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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
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