|MadSci Network: General Biology|
Natalie, this is a very good question and one that has been discussed by gerontologists (people who study aging) for the past 100 years. While there are some general rules that can be applied to maximum lifespan, they are not concrete and there is much variance from theoretically predicted lifespan to observed lifespan among species. We usually think of aging and senescence when we think of maximum lifespan. But the concept of aging and what constitutes aging has been hotly debated over the years. There is no agreement as to why we age in the first place. You have to remember that most individual organisms never reach a point in the wild when they start to show signs of senescence (I will define senescence as the decline of physiological function over time). However, almost all organisms show signs of aging, with the possible exception of some single-celled organisms. One of the first global generalities we can assign to maximum lifespan is the correlation between fecundity and longevity. Basically, the more offspring an individual of a species is capable of producing, the shorter the maximum lifespan of that species. Evolutionarily speaking, two basic reproductive methods have evolved to increase the likelihood that an individual will contribute their genes to the next generation. The first is called prodigal (or r-selected, where r is the mathematical symbol for the rate of population increase) and is associated with high fecundity and high mortality. The second is called prudent (or K-selected, where K is the mathematical symbol for the carrying capacity of the environment) and is associated with small numbers of offspring coupled with lower mortality. These differences are obvious when one compares flies to humans. The common housefly only lives for a few days but on average lays 120 eggs. In a given year there will be an average of 7 generations of houseflies born. If you had a fly in your house in January that laid its eggs and if all the subsequent generations survived at the end of the year there would be 5,598,720,000,000 flies in your home! A fly has a lot of offspring but doesn't stick around too long to make sure they live. The vast number of flies born will ensure some will live. Humans on the other hand give birth to relatively few babies during their lifetime but spend a lot of energy making sure each offspring survives. This correlation between reproduction and lifespan is obvious when we discuss organisms as different as flies and humans but it generally holds true for more related animals. If we look at only mammals this correlation between fecundity and lifespan also holds true (dogs and cats can have several litters of multiple offspring and they rarely live more than 2 decades) there is a correlation between the age of the animal when it is first capable of reproducing (reaches sexual maturity) and its maximum lifespan. The longer the developmental period (that time between birth and sexual maturity) the longer lived the animal is. Of all animals, humans have the longest developmental period and are among the longest lived of all animals. Another correlation with lifespan is body size. Generally speaking the larger the animal and the larger its brain, the longer lived. This is generally believed to be the result of the metabolic rate and is best exemplified in mammals. Small mammals have a large surface area compared to their weight and lose body heat at a greater rate than a large animal. In order to maintain their body temperature they have a higher metabolic rate (i.e. they burn food faster). This tends to "burn out" their bodies quicker. Indecently, if warm-blooded animals are raised in cold environments they have a shorter lifespan because their metabolic rate is higher; cold-blooded animals show the opposite affect. One of the major exceptions to the body size rule are humans. Humans have a much longer lifespan than their body size should allow (we should live about as long as most pigs). This lead to the idea that brain size must be just as important as body size. Generally, the greater the brain/body size ratio, the longer lived the animal. Humans have the biggest brain to body size ratio and live a long time (accepted maximum is about 122 years). Lastly, there appears to be a correlation with lifespan and the number of hazards in an animal's normal environmental niche; the more hazardous the environment the shorter the maximal lifespan. This may be due to the necessity of reaching sexual maturity quicker in these animals to ensure their reproduction. I hoped this answered your question. If you have any other questions please feel free to contact me.
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