My first question in this forum so if I have made a mistake I apologize in advance.

My World is Earth, but it's changed based on the following four premises.

  1. In 1000 AD a virus killed 90% of all males of every animal species.

  2. The females were immune but benefited with longer lifespans (four to five times normal) and immunity to all diseases.

  3. 90% of all male newborns die shortly after birth since the virus struck worldwide.

  4. It's now around 1800 AD

Without considering the impact on Humans:

Question: Would the wild animal populations be able to maintain sustainable populations? Would predation make it impossible for wild species to survive?

My assumption is that even one male in ten can still inseminate as many as ten males could, so while population drops, the longer lifespans, and continued breeding maintains smaller but still sustainable populations. I'm not sure of the "math" when it comes to predation.



I received a lot of great feedback, which was instrumental in tuning of the world. Thanks to all those who replied.

In the world I'm building, which is our Earth, a meteor strikes the west coast of north America in 1000 AD. This is a transpermia event in which a kind of virus from some other distant world (far, far away and a long time ago) infects all mammals. The virus weaves itself into the Y chromosome in the germ cells (sperm) so males only pass it to male offspring. It doesn't do anything to the adult host, only its descendants. The virus can infect cross-species and as it does it carries with some of the DNA of the infecting host. This DNA is then somehow incorporated into of the germ cells which are then passed on to male infants.

The virus makes somewhat random attempts to weave in the genetics it carried from the original host to the offspring of the infected resulting in mutations most of which results in miscarriages or death after the birth of the offspring of the male. Those male offspring that survive are chimeras - a mix of phenotypes from different species. The result might be as simple as a horse that grows deer antlers, or it can be far more disturbing.

Again, most of the mutations are fatal, but the virus isn't completely random and does attempt to make changes in line with natural development. When it hits on a success - a male that survives and in turn breeds - it preserves what works but will add some changes to its own offspring.

All the mammals that come in contact with each other exchange chimera viruses resulting in more and more diversified phenotypes in the offspring.

So, for anyone one who was curious or wants to provide more feedback, there you go.

Thanks again to everyone. I'll do my best to help others in the same way in this community.

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    $\begingroup$ Some species have been observed changing sex. How does the virus affect them? $\endgroup$
    – Frostfyre
    Dec 2 '16 at 13:50
  • 1
    $\begingroup$ May I recommend an edit to avoid the trivially obvious answer? Change it to "Would all sexually reproducing species on Earth..." Otherwise the answer is obvious because of all the asexual single celled organisms out there. $\endgroup$
    – Cort Ammon
    Dec 2 '16 at 15:11
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    $\begingroup$ what about hermaphrodites? $\endgroup$
    – John
    Dec 2 '16 at 15:23
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    $\begingroup$ monogamous or endangered species will go extinct very quickly, Its too fast an effect to evolve around. $\endgroup$
    – John
    Dec 2 '16 at 15:26
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    $\begingroup$ In many mammal species - horses, for instance, or lions - one male keeps a herd/pride of females and does most of the mating. ("Batchelor males" get mating opportunities only by being sneaky :-)) So for these species, nothing much would change. $\endgroup$
    – jamesqf
    Dec 2 '16 at 18:39


Some species change sex (including clownfish), often due to lack of a male in the immediate environment. Komodo dragon females can reproduce by parthogenesis, all viable offspring of this will be male. Many insects only briefly have males during breeding season, the rest of the year it's female only. Herd mammals tend to have single males with large groups of females, you'd have to look at each one in turn to see what percentage of males actually reproduced in any year. Cats (big or small) also have one male to many females. The great apes also, in fact anywhere the term alpha male or dominant male is used.

Only pair bonding species, including many dogs, birds and humans are going to suffer.

You ask about predation, but that's unlikely to be a problem. In the case of birds, both predators and prey pair bond. In the case of mammals, the prey tend not to pair bond and most of the predators don't either. You'll probably end up with a massive boom in prey mammals


Seahorses, some frogs and a few birds, the father is the one that cares, the mother lays eggs and wanders off. Some spiders where the male dies or breaks off a penis in the female or have other limiting factors on the number of females they can impregnate. Preying mantises could adapt but would suffer a large loss in population first as not all males heads are bitten off.

Long term

A hundred generations of a specific virus/disease/condition and only the survivors reproducing. Things will settle out to an ever higher percentage of survivors. Eventually all will be forgotten.

