Why isn't it a big bang?
In biology, "big bang" stands for the more familiar kind of banging:
...For many species of fish, insects, and plants, a single reproductive episode occurs before dying: this strategy is termed as semelparity. For others, such as almost all mammals, reproductive episodes are spread over the course of their lifetime: a strategy called iteroparity. Evolutionary ecologists have tried to find explanations for this difference in life-history. The first investigation into these strategies was by Cole (1954) who compared the intrinsic growth rates of semel- and itero- parous species and concluded: "for an annual species, the absolute gain in intrinsic population growth which could be achieved by changing to the perennial reproductive habit would be exactly equalent to adding one individual to the average litter size. Considering the ease at which an organism could increase offspring number by one, Cole reasoned that selection should favor semelparity.
...In nature, however, iteroparous species abound; this apparent contradiction between theoretical prediction and natural occurrence has been known as Cole’s paradox. The possible selective advantages of iteroparous strategies are that growth rate is not always the most important factor in the growth and maintenance of a population, and that selective pressure toward an iteroparous strategywould be generated when survival from zygote to first maturity became uncertain. Charnov and Schaffer resolved the apparent paradox by showing that optimal life history is a function of both fecundity and the relative survival rates of juveniles and adults. Thus, relatively low adult survival and high juvenile survival are expected to select for semelparity, and the reverse conditions should select for the iteroparity. http://www.nature.com/scitable/knowledge/library/semelparity-and-iteroparity-13260334
The problem with the demographics arguments is that a change in the survival rates can easily change the fine balance, whereas the reproductive pattern is much harder to change. Cephalopods have reproduced for hundreds of millions of years while their ecology changed dramatically over this period of time. Yet all of them are semelparous. Unlike the evolution theorist, an animal cannot study survival odds and decide whether to turn on or off built-in mechanisms for obligatory senescence. On the time scale required to develop such mechanisms, the conditions would surely change beyond recognition, upsetting the very odds that are claimed to be the deciding factor for the chosen reproductive strategy. Naturally, the empirical support for such "smart" theorizing is thin to nonexistent.
Strategically speaking, semelparity makes a lot of sense for animals with low parental investment: all available energy is exerted in a spectacular bang, which yields plenty of offspring, after which the parents are redundant. Many invertebrates reproduce in this way, and the familiar examples tend to be Pacific salmon or insects, that is, animals that are not like us, mammals. It is considered to be nearly self-obvious that mammals should be iteroparous by default, because parents are needed to care for their young. This is correct, but that does not require both parents. In fact, some marsupials (e.g., opossums) do have semelparous males. http://bio.research.ucsc.edu/people/doaklab/extras/xtreme/Cockburn01.pdf
After a short rut, the males die off before the females give birth. The body of the males secretes corticosteroids, and they die of acute gastrointestinal haemorage, in horrific pain. Why? It used to be believed that this secretion increases catabolism of stored protein into sugars to provide calories for the final burst of sex activity.
...The argument goes that the small size of these marsupials (they weigh up to 300 g), together with winter food shortages, means that they can't lay down enough energy reserves in the form of body fat. By using body protein as an energy source instead, the males can survive long enough to mate. The physiological consequences, however, are severe, and include a general decline in body condition, as well as impaired immune and inflammatory systems. Moreover, the increased concentrations of stress hormones are usually associated with gastrointestinal ulceration. It is these consequences that ultimately kill the animals. http://www.nature.com/nature/journal/v410/n6830/full/410758a0.html
However, there are marsupials in which catastrophic senescence is not triggered hormonally or preceded by weight loss. Another "explanation" is that high female mortality drives extreme male promiscuity (to improve one's chances of proliferation) which in turn required going all-out, which in turn led to semelparity.
http://rspb.royalsocietypublishing.org/content/270/Suppl_2/S251.full.pdfThe problem with such rationales is that these are almost impossible to test.
How about the placental mammals? There is a semelparous woodrat that lives in the Death Valley, CA. It is believed that the extreme heat stess (it is 46 C mean temperature in July) forced this rat to evolve semelparity. The rats grow during the fall and winter, mate, and then die by the early summer, because only the smallest rats can survive the scorching heat. These woodrats have remained in the valley from a cooler era; instead of developing dwarfism to stand the extreme heat, they've developed semelparity instead. http://biology.unm.edu/fasmith/Web_Page_PDFs/Smith_Charnov_DV.pdf
Does semelparity have anything to do with fitness and natural selection -- or is it, as some claim, a phylogenetic burden that has no adaptive value (it is retained because evolution can't always find a way out of the set reproductive pattern)?
...is this life-history strategy a (perhaps unwanted) legacy from an ancestral species? Or is it an adaptation to specific, and as yet undefined, environmental or ecological circumstances unique to these groups? If semelparity is to be expected as a result of a phylogenetic predisposition, then subsequent evolution, constrained by history, may tend to be limited to making the best of a bad job. But if semelparity is an adaptation, what benefits does it provide, and how did it evolve? Sexual selection might be the driving force: females might be biased towards young, vigorous males, meaning that there would be little reproductive benefit to the survival of older males. Alternatively, circumstances might be such that the intensity of fighting between males during the rut means that only the youngest and fittest can win and survive to mate. The cost of such fighting might be reduced lifespan. These ideas, although speculative, illustrate the need for further studies to determine which ecological or evolutionary circumstances promote this extreme, and extraordinary, reproductive tactic in a relatively large mammal.
http://www.nature.com/nature/journal/v410/n6830/full/410758a0.html
I think it might be the legacy pattern; the current odds do not really matter. There is no existing "reason" why some mammals are semelparous whereas most of us are not. We could've all been like these opossums, and the picture below illustrates the selective pressures the later mammals have been spared.
I consider it to be my luck that somewhere on the way from marsupial to placental mammals, one of my ancestors switched from semelparity to iteroparity, for reasons remaining obscure. Looking at the octopi one realizes that it is entirely possible that intelligence could be locked in a male body that is programmed to self-implode after the one and the only mating bonanza. When it happens, deleterious mutations affecting later life accumulate because there is no "later life" and this "later life" gradually becomes impossible.
Imagine semelparous mankind... Creepy, isn't it?
Yet, logically, this can happen. In the olden times, people made many children over their entire (admittedly, short) life span and their iteroparity was enforced, by stretching the reproductive cycle: many mating episodes worked against deleterious mutations leading to high mortality in the older age, as the ones carrying such mutations had lower fecundity towards the end of the reproductive age and so were outbred. Now many people have only few children in 1-2 births that occur relatively early in life. In this way, deleterious mutations affecting health past this de-facto reproductive age can be passed to one's children, as there is no selective pressure against such mutations. This is the classical scenario for developing semelparity. What occurred under the extreme duress can also be introduced artificially. We (at least, men) might be joining the octopi.
Why is there no "big bang" for humans? Is it probable that we'll become semelparous?
...For many species of fish, insects, and plants, a single reproductive episode occurs before dying: this strategy is termed as semelparity. For others, such as almost all mammals, reproductive episodes are spread over the course of their lifetime: a strategy called iteroparity. Evolutionary ecologists have tried to find explanations for this difference in life-history. The first investigation into these strategies was by Cole (1954) who compared the intrinsic growth rates of semel- and itero- parous species and concluded: "for an annual species, the absolute gain in intrinsic population growth which could be achieved by changing to the perennial reproductive habit would be exactly equalent to adding one individual to the average litter size. Considering the ease at which an organism could increase offspring number by one, Cole reasoned that selection should favor semelparity.
...In nature, however, iteroparous species abound; this apparent contradiction between theoretical prediction and natural occurrence has been known as Cole’s paradox. The possible selective advantages of iteroparous strategies are that growth rate is not always the most important factor in the growth and maintenance of a population, and that selective pressure toward an iteroparous strategywould be generated when survival from zygote to first maturity became uncertain. Charnov and Schaffer resolved the apparent paradox by showing that optimal life history is a function of both fecundity and the relative survival rates of juveniles and adults. Thus, relatively low adult survival and high juvenile survival are expected to select for semelparity, and the reverse conditions should select for the iteroparity. http://www.nature.com/scitable/knowledge/library/semelparity-and-iteroparity-13260334
The problem with the demographics arguments is that a change in the survival rates can easily change the fine balance, whereas the reproductive pattern is much harder to change. Cephalopods have reproduced for hundreds of millions of years while their ecology changed dramatically over this period of time. Yet all of them are semelparous. Unlike the evolution theorist, an animal cannot study survival odds and decide whether to turn on or off built-in mechanisms for obligatory senescence. On the time scale required to develop such mechanisms, the conditions would surely change beyond recognition, upsetting the very odds that are claimed to be the deciding factor for the chosen reproductive strategy. Naturally, the empirical support for such "smart" theorizing is thin to nonexistent.
