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Current time:0:00Total duration:11:47

Life history strategies and fecundity

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Video transcript

we're going to talk about in this video is what I consider one of them the most fascinating subjects in biology and that's the variation we see from species to species in life histories and lifespans and their rate of reproduction for example we have three different species here on the Left we have an African elephant an African elephant you might know can live a long time especially out in the wild it could live many decades even 40 50 60 years and their life history actually parallels human at least modern human life history in a lot of ways the first ten years of their life they are very dependent on their parents after that they're kind of enter into a bit of an adolescence very similar to how humans do were in theory they could reproduce but they don't tend to and they are still somewhat dependent and then they move into a phase when they do reproduce and they will reproduce on the order of once every two to four once every two to four years a female African elephant will reproduce their gestation period the amount of time that the the baby elephant will be in the mother's womb is on the order it's actually longer than four humans humans you probably knows nine months for an African elephant it is 22 months and so because of that they can reproduce about once every two to four years now another example and these are actually after elephants and and rabbits might not look closely related to you but they are actually still pretty closely related if you think about the entire Tree of Life they are both mammals and we're actually all everything we're considering here are animals we're going to consider African elephant rabbit and we're going to consider salmon but what I'm talking about applies to all life it applies to bacteria it applies to trees that there's a huge variation in in their fecundity the rate at which they reproduce let me write that word down fecundity the Kunda D the rate at which they reproduce and also variation in their actual lifespan whether you're talking about a tree or a bacteria or a fish or a mammal but just going from one mammal to another let's go to a rabbit and depending on on which type of rabbit you're talking about but a rabbit could lifespan is in the single-digit years but unlike an elephant an elephant the first 10 15 20 years of their life they aren't in that reproductive phase of their life a rabbit enters into that reproductive phase of their life within several months within four or five months of birth and so and once they enter into that reproductive phase and I'm showing the reproductive phase in magenta here they can reproduce a lot they have - gun daddy they have a very high reproductive rate every time a female rabbit has a litter it can have it can have many many baby rabbits in it the numbers I found were one to fourteen one to fourteen rabbits and not only can they have one to fourteen rabbits every time they have a litter but they can have a they can they can do this on the order of once a month so every every month so even though the lifespan of that female rabbit depending on which type of rabbit you're talking about might be it might be three four five six years depending on the type of rapid you're talking about you can imagine if they're producing let's say ten ten rabbits every month per year they could produce 120 rabbits or if they can produce 10 rabbits per month 12 months a year that's 120 rabbits a year over several years and then you can imagine those rabbits very quickly the female ones the the if we assume roughly half of them are female that half can very quickly get into that reproductive phase and then start reproducing at a similar rate so on an individual level a female rabbit has - daddy and then as on a on a population level that group of rabbits will also have very very high fecundity and then we could look at another example and this is the example of salmon and there's many types of salmon but the general way that salmon the the general life cycle that salmon go through is they are born then and they are usually born up some stream and usually some water that is that where there there isn't a strong current and then once the baby salmon are born and they could be born in groups of hundreds or thousands they make their way down that River down that stream into the ocean and then they have many years of a growth phase in the ocean where they get larger and larger they're not reproducing then and then when they are ready to reproduce they fight their way back up the same stream that they were born in or the same river that they were born in they fight their way back up to it and they reproduce and this is both the males and the females the the male's fertilize the females produce the eggs the male's fertilize the eggs and then they die so they have one reproductive event so you have one reproductive event and then death and then they kill that they just die and people are still understanding why exactly does this happen so one reproductive event reproductive event and then and then they die and there's actually a technical term for species that do this the salmon isn't the only one where they have that one they go out with you can kind of view it as a Big Bang where they they have that one reproductive event where they might have hundreds or even thousands of eggs but then they die this is called Cemil parity let me write this down so this is called Cemil Cemil parity Cemil comes from the latin for once parity comes from the Latin for to beget so to beget once you're reproducing once and then in the case and then in the case of salmon you are dying and you might say okay if that Cemil parity what would we call an elephant or rabbits rabbits for short and elephants as well that can have multiple reproductive events well there that is called it Errol parity it hero it Errol parity you might have heard the word iterate that means to repeat something or to do something over and over again it Errol is the root for and means repeat so it Errol parity beget repeatedly so that's what animals like elephant and for sure rabbits are actually doing and what's fascinated about all of this is and it is a question that I've I've wondered many you know since I first realized when I was young that Wow why is there so much variation here is why has nature selected for or why have these species found niches in which they can operate in which it makes sense where natural selection has selected for these very different lifespans these very different reproduction rates this variation in fecundity this you know sometimes it erro parity sometimes Cemil parity and it is a bit of it's not a mystery people are studying this and they have good hypotheses but we don't know for sure especially from species to species and a framework you could use to think about it is is a a species they're trying to optimize survival and not even of the individual they're trying to optimize survival of really their genetic information that's what and it's not like the species or the genes are actively trying to do it but natural selection is is doing that for them so let's call this box a natural selection natural selection and so what you have coming out of this is the fittest fittest genes and when we talk about fittest genes we're not talking about somehow that some are better than others we're just saying for that environment the ones that seem though the the genes that produce the traits that are most suitable to survival and most suitable towards reproduction and then the inputs that are going into this natural selection box are things like availability of energy of food of well I'll call it free energy availability it's not just obviously plants can get that free energy from the Sun availability ability of energy we could talk about the predatory environment predatory predatory environment we could talk about disease disease every moment that an organism is alive it has to worry about these things it has to worry about finding food or competing for food it has to worry about predators it has to worry about disease and once again the individual organism is not sitting there it's not necessarily that these salmon are like oh I hope I don't catch a disease or or they might not even you know you be stressed about the Bears that might try to grab them as they go upstream but these are the factors that play into how how or how how what gets selected for I guess is is is the best way to phrase it and in terms of from a species point of view the various dials well these are things like reproduction like what is it what does the species decide to do given these constraints and so the various dials are fecundity actually let me write it this rate of reproduction age of reproduction when these are related age of reproduction things like lifespan and these are also all related in some way lifespan growth growth health and a species and an organism and is making trade-offs all of the time do we do you know the salmon goes through that huge phase where it's deciding to apply most of its energy towards growth and survival and then all of a sudden it kicks into another gear where it actually uses a lot of that energy that was stored up to go upstream and it goes into a reproductive phase and then it dies and natural selection has has this has happened arguably because that somehow helps the Salmons DNA to spread more maybe somehow it adds nutrients to the water or you know they put all of that energy to go upstream so that their offspring will have an easier time going downstream but there's also other trade-offs you could have things that lay a salmon might a female salmon might lay thousands of eggs but very few of those actually make it make it through the full cycle they asked me I've seen is out of those thousands of legs eggs that get laid only about three make it back this was the example I saw for sockeye salmon on average only three of them make it back for the next year so you have a huge amount of I guess you could say attrition while in the case of an elephant they invest more per offspring and you have a much higher probability that each of those offspring will survive so there's all sorts of interesting trade-off to think about when you think about life history a lifecycle lifespan and things like fecundity and how organisms reproduce
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