival of the animal, but it may be the result of biophysical restraints on the animals’ cell growth. Growth rates of the common lizard, Lacerta vivipara, increase when reared in hot environments due to higher internal body temperatures. Early maturation for juveniles can be advantageous if their reproduction season is short. The ability to mature early and increase reproduction rates during a short window of opportunity will increase the chances of passing on genetics to the next generations.

Some juvenile ectotherm growth rates slow in environments with low ambient temperatures, thus resulting maturation takes longer to reach. The resulting adults, though slow to mature, develop larger bodies than juveniles raised in high temperatures. A larger body size is typically indicative of higher fecundity. However, the decreased growth rates of ectotherms in cold environments limit the ability to produce offspring earlier. The extended window for mortality may hinder the passing of genetics if maturation is not reached in time for reproduction. Nonetheless, there may be higher benefits of maximum fecundity when maturation is eventually reached, as reproduction events are often less frequent in colder environments. Larger body size may indicate better parental care, thus resulting in a higher percentage of surviving offspring. As well, larger adults typically produce offspring of larger size.

Pros and cons

A 1.8m southern black racer basking in the Inverness, Florida sunshine on a cool morning.

Ectotherms rely largely on external heat sources such as sunlight to achieve optimal body temperature for various bodily activities. Accordingly, they depend on ambient conditions to reach operational body temperatures.

Endothermic animals maintain nearly constant high operational body temperatures largely by reliance on internal heat produced by metabolically active organs (liver, kidney, heart, brain, muscle) or even by specialized heat-producing organs like brown adipose tissue (BAT). Ectotherms typically have lower metabolic rates than endotherms at a given body mass. Consequently, endotherms generally rely on higher food consumption and commonly on food of higher energy content. Such requirements may limit a given environment's carrying capacity for endotherms compared to its carrying capacity for ectotherms.

Because ectotherms depend on environmental conditions for body temperature regulation, animals in cooler climates are typically more sluggish at night and in early mornings. When they emerge from shelter, many diurnal ectotherms need to heat up in the early sunlight before beginning their daily activities. Therefore, most vertebrate ectotherms are restricted to engaging in foraging activities only during the day. For instance, most nocturnal species of geckos rely on "sit and wait" foraging strategies (see ambush predator). Such strategies do not require as much energy as active foraging, and they do not require hunting activity of the same intensity. For ectotherms, sit-and-wait predation may require very long periods of unproductive waiting. Endotherms cannot 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 activity patterns.