Animal Physio Lecture 4

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University of Toronto Scarborough
Biological Sciences
Jason Brown

Lecture 4 Temperature Homeothermy: Bobcat the environment changes but body temperature relatively constant. Endothermy: Via metabolic heat production. Generate large amounts of heat and use that to regulate body temperature. Fish and reptiles. Bees, insects. Largely mammals and birds UNIQUE can produce enough even at rest since they don’t use skeletal muscle. Use tissues other than skeletal muscle tissues to produce heat. Can be endothermic even when they are not using their skeletal muscle and homeothermic because can produce large amounts of heat, use that as to regulate their body temperature, even when at rest. By contrast, insects, types of fish, reptiles, they are generating metabolic heat via skeletal muscle. Only able to use that metabolic heat contribute to body temperature with their active. Birds and Mammals come from separate evolutionary lineages. There last common ancestor was diverged 325 MYA. Leading us to only a mammal lineage and a reptile lineage, and the birds evolved a reptile lineage- “birds are classified as part of reptile lineage”. Both groups still evolved independent the homeothermic and endothermic trait. Common ancestor of mammals and birds could have been homeothermic but birds kept it when diverged. Reptiles lost homeothermic. Birds gained homeothermic. But mammals and birds evolved independently. Mammals and Birds must faster rate of metabolism. Eat a lot more because convert that into heat and using it as energy. Man sitting on step, Alligator. Same body size (60kg animals). BMR of a human is higher than alligator. [BMR and SMR is a similar term, one just applies to human & the other to alligator] BMR is very high in mammals and birds. Alligators low SMR, don’t produce a lot of heat to regulate body temperature. EX: if you and the alligator decides to run, the alligator would be able to keep up with you approximately one minute, because the capacity to generate energy, capacity to consume oxygen, capacity to do work, is about the same in the mammals and reptiles in very short durations. But if you decide to run a marathon with the alligator, chances are that you will be the alligator because over longer durations we can maintain higher rates of metabolism than can the alligator. This is the advantage to high BMR. High resting capacity to convert energy to heat or work and using this for long durations of activity to sustain long activity. Alligator have a very low standard MR and low capacity then to ramp up the metabolism on any kind of sustained basis. They can ramp it up on very short term using largely anaerobic pathways, don’t have aerobic capacity to do work over long time (burst of metabolism for long periods of time) like how mammals are birds do. Question: thinking back to evolutionary time, the origins of homeothermy/ endothermy, was this ability to produce lots of heat evolved first, and then later sustain the activity to do in longer periods of time? Or was it the opposite? So, able to produce lots of MR to do activity for a long duration, and then the consequence, where they would start producing heat. Question: Why did the mammals and bird evolve to have a high MR in the first place? Thermoregulatory Hypothesis Natural selected acted directly to increase body temperature and/or internal thermal stability via incremental increases of metabolic rate. The major advantage for homeotherms and endotherms was that the body temperature constant and high and this was achieved by being warmer providing advantage. Animals had slight increase in metabolic rate which slightly warmed them up... gave advantage over other predators, and they were more successful. What would those advantages be? Having constant body temperature advantage: Create membranes and enzymes that allow you to function at that one temperature. Avoid fungal infections. Most fungal grow 0-30 degrees but over 30 degrees growth is inhibited and that’s where most body temperatures for mammals ad birds exist (30-45). So another advantage could be that high body temperature minimizes fungal infections. Experiment to test hypothesis: Took lizard and measured when he wasn’t eating and his metabolic rate (low) and body temperature(high body temperature 30 degrees). Test specific dynamic action… Feed reptile to increase MR so body temperature should go up. Problem for hypothesis if reptiles increase MR it wasn’t increasing in body temperature. Not an incremental increase, huge increase. The hypothesis is dead, not true, not reason body temp high in mammals. To address flaws. Put fur coat on lizards. Not increase in Mr that increased body temp but they just got more and more insulated. Also unsuccessful fur coat didn’t increase body temperature. Alternative hypothesis: Aerobic Capacity Natural selection acted directly to increase maximum metabolic rate (carry out metabolism long term basis) and associated behaviours (capturing, and avoiding predators); increased basal metabolic rate evolved as a correlated response Idea was natural selection favours higher metabolic rate and increase BMR evolved in correlated response which led to indirect result of higher body temperature. Test this hypothesis: swimming mice Took mice pop and threw them in water where they swam. Took mice that could swim the longest. So had control population and a population of mice that could swim well. They measure maximum MR while swimming of the two different groups. Control had a lower max MR than the swimmers. Seemed that selection regime was efficient in the best swimmers than the control mice. Did same with wheel, ones selected to be good runners had high MR. But is there an increase in BMR? No regardless control or treated, BMR was same between both. So natural selection could’ve increase max MR( catch prey more easily) but necessarily didn’t increase body temp or BMR. Parental Care Hypothesis Recent. Other major trait that mammals and birds share in common is that they offer parental care relative