Essay C, Student 10, Marker 4
Describe and discuss the influence of environmental temperature on metabolism
in poikilotherms and homeotherms.
Animals can be grouped in many ways. One way is to separate them depending
on their method of thermoregulation. This is how an animal maintains its internal
temperature. Animals that have a relatively constant internal temperature and are
known as homeotherms, from the Greek ‘homoios’ meaning similar and ‘thermos’
meaning temperature. This stability requires internal heat generation that comes
from the animal’s metabolism. Animals whose body temperature varies
depending on the external environment are known as poikilotherms, from the
Greek ‘poikilos’ meaning varied or changeable.
In general mammals and birds are considered to be homeotherms and
amphibians, reptiles and fishes are considered poikilotherms. There are some
animals that show characteristics of both; for example, some deep-sea fish have
a constant body temperature their entire life, varying only by +0.5˚C, as the
ambient temperature varies very little. Larger fish like some species of shark and
tuna are able to maintain certain parts of their body at temperatures up to 10˚C
above the ambient temperature. (Sadava et al, 2011) The red swimming muscles
are warmed by a complex circulatory system in which arteries and veins run
parallel allowing the arteries to return heat from the working muscle back into the
muscle. This increases power and speed, giving the fish a distinct advantage
over smaller prey animals that do not possess this system.
Some homeothermic animals show poikilothermic behaviour. For example,
hibernating mammals act as homeotherms in the summer and during hibernation
in the winter their metabolic rate decreases and their body temperature drops to
near the ambient temperature. Animals which display both homeothermic and
poikilothermic behaviour as known as heterotherms.
Homeotherms have a higher metabolic rate than poikilotherms even though both
groups’ metabolic rate varies as the external temperature changes. As ambient
temperature decreases the metabolic rate of poikilotherms also decreases.
Essay C, Student 10, Marker 4
However, as ambient temperature decreases the metabolic rate of homeotherms
increases, producing enough heat to prevent a decrease in its body temperature.
The metabolic rate decreases as ambient temperature increases to the lower
critical temperature when metabolic rate is at its lowest, known as the basal
metabolic rate. This temperature varies among animals; it may be as low as –
30˚C in some large arctic mammals or as high as 30˚C in sloths and other
tropical mammals. (Hill, Wyse & Anderson, 2008) As the ambient temperature
increases past the upper critical temperature the animal’s metabolic rate
increases. Above the upper critical temperature the animal must actively cool its
body. Animals may sweat or pant. This cools the body down by evaporation.
When there is a plentiful supply of water to the animal this is an effective method
of cooling. However, in areas where there is no source of water, cooling by
evaporation can be dangerous as so much water is lost from the body. Animals
in hot, arid climates use this only as a last resort. Many animals in hot climates
have large ears that pass heat from the animal’s blood into the surrounding air.
Conversely, animals in colder climates tend to have smaller ears.
If the ambient temperature decreases below the lower critical temperature the
animal must create heat to warm up the body. In a cold environment, animals
may produce heat by shivering. This is very energy intensive and, as the animal’s
metabolic rate is already high, these animals must have a very high food intake
to replenish the lost energy. Aquatic mammals prevent heat loss into the
surrounding water by having a thick layer of fat underneath the skin.
Between the upper and lower critical temperatures, the body temperature is
regulated by blood flow to the skin.
Many species go into torpor, a state of low activity and metabolic rate, in order to
save energy while avoiding difficult conditions such as extreme temperatures or
low availability of food. Daily torpor is exhibited in some species of birds and
small mammals. Hummingbirds have an extremely active lifestyle and must save
energy wherever possible. They go into torpor at night and their body
temperature may decrease by 25˚C or more. (Campbell et al, 2008). Hibernation
Essay C, Student 10, Marker 4
is a long-term torpor in which an animal goes into torpor to avoid the winter. The
animal’s body temperature decreases to close to the ambient temperature as the
metabolic rate decreases. Upon arousal, when the animal awakes, the metabolic
rate increases and heat is produced to warm the body to normal temperature.
Poikilotherms have no way of regulating their internal body temperature other
than using and external heat source and for this reason they cannot survive
extreme temperatures for long periods of time. Terrestrial poikilotherms risk
desiccation if they stay out in the sun too long and cannot absorb sufficient
energy in cold environments. Thermoregulation is much more difficult in aquatic
poikilotherms as the thermal conductivity of water is twenty-three times that of air
heat is lost much faster in water than air.
Homeotherms are able to keep their body at an optimal temperature in order for
enzyme-catalysed reactions to occur at the optimum rate. Extreme ambient
temperatures lead to the animal’s death by hypothermia (extreme cold) or
hyperthermia (extreme heat). This highlights a disadvantage of being
homeothermic. These animals have a narrow enzyme temperature range so a
fluctuation of only a few degrees Celsius from the optimal internal temperature
results in the denaturation of the enzymes. Poikilotherms have a much wider
enzyme temperature range so their internal temperature can fluctuate much
more without any damage to the body. Due to the ability to maintain a constant
body temperature, homeotherms can live in colder climates and at higher
altitudes than poikilotherms. They can also be nocturnal.
The metabolic rate of a homeotherm at basal level is still between four and ten
times higher than a poikilotherm of equal mass and internal body temperature.
Perhaps because of this much lower metabolic rate, poikilotherms (especially
reptiles) tend to live much longer than homeotherms of the same size. Certain
species of tortoise have life spans of well over a hundred years.
Essay C, Student 10, Marker 4
References:
Campbell, N. A. et al. (2008) Biology, Eighth Edition. Pearson Benjamin
Cummings, San Francisco.
Hill, R.W., Wyse, G. A. & Anderson, M. (2008) Animal Physiology, Second
Edition. Sinauer Associates, Inc., Sunderland, Massachusetts.
Sadava, D. et al. (2011) Life: The Science of Biology, Ninth Edition. Sinauer
Associates, Inc., Sunderland, Massachusetts.