29-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 29: Animals responding to environmental stress

29-2Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Phenotypic plasticity

• Some organisms can modify phenotype to accommodate changes in environment

– phenotypic plasticity

• Such changes are categorised as– acclimatisation: accommodating several changes in

environment– acclimation: accommodating one change in environment– hardening: acclimation in plants

29-3Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Stresses of the arid zone

• Arid zone of Australia characterised by – high daytime temperatures– extended periods of low rainfall– intermittent floods

• Most animals avoid high temperatures rather than tolerate them

– burrow– active during cooler periods (dawn, dusk, night)

• Most animals do not require drinking water

29-4Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Termites in arid Australia

• Termites build substantial nests above ground• Maintain constant temperature

– thick wood-pulp walls insulate nest– north-south orientation of ‘magnetic’ termite mounds

reduces exposure to midday sun– internal passages allow air to circulate– high humidity is maintained by transporting water from

water table

29-5Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Stresses of low temperatures

• Mountain and polar regions are characterised by – low temperatures– increased solar radiation

• Animals of cold regions are more tolerant of low temperatures than other animals

• Ice formation damages cells by concentrating cytoplasm and dehydrating proteins

29-6Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Do animals freeze?

• Animals in areas that experience low temperatures may

– avoid freezing by supercooling– tolerate freezing

• During supercooling, the temperature may drop below 0°C, but ice does not form in the animal’s tissues

– evacuate body of material that might seed ice formation– produce antifreeze that prevents ice formation and/or

lowers freezing point

29-7Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Insects at low temperatures

• Insect species that live at high altitude tend to be small and wingless

– able to make use of sheltered microhabitats

• Many species exhibit thermal melanism– dark coloration absorbs heat

• Basking in sun increases body temperature– some insects bask in light-coloured flowers that reflect

heat

29-8Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Temperature coefficient Q10

• Biochemical processes take place within a range of temperatures

– rate of activity increases with temperature

• Temperature coefficient, Q10, models the rate of reaction for a 10°C rise in temperature

• Quantifies effect of temperature on biochemical processes

10RT

RT10Q

29-9Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 29.4: Relationship between process and temperature

29-10Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Temperature regulation

• Endothermic animals (birds, mammals) maintain a constant body temperature by deriving heat from internal or metabolic processes

• Ectothermic animals cannot regulate body temperature through those processes, but can reduce fluctuations in body temperature by adjusting behaviour

29-11Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Thermal acclimation

• Although metabolic processes in ectothermic animals tend to increase with temperature, thermal acclimation means that metabolic rate may change between seasons

– cold-water fish may have higher metabolic rate in winter than they have in summer

• Seasonal metabolic compensation– different sets of summer and winter enzymes with

different optimal temperatures– animals may be more active in winter than summer,

despite lower temperature

29-12Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Metabolic depression

• Facing extreme conditions, many animals undergo a reduction in metabolic rate (metabolic depression)

• Some organisms can reduce metabolic rate to less than 1 per cent of normal resting metabolic rate

• Animals survive by dehydrating as larvae or adults, aestivating or becoming inactive

29-13Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Hypothermia and torpor

• Endothermic animals undergo – prolonged hibernation during winter– prolonged aestivation in dry conditions– shorter periods of torpor

• Body temperature is reset to a lower level– hypothermia– metabolic processes drop as a result– decreased responsiveness to stimuli

29-14Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Risks of torpor

• Freezing– use of cryoprotectants such as glucose to prevent

freezing

• Lack of oxygen– many animals can tolerate anoxia

• Exhaustion of energy supply– breakdown of lipids using anaerobic pathways to avoid

using O2

• Desiccation

29-15Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Torpor

• Many small mammals and some birds reduce metabolic rate and enter torpor in response to low temperatures

• Body temperature is regulated during torpor– if it drops too far, animal becomes active for a period

before re-entering torpor

• Blood flow to skin and extremities is reduced during torpor

29-16Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 29.8: Rate of O2 consumption in dunnart

29-17Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Environmental oxygen stress

• Hypoxia is decrease in partial pressure of O2 from normal levels

• Occurs when rate of O2 consumption exceeds replenishment

– caves, burrows– swamps, water-logged soil– tide pools

• Occurs at high altitudes where PO2 is low

29-18Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Adaptations to low oxygen

• Low environmental PO2 reduces the gradient

essential for diffusion of O2 across membranes

• Physiological and behavioural characteristics compensate for low PO2

– tolerance to anoxia– haemoglobin– high erythrocyte counts

– low rates of O2 consumption

– burrow ventilation

29-19Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Responses to high altitude• Lower partial pressure of oxygen in high altitudes

produces altitude hypoxia• Hyperventilation is a response to low PO2

– increases O2 content

• Hyperventilation also eliminates CO2 from body, causing high blood pH

• Erythrocyte count may increase as a result of acclimation

– higher levels of haemoglobin

29-20Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Oxygen stress

• When faced with a thermal gradient, animals have a preferred temperature

– behavioural thermoregulation

• When exposed to hypoxia, animals choose a lower temperature

– hypoxia depresses thermogenesis (metabolic heat production) in endotherms

– set point of body temperature lowered

(cont.)

29-21Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 29.16: Distribution of Daphnia carinata

29-22Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Oxygen stress (cont.)

• Reduced temperature decreases metabolic rate (Q10 effect)

• Decreased requirement for O2

• Reduction in temperature increases O2 affinity of haemoglobin

• Hyperventilation and increased cardiac output are avoided

(cont.)

29-23Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Oxygen stress (cont.)

• Depression of thermogenesis in response to hypoxia is more common in small animals than in large animals

– large animals have a lower mass-specific metabolic rate, so use proportionately less O2 to maintain body temperature

– smaller surface area in relation to body volume means that large animals do not absorb or lose heat as rapidly

29-24Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Global warming

• Increase in temperature as a result of global warming may stress less thermally-tolerant organisms

– increased sea temperatures have a negative impact on penguin species

– migrations of some bird species start earlier in the year– breeding is brought forward or delayed– species’ ranges are extended or retracted

29-25Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 29.18: Mean values of spring phenological shifts