What drives male fish to spray their milt?

Question

In mammals the sexdrive of the males is because the intercourse provides a satisfaction where after, the sperm is ejaculated into the female. However, the male sperm doesn't enter the female fish but reproduces by spraying this fluid which contains the sperm, onto roe (fish eggs). But what kind of (physical) drift is urging them to spray, while they probably don't have an orgasm, or do they?

Answer 1

From a human perspective, it's easy to understand how feelings of sexual attraction and an urge to have sex result in people reproducing. For men, this may be most apparent in the extreme pleasure found at the moment of orgasm, which coincides with insemination. However, there are many behaviors that people have a tendency to perform that don't involve any sort of immediate pleasure comparable to an orgasm. For example, if someone cuts their finger, often their initial instinct is to stick their finger in their mouth. This is a type of wound licking, which is observed in a number of different species of mammals and is believed to promote wound healing due to clotting factors, growth factors, and/or antimicrobial factors present in saliva. However, a young child who cuts his finger sticks it in his mouth without any knowledge of these biological processes. It's a behavior that we are genetically programmed to perform because it promotes healing and reduces the risk of infection, thus promoting survival, thus promoting eventual reproduction. It has therefore been favored by natural selection.

There are many behaviors in animals that are clearly genetically programmed, and which animals perform consistently given the right stimuli (i.e., the right "triggers," whether they be visual or via other senses). Here are a few examples off the top of my head:

In birds, the process of mating and raising young is long, complex, and laborious. The process begins well before and continues well beyond the actual act of copulation (when insemination occurs). This suggests that there is some sort of more general internal mechanism that motivates the birds to go through these costly, time-consuming, stereotypic behaviors, generation after generation, in order to reproduce.

As a quick overview of some of these reproductive behaviors:

Many birds go through the effort of migrating long distances to very specific locations that they use as their breeding grounds and/or nesting grounds. The travel alone is energy-intensive and can be dangerous or even fatal. Mating is often preceded by complex courtship behaviors, sometimes involving elaborate song-and-dance performances by the males (video), in an attempt to impress the typically picky females. In some species, such as manakins, juvenile males may practice their courtship performance (video) for several years before using it to actually find a mate, as the execution needs to be flawless in order to sufficiently impress a potential mate. The young males use their male peers as stand-ins for the female during these practice sessions or rehearsals.

If the female is satisfied with a male suitor, then they may pair up and create a nest. The parent birds build their nests in very exacting and particular ways, even though they themselves don't even live in a nest the rest of the time. It's constructed purely for the purpose of raising chicks, and is abandoned after the chicks leave the nest. Yet the parents have some desire or "push" to build it, and to do so in very precise and structurally sound ways (in terms of climate, location, distance from the ground, materials used, size, shape, etc.). Eggs are laid, and when they hatch, a great deal of effort goes toward feeding the hungry chicks with their gaping mouths and helpless cries for more food, day in and day out, often for several weeks or months. I doubt that the motivation of either the mother or father bird is comparable to the human experience of sexual pleasure, so there is something else that prompts them to go through this ordeal, year after year.

Would we say the parent birds experience intense pleasure that motivates them to do all of this, in the right order, at the right time, and with such precision? It's impossible to know what a bird is "thinking," but remember that there is a danger in trying to project human emotions onto animals, whose brains may function in ways rather different from our own.

Interestingly, the sight of a young bird's open mouth is apparently a strong trigger for the parent bird to "want" to put food in that mouth. Evidence of this includes the fact that some bird species don't build their own nests or raise their own young at all, but rather lay their eggs in other birds' nests (quickly and sneakily, while the parents are away). The parent birds residing at the nest (i.e., the hosts) often don't notice the difference between their real offspring and the intruder that's a different species (i.e., the parasite), even though the difference is often obvious to any human observer. As a result, the the unwitting host parents feed and raise the parasite bird as if it were their own, often to the detriment of their actual chicks. The parasite chick species is often larger and grows faster than the host species, so the parasite outcompetes its host siblings for food (and sometimes pecks them to death or pushes them out of the nest). The parasite often grows much larger than its parents, yet the parents continue to expend lots of energy feeding it, as if they are unable to override whatever program that normally guides their behavior, because the sight of the parasite chick's open mouth is such a strong trigger.