Strategically speaking, semelparity makes a lot of sense for animals with low parental investment: all available energy is exerted in a spectacular bang, which yields plenty of offspring, after which the parents are redundant. Many invertebrates reproduce in this way, and the familiar examples tend to be Pacific salmon or insects, that is, animals that are not like us, mammals. It is considered to be nearly self-obvious that mammals should be iteroparous by default, because parents are needed to care for their young. This is correct, but that does not require both parents. In fact, some marsupials (e.g., opossums) do have semelparous males. http://bio.research.ucsc.edu/people/doaklab/extras/xtreme/Cockburn01.pdf
After a short rut, the males die off before the females give birth. The body of the males secretes corticosteroids, and they die of acute gastrointestinal haemorage, in horrific pain. Why? It used to be believed that this secretion increases catabolism of stored protein into sugars to provide calories for the final burst of sex activity.
...The argument goes that the small size of these marsupials (they weigh up to 300 g), together with winter food shortages, means that they can't lay down enough energy reserves in the form of body fat. By using body protein as an energy source instead, the males can survive long enough to mate. The physiological consequences, however, are severe, and include a general decline in body condition, as well as impaired immune and inflammatory systems. Moreover, the increased concentrations of stress hormones are usually associated with gastrointestinal ulceration. It is these consequences that ultimately kill the animals. http://www.nature.com/nature/journal/v410/n6830/full/410758a0.html
However, there are marsupials in which catastrophic senescence is not triggered hormonally or preceded by weight loss. Another "explanation" is that high female mortality drives extreme male promiscuity (to improve one's chances of proliferation) which in turn required going all-out, which in turn led to semelparity.
http://rspb.royalsocietypublishing.org/content/270/Suppl_2/S251.full.pdfThe problem with such rationales is that these are almost impossible to test.
How about the placental mammals? There is a semelparous woodrat that lives in the Death Valley, CA. It is believed that the extreme heat stess (it is 46 C mean temperature in July) forced this rat to evolve semelparity. The rats grow during the fall and winter, mate, and then die by the early summer, because only the smallest rats can survive the scorching heat. These woodrats have remained in the valley from a cooler era; instead of developing dwarfism to stand the extreme heat, they've developed semelparity instead. http://biology.unm.edu/fasmith/Web_Page_PDFs/Smith_Charnov_DV.pdf
Does semelparity have anything to do with fitness and natural selection -- or is it, as some claim, a phylogenetic burden that has no adaptive value (it is retained because evolution can't always find a way out of the set reproductive pattern)?
...is this life-history strategy a (perhaps unwanted) legacy from an ancestral species? Or is it an adaptation to specific, and as yet undefined, environmental or ecological circumstances unique to these groups? If semelparity is to be expected as a result of a phylogenetic predisposition, then subsequent evolution, constrained by history, may tend to be limited to making the best of a bad job. But if semelparity is an adaptation, what benefits does it provide, and how did it evolve? Sexual selection might be the driving force: females might be biased towards young, vigorous males, meaning that there would be little reproductive benefit to the survival of older males. Alternatively, circumstances might be such that the intensity of fighting between males during the rut means that only the youngest and fittest can win and survive to mate. The cost of such fighting might be reduced lifespan. These ideas, although speculative, illustrate the need for further studies to determine which ecological or evolutionary circumstances promote this extreme, and extraordinary, reproductive tactic in a relatively large mammal.
http://www.nature.com/nature/journal/v410/n6830/full/410758a0.html
I think it might be the legacy pattern; the current odds do not really matter. There is no existing "reason" why some mammals are semelparous whereas most of us are not. We could've all been like these opossums, and the picture below illustrates the selective pressures the later mammals have been spared.
I consider it to be my luck that somewhere on the way from marsupial to placental mammals, one of my ancestors switched from semelparity to iteroparity, for reasons remaining obscure. Looking at the octopi one realizes that it is entirely possible that intelligence could be locked in a male body that is programmed to self-implode after the one and the only mating bonanza. When it happens, deleterious mutations affecting later life accumulate because there is no "later life" and this "later life" gradually becomes impossible.
Imagine semelparous mankind... Creepy, isn't it?
Yet, logically, this can happen. In the olden times, people made many children over their entire (admittedly, short) life span and their iteroparity was enforced, by stretching the reproductive cycle: many mating episodes worked against deleterious mutations leading to high mortality in the older age, as the ones carrying such mutations had lower fecundity towards the end of the reproductive age and so were outbred. Now many people have only few children in 1-2 births that occur relatively early in life. In this way, deleterious mutations affecting health past this de-facto reproductive age can be passed to one's children, as there is no selective pressure against such mutations. This is the classical scenario for developing semelparity. What occurred under the extreme duress can also be introduced artificially. We (at least, men) might be joining the octopi.
Why is there no "big bang" for humans? Is it probable that we'll become semelparous?