  • 2
    $\begingroup$ What on Earth makes you think humans (or dogs FTM) are an inherently pair bonding species? There's plenty of evidence to show that a good many human males are, despite considerable social, religious, and economic pressure, willing to mate with as many willing females as they can find. Indeed, a couple of them have been elected President. $\endgroup$
    – jamesqf
    Dec 3 '16 at 4:39
  • $\begingroup$ DNA analysis shows that 1 in 10 human beings is not the child of their legal/supposed father. $\endgroup$ Dec 3 '16 at 8:26
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    $\begingroup$ Very few "pair bonding" species are truly exclusive. When we talk about "pair bonding" species we generally mean species that sometimes or often form exclusive pairs, instead of rarely or never which is the norm among most animal species. $\endgroup$ Dec 6 '16 at 6:39

You will need to have a good, hard think about the mechanism of your Doom Event, such that it would continue to kill 90% of males, generation after generation.

Assuming you aren't having magic as a factor, and you want a scientifically believable mechanism, then you are looking at, for example, something that has a protective gene on the part of the X chromosome that is not duplicated in the Y chromosome.

And then 1 in 10 males have this protective factor on their Y chromosome, so they survive.

But once they breed, the percentage that is protected will rise rapidly, because only the protected males get to reproduce. In two generations or so, the only male to survive will be the immune ones, and normality will be restored.

Alternatively, say it is a bacteria or virus - the 10% who survive are the most resistant. Their offspring will be more resistant on average, so the next generation, 20% or 30% or even more will survive.

You could say "no, no, it is triggered by testosterone", in which case it would kill the unborn males in utero, when they get their first flush of testosterone and start forming male genitals. But the 10% that survived would be the ones with the lowest testosterone levels at that time, and their sons would also have low levels ...

Evolution rubs its hands at this kind of sorting event. The fittest (those who can survive the sorting) are the ones who breed.

You will need ongoing handwaving if you want evolution to stop functioning as it always has. The 1:1 gender ratio is not arbitrary - there are evolutionary reasons why the ratio tends back to 1:1 any time it gets out of whack.

  • $\begingroup$ And indeed, we see this in the species that have one male per herd/pride of females. Even though only 1 in 10 males get to do most of the mating (I'm oversimplifying the true situation, of course) the sex ratio at birth is still close to 1:1. $\endgroup$
    – jamesqf
    Dec 3 '16 at 18:37
  • $\begingroup$ Actually, bacteria and viruses are very, very bad examples. Neither reproduce sexually, so there are no offspring to inherit resistance. $\endgroup$ Dec 4 '16 at 14:55
  • $\begingroup$ I think you have misinterpreted the answer, @WhatRoughBeast. Bacteria and viruses are discussed as possible CAUSES OF DEATH of males, not as males themselves. $\endgroup$ Dec 12 '16 at 2:00

It seems to work for African lions.  The bulk of their breeding occurs in extended family groups (called “prides”) where the female/male ratio is somewhere around 3:1 to 6:1.  When cubs are mature enough to breed, the males are expelled to live on their own, but the females stay in the group.  The only way a male lion gets to breed is if he kills the dominant male of a pride.  The other males might as well be dead as far as reproduction is concerned.

ISTM that this could work with a 10:1 ratio.  ISTR that there are other species (smaller animals, like birds and insects, maybe? but also deer/moose, sea lions (!) and walruses?) that do a similar thing with a much higher ratio — e.g., out of a generation of 10 or 100 or more individual animals, only one male wins the right to breed with all the females — but I’m not sure about that, offhand.

A couple of issues:

  • In our real world (in Africa), only the top n% (with n ≈ 10?) of the male lions (based on fighting ability) earn the right to reproduce.  The other species that I mentioned operate similarly.  This, obviously, is natural selection — “survival of the fittest”.  If all the males are able to reproduce (because there are only 10% as many), you lose this mechanism for keeping the genome strong.  (Then again, most species allow all animals that survive to adulthood to breed, and that model seems to work, too.)
  • Some male lions kill their fathers (and, perhaps, end up mating with their sisters, aunts, and even their mother).  Most migrate across the countryside, and, if they take over a pride, it’s an unrelated one.  This, obviously, maintains genetic diversity.  If you take the lion model and just stipulate that the male cubs stay in the pride, that might lead to inbreeding.  But that feels like an oversimplification.

YES, depending on how many species you are affecting. If you are counting all plants, microorganisms (for whom the concept of sexuality is ambiguous), and insects then you will collapse the entire food chain. Even if larger animals survive, their ecosystems will be destroyed. The surviving birds won't find appropriate food, gut bacteria wouldn't proliferate properly, fish wouldn't have an intact food chain. That level of constant mortality (if it targeted anything approaching "male") would wipe out so much life the rest would starve for lack of being able to find each other.