to most animals. Natural selection directed to increase both increase BMR and body temp because these two together increases likelihood that offspring would survive. Experiments: Mammal and Bird parental care before offspring is born. For example the cat is trying to keep embryos/fetus warm. The bird is keeping the eggs warm. Incubating leads to better development, avoid disorders. If you take a cat embryo, and incubate at a cold temperature, you tend to get more spontaneous disorders occurring, than if you take that same embryo and incubate it out at warm temperatures. Therefore, advantage to cats and birds. Advantage to having high body temp because it could incubate embryo and decrease developmental abnormalities and speed up development. So advantage to having high body temp if you’re going to give parental care. The faster you can get the milk to the cows mouth, the faster the cows grow, more likely that cow will survive. The faster the bird to get food for its offspring, the more likely they are going to survive. Advantage here is that did not have to pick temperature of MR, takes both equally important. Criticism: why would you have high BMR and body temp if no parental care. Only works for females who incubate offspring. Males don’t need it because they are no incubating embryos or producing milk. No hypothesis why mammals and birds evolved homeothermy and endothermy, Why is body temperature at 37 degrees? Not typical of all mammals and birds. Hedgehog a little lower. Birds (small birds) have higher than mammals 39-42. All generally the same though. Why did natural selection favour a high temp? 37 degrees allows reaction rates to run quickly but beyond that point enzymes denature. Certain bacteria that can have enzymes working optimally well at 70 degrees. All enzymes that denature at 37 is not true. PCR reaction, uses enzymes from microbal bacteria because they are alive at desire temperature (high temperatures likea round 70-80 degrees). Change the question to: Why not have a body temperature of 80 degrees? Map: Mean distribution of temperatures on globe in January. Below 0. If you had body temperature at 80 degrees in Toronto in winter. Problem? YES. Have this 80 degree thermal gradient between body and environment. Lose heat so have to eat to maintain body temp that high. Just like if you have a leaky house at home, it would be absurd to say to turn the thermos to 80 degrees. Why is that absurd? Well you will be losing too much heat out the windows, doors.. heat is leaking. Spending fortunates on your gas bills just to keep power it. So it makes sense, if you get a leaky house, you would turn the heat down a bit to maybe around 20 degrees. Keep this gradient (between my house and the environment) low as possible. Similarly, keeping the gradient between your body temperature and the environment as low as you can. Why not have my body temperature ten or 20 degrees? Map of july, temperature at 25 to 30 degrees. So if I have my body temp at 20 degrees, so that means during the winter time I won’t have to lose so much heat to the environment. Problem: warmer outside then body, heat constantly moving in my body through conduction. Only option to get rid of the excess heat coming in is evaporation; is to sweat or pant. Problem to sweat or pant is that it takes a lot of water. Constantly losing water, dehydrate. Idea is that body temperature should be set just above the likely maximum environment temperature than what animal is going to face. Small gradient between self and environment and body temperature always above ambient temperature. Can shed heat through non evaporative ways (conductance, convection, radiation). Bird over large range of ambient temperatures (from 40-50C). Over 90 degree range, its body temperature remain relatively constant. So if its -41 degrees out, the bird’s body temperature would be 40 degrees no matter what the environment temperature is. Let’s look at the metabolic rate. Its metabolic rate (purple) get U- shaped pattern. At 5 degrees up to 35 degrees its MR is constant over entire range of temperatures. This range is called the thermoneutral zone. For mammals and birds, the thermoneutral zone is the range of ambient temperatures of which the MR remains constant. In this range of temperatures over at which the metabolism doesn’t show any change. Once you get lower at thermoneutral zone, or once the temperature is above the thermoneutral zone, metabolism increases on both ends. So the thermoneutral values are valid through 2 critical values. The lower critical temperature and the upper critical temperature. Once the ambient temperature drops below the low critical temperature, metabolism starts to increase in this ambient. Once the ambient temperature the above the upper critical temperature, metabolism starts to increase. So why is this a case? How can we explain the body temperature, metabolism and the ambient temperature. Body temp of any animal is equal to average ambient temperature (that’s the starting point; you can’t be cold nor warm) plus whatever heat produced by metabolism divided by thermal conductance of animal. This equation also helps understand, the U shape relationship. Tells us that we can’t have a constant body temp value despite a change (no change in materbolic rate) if we do modify how thermoic conductive we are to the environment. In order words what animal are doing in their thermal neutral zone, how conductive they are? How fast are they losing heat to their environment. Lower critical temperature represents the minimum conductance, so we’re maximmaly insulated at this point. Upper critical temperatures represents the minimally insulated we could be. Once you pass those points, once I get insulated as fast as i could be, and now if it gets even colder yet, i have no choice but to increase my MR heat production. How can a mammal regulate its body temp by changing its thermo conductance to the environment? Pilometer /tilomotor response. (hair) Left arm goosebumps keeping hair stand up. Mouse hai
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