Hard SF : Aliens : Alien Reproduction
We can make the best guesses about what intelligent extraterrestrials would be like in areas related to their minds – for the simple reason we are discussing life forms with intelligence. Using definitions of life, evolution and intelligence, we can estimate some likelihoods. But only knowing something is alive and intelligent won’t give us much to go on to guess about it’s breathing apparatus, its digestive system or its reproduction. It does seem a fairly safe guess that reproduction will be an important part of life on any world. What sort of reproduction they employ may, then, influence the thinking of extraterrestrials, perhaps in different ways but being as major an influence as sex is in the brains of humans.
Although the two-gender reproduction on Earth has shown itself to have its benefits, that does not mean evolution on other worlds cannot find other successful options. Below are speculations on other possibilities, although maybe rare ones.
In a two-gender system there could be a "sperm-laying" gender. This gender could deposit a sperm capsule (what we might think resembles a lizard egg). Pheromone-type chemicals attract a "wombed" gender who inserts the sperm capsule into its womb. In the womb, the capsule could either have a reproductive cell from the womb attach to the capsule for fertilization, or the capsule could open, releasing sperm-like cells to search for something to fertilize.
Perhaps, it would be possible to get more-or-less the benefits of multi-gender reproduction with just one gender. Suppose their equivalent of DNA has two kinds of data, A and B. The A strand defines the current individual. The B strand is structured differently, indicating the values for each gene its ancestors have had. For instance, it might say that possible values 1, 3, 7, 12, 13 and 18 have been used by ancestors. When creating an offspring, it will use a method to choose one of those tried-and-true values for that gene from the B strand. Then choose between the A strand gene and the B strand gene using a method analogous to how Earth life chooses between the gene from the sperm and the gene from the egg. This would give the individual options other than duplicating itself or random values. It would allow using values for genes each of which the current individual or some ancestor who successfully reproduced had, but not necessarily the particular combination of genes previously used by a single ancestor. If all the gene choices are to come from one individual and its ancestors, in order to produce a variety of gene values to choose from such a species might need to have a higher mutation rate.
Mutual impregnation by a species of hermaphrodites might be possible. It has the advantage / disadvantage that you get two children for the price of one mating (although at the price of two gestations). This could potentially make both mates more vulnerable during gestation; perhaps evolution would find it desirable to have local groups of the species act as a community that helped support those gestating. Would such a species be more likely to group in families of two adults and two children, two families of one adult and one child, or something more communal?
More than two genders may be possible, but I suspect the complexity involved in that would discourage its development. The need for three or more adults to be involved would restrict the number of successful matings to some degree. Choosing between copies of genes from three or more mates would be a more complex process. It might potentially give more chances to select a superior gene or to end up with a mutant gene. Perhaps on a planet where the mutation rates of early life had been relatively high or low such a mechanism would evolve to play a moderating role. As a whole, it seems extra genders would be uncommon.
There might be a logic to pheromone-like attractants drawing young females to the oldest males (those who have demonstrated the longest survival). However, the oldest females, even if technically fertile, may not be in the best health for pregnancy. Perhaps on some world reproduction would evolve a way for old females to place their embryo [either like reptile eggs or like the immature kangaroo going into the pouch] in the womb / pouch of a young female for the gestation period. If that was possible, it could maximize use of both males and females who had survived longest. There are two issues with the "old female’s egg in young female" approach. First, this only works as a group strategy, it doesn’t work for the young female’s genes that want to copy themselves. Also, there could be an issue of the young female’s immune system not accepting the foreign egg.