But it sounds like you may only be considering larger animals, perhaps only mammals and birds. In which case the lower organisms will persist and the larger ones would have some time to evolve in order to drastically increase male offspring reproduction so that at least some survive. I suspect that for many larger species (like lions, elk, etc) only a few males successfully produce offspring anyway so other animals could replicate their strategy (one male with a harem of females).

  • $\begingroup$ it states only animals. $\endgroup$
    – John
    Dec 2 '16 at 15:25


Cattle already maintain a high bull:cow (male:female) ratio. Expect twenty to thirty cows per bull, so this would still leave bulls in excess.


As discussed in the Dick Francis book Banker, one stallion (male) can cover about forty mares (females) a year. Some manage more. Some less. Allowing for less, a thirty to one ratio seems safe and leaves stallions in excess.


The rule of thumb is one rooster (male) for ten hens (females). But some go with lower or higher numbers of hens per rooster. Eating the excess hens rather than the no longer in excess roosters could bring this back into balance for those who prefer a ratio less than ten to one.


Bee reproduction is self-regulating. Drones (male) come from unfertilized eggs, having just one parent (a mother, a queen). Female bees (worker or queen) come from fertilized eggs (two parents, a drone and a queen). So if nine in ten drones die before mating, bees would make more drones.

You could also exempt bees as "not animals", but even if you do include them, it doesn't end the world. It does increase the egg laying requirements slightly.

Bees are interesting because they are really important for crop pollination. Ending bees might end the world, so it's good to see that that wouldn't happen.


I'm going to assume that any species that has both male and female organs in the same organism will not be affected. So most plants and earthworms would be safe. Also, those sex changing fish. Asexual production for bacteria and algae would be unaffected.

Species die off

It is quite possible that some individual species might not be able to survive this. Some may have consequences. Bees and earthworms were the two most wide ranging problems, but bees should be OK and earthworms would reasonably be exempt. Domesticated species should be able to handle things, as they have excess breeding capacity.


Would the wild animal populations be able to maintain sustainable populations? Would predation make it impossible for wild species to survive?

NO to the first question. Here's why.

The biggest issue I see here is that you've said all Animals. Scientifically, insects are Animals, and they DO have gender. This is a massive problem. Insects are kind of like a base in the pyramid. Take most of the species away, and you've got eco-system collapsing problems. Plants, for instance, depend on them. And many animals eat bugs. This is a bottom up kind of thing.

Now, were I you, I would limit the groups effected to avoid this. So in the Animal Kingdom you have two groups: Vertebrates & Invertebrates.

Vertebrates include these classes: fish, amphibians, birds, reptiles & Mammals

Invertebrates include these classes: all insects, mollusks, crustaceans, corals, arachnids, velvet worms & horseshoe crabs.

I would have the species affected, at the very LEAST be limited to Vertebrates.

Limit it more to make the problem devastating, but maybe not resulting in the complete collapse of Animal-kind. If you leave the bottom of the pyramid intact, it's more likely to survive, but if you just limit it to--mammals for instance, it's still going to be a big deal, but the earth won't become a barren wasteland.

Under Vertebrates, having all fish affected by this might well kill the oceans and water entirely. Contaminating nearly all of the earth's water supply. This would be the fish kill to end all fish kills. The rotting corpses of the boy fishes would result in a massive algae bloom, cutting off the oxygen for most of the remaining fish, who would also die, and contributing to more of a bloom, which will kill off all the coral and all the crustaceans, and pretty much all sea life and any mammal/bird that depends on the sea for food. Also, you know, very little water now available to drink. So massive die-offs on land for that.

Any species that can change gender of course, has a great biologic advantage, and the excluded classes of animals will also thrive in unexpected ways.

I am not sure where predation comes into the equation. There's enough trouble already with what's going on. There will be less predators chasing the prey, because the number of those will also go down.

Your stumbling block is actually genetic stability, not predation. Lots of inbreeding going on here...

EDIT: With the 90% number, there just isn't enough genetic diversity for most animals to survive. Instead, I would vary it according to species. There's really very little out there which would affect the entire biome like this. Even a 50% drop would be huge for most species. A 90% die off that was non-gender discriminating would be tough for a lot of species, but the fact that it's just males actually makes the difficulty of the species surviving even higher. Take a look at Minimum Viable Population research to get you started and keep in mind that most animals don't check to see if they are genetically compatible and many stay to a certain area, so even if a world wide population is high, if the communities are isolated by geