All living things reproduce. Organisms on Earth have evolved a dizzying array of reproductive habits and anatomy. I'm just going to look at the very basics in this post, and next time we'll look at how reproduction might affect the mentality and society of alien beings.
Strategies: Biologists have identified two basic reproductive "strategies" for living things on Earth — though of course there are plenty of variations within the broad outlines. Basically one can classifty species as "K-strategy" or "r-strategy" in reproduction. (And yes, the K is capitalized but the r is lower case.)
K strategists put a great deal of effort into supplying and supporting their offspring, trying to guarantee their survival and thereby maximize reproductive success. Obviously this emphasis on "quality" means K strategists tend to have fewer young. Elephants, humans, and orcas are K strategists, with immense parental and social investment in the care of young, and typically give birth to only one or a few offspring at a time.
Conversely, r strategists emphasize quantity. They have large numbers of young — sometimes extremely large numbers — and as long as one or two survive to adulthood, it counts as success. Maple trees, lobsters, and shad all have enormous numbers of offspring. Trees release thousands of airborne seeds, lobsters fill the water with gametes and hope they find each other, and shad lay thousands of eggs at a time. Once they've released their young into the world . . . they're on their own.
Obviously there are intermediary or relative cases. Compared to most fish, mammals are obsessive about caring for their young, but among the mammals you have humans and elephants devoting years to raising a single offspring while possums have a dozen every year.
Sex: The vast majority of organisms on Earth reproduce very simply. They split in half and the two new entities go their separate ways.
Asexual reproduction is universal among single-celled organism but it shows up in complex multicellular creatures as well. Often it can persist alongside sexual reproduction in the same organism. Many plants, for instance, can not only send out seeds or create fruit, but also bud off clones of themselves from roots or branches.
Sexual reproduction requires two parents (and there are reasons why two is likely to be the maximum) who combine their genes to create a distinct new organism. They don't ever have to see each other, but their gametes do. Sexual reproduction is slightly more difficult to arrange than asexual, but the greater genetic diversity resulting from sex means that when conditions change the species can evolve to adapt faster.
Sexual reproduction has two main modes. In external fertilization, organisms release their gametes into the outside environment and hope that complementary gametes will find them. As one might expect, this is typically part of an r strategy approach, since there isn't really any way for a parent to care for its young when it has no idea where they might be or whether they even exist.
Conversely, internal fertilization needs the parent organisms to actually come into physical contact. This has benefits and drawbacks: mating is more of a "sure thing" than just spewing pollen into the air and hoping it finds its mark, but of course this means an organism must locate a potential mate and secure its cooperation.
As I mentioned, there is a staggering variety of mating arrangements among animals, and there is no way I can summarize them all in a blog post.
Males produce "cheaper" or less energy-intensive gametes, though typically their gametes are more mobile. The goal is to maximize the number of chances to get a successful fertilization.
Females produce gametes which have a support system — either in the form of a yolk sac for eggs, nourishment from her own body for live-bearing females, or other mechanisms. Their gametes have a better chance to gestate successfully.
Some species — like snails — are hermaphrodites, with both male and female gametes. They mate and both get fertilized eggs. Others shift from male to female depending on conditions.
Gestation: Again, there is tremendous variation, but how the young develop can be divided into live-bearing and egg-laying.
Live-bearing organisms gestate their young within the mother's body. This is a comparatively K type method, though some mothers can support dozens of young at a time. But live-bearing represents a huge investment in the survival of the offspring, and is often accompanied by some degree of post-natal care by the parent.
Egg-laying organisms (and, if you squint, seed-bearing plants) release the young in a container of some kind, along with a supply of nutrient which can support the young through development until hatching. As one would expect, it lends itself to r strategy approaches, although there are plenty of counter-examples of egg-layers who lavish care on their young.
Care: Humans are probably the kings of giving care to young, keeping ours around for a decade or more before they're ready to live on their own. Most other mammals spend only a year or two at it. Even so, caring for one's young is a large investment of resources but pays off with a higher survival rate. Often this requires the development of complex social structures so that members of a pack or herd can help feed and look out for the young ones.
Hives: The social insects (and a few other species) have come up with a method that combines the massive birth rate of an r-strategy organism with the high survival rate of a K-strategy species. Essentially one or more "queens" does nothing but reproduce, while her sterile sisters and daughters work to get food, defend the hive, and care for the young. Since they're all related, the workers' genes are carried on via their fertile sisters. I'm listing it as a "parenthood" strategy rather than a social system, because it's pretty hard-wired. You're not going to see ants leaving their hive and going off to raise a family alone.
As I keep repeating, this just scratches the surface, and a little research into the variety of life on Earth will teach you about much weirder reproductive systems than anything you can imagine on your own.
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