Even stranger, perhaps, is the fact that multiple different species of birds have been observed feeding fish (often koi/goldfish in ornamental ponds or parks) as if the fish were their own offspring. The fish swim to the water's surface and open their mouths, and the sight of this drives certain birds to go find food and bring it back to feed to the fish. According to one source (in the following video), the feedings occurred up to six times per day. Here's that video: A cardinal feeding several hungry goldfish.

Enough about birds, though; let's talk about bees. Actually, not bees, but their close relatives, wasps. Among wasps, parasitoid wasps are a large group estimated to contain over 600,000 different species worldwide. An example that you may have heard of is the tarantula hawk wasp, which attacks, stings, and paralyzes (but does not kill) a tarantula "victim," then drags the tarantula to a prepared burrow/nest location, lays an egg on the tarantula's body, seals off the nest with the paralyzed (but still living) tarantula and wasp egg inside, and then flies off, never to see the offspring again. When the wasp egg hatches, the wasp larva bores a hole into the spider's abdomen, eating the living spider's insides while carefully avoiding the vital organs for as long as possible. This keeps the spider alive as it gets eaten from the inside out, and thus prevents the larva's food from rotting. Interestingly, as adults, these wasps subsist on nectar and plant material. The males don't hunt prey at all, and the females go through the process of attacking tarantulas (which are much larger than themselves) just so that they can drag the spider to the prepared burrow for their offspring to eat later. They don't even witness the fruits of their labor — but they are motivated to go through this process nonetheless.

While the process would seem to require some skill and intelligence on the part of the mother wasp, in some ways it seems overly pre-programmed and inflexible. For example, consider the following description of the behavior of the parasitoid wasp genus Sphex, relayed by D. E. Wooldridge in The machinery of the brain in 1963:

When the time comes for egg laying, the wasp Sphex builds a burrow for the purpose and seeks a cricket which she stings in such a way as to paralyze but not kill it. She drags the cricket into the burrow, lays her eggs alongside, closes the burrow, then flies away, never to return. In due course, the eggs hatch and the wasp grubs feed off the paralyzed cricket, which has not decayed, having been kept in the wasp equivalent of a deepfreeze. To the human mind, such an elaborately organized and seemingly purposeful routine conveys a convincing flavor of logic and thoughtfulness — until more details are examined. For example, the wasp’s routine is to bring the paralyzed cricket to the burrow, leave it on the threshold, go inside to see that all is well, emerge, and then drag the cricket in. If, while the wasp is inside making her preliminary inspection, the cricket is moved a few inches away, the wasp, on emerging from the burrow, will bring the cricket back to the threshold, but not inside, and will then repeat the preparatory procedure of entering the burrow to see that everything is all right. If again the cricket is removed a few inches while the wasp is inside, once again the wasp will move the cricket up to the threshold and re-enter the burrow for a final check. The wasp never thinks of pulling the cricket straight in. On one occasion this procedure was repeated forty times, always with the same result.

On a similar note: Normally, the female wasp grabs the paralyzed cricket by its antenna to drag it into the burrow. However, if both antennae are cut off while the wasp is briefly away, then she is unable to pull the cricket inside and complete her task. She doesn't even attempt an alternative, such as pulling it by one of its legs, even though that would seem like an obvious backup plan to the human mind.

So, what motivates the female wasp to go through all this trouble in this way? Is this even an example of true "motivation," or is it simply a form of genetic determinism in which she is an automaton, unable to modify her actions? It's food for thought.

Answer 2

Assessing the motivation of gamete release in fishes is difficult, but most if not all organisms have a drive to reproduce, which likely instills some feeling of satisfaction if the species is capable of such feelings. From numerous observations of fish spawning, I am not sure that I would use the word spray for the release of milt. Such fishes are only capability of external fertilization, where females either deposit eggs or release them into the water column. Males will release milt either over deposited eggs or pelagically at the same time of egg release. The observed spraying is caused by the release of milt into a fluid that disperses and mixes it in the water column with eggs for fertilization. Water movement coupled with the movement of the male fish results in the perceived "spraying." The functionality of this type of dispersal is to mix the sperm with eggs, both of which are subjected to fluid dispersal. Numerous videos show this behavior in Rowell et al. 2018 supporting info videos https://onlinelibrary.wiley.com/doi/10.1111/jfb.13888 There are numerous other videos out there as well.