Lecture 10

Lecture 10: Temperature Regulation

Reading: Economy of Nature, pp. 320-323.

Functional Significance of Endothermy

	The energy costs of avian-mammalian endothermy is ten times greater than the cost of ectothermic metabolism for animals of the same body mass. Ectotherms are very efficient in their energy use.
	Endotherms can sustain longer periods of active metabolism than can similar sized ectotherms, but the cost of endothermy applies at all times including resting metabolism.
	Endotherms have accelerated development rates and begin reproduction earlier than ectotherms of the same body size. Age at first reproduction is shown as a function of body mass for ectotherms and endotherms. (Figure from Downhower and Blumer, unpublished)

AFR_vs_BW.jpg (29062 bytes)

Temporal Responses to Temperature

Behavioral and Physiological Responses to environmental conditions or resources

Acclimatization: Response to multiple factors (under natural conditions)

Acclimation: Response to single factor (under laboratory conditions)

Both acclimatization and acclimation (collectively termed acclimation by Ricklefs) involve changes in behavior, structure, physiology, and biochemistry in response to continued prevailing environmental conditions or resources. When environmental temperatures change acclimatization or acclimation can result in a change in metabolic rate responses to temperature.

Swimming speed of goldfish changes with ambient temperature, but the position of the swimming speed versus temperature curve is dependent on the temperature at which the fish were living (acclimated) prior to the test (Ricklefs, 1996, p 219, Fig. 10.12, also see p 76, Fig. 3.17).

Acclimation occurs in ectotherms, such as goldfishes, and in endotherms as shown below for ptarmigan. Metabolic rate responses to temperature are different in summer and winter-acclimated willow ptarmigan (Ricklefs, 1993, p 189, Fig. 10.11).

Acclimation responses to temperature are also observed among plant species (Ricklefs, 1996, p 220, Fig. 10.14).

Acclimatory responses are defined as reversible and therefore plastic, but some responses to environmental conditions or resources are irreversible. Irreversible responses are part of the developmental process in plants and animals.

Growth and photosynthetic rate responses by seedling trees change with the availability of light. Observed differences between sun and shade-grown seedlings are developmental responses (Ricklefs, 1996, p 221, Table 10.2).

Characteristic	Shade-grown	Sun-grown
Percentage of dry weight, 6-month old seedlings
Roots	35	52
Needles	47	37
Stems	18	11

Photosynthetic rate, 4-month-old seedlings
Low light intensity (500 fc)*	1.9	1.0
Moderate light intensity (1,500 fc)	4.6	4.0
High light intensity (4500 fc)	7.2	6.6
fc = footcandles Source: After F.H. Bormann, in D.V. Thimann (ed.), The Physiology of Forest Trees*. Ronald Press, New York, (1958), pp. 197-215.

Both acclimatory and developmental responses to environmental conditions and resources cause phenotypic variation in populations that are a consequence of differences between the environments in which organism live. Understanding the causes for variation is important both for interpreting the significance of differences between individuals in one species, and evaluating the potential for natural selection and evolution. Recall that only variation that has a genetic component can yield evolution by natural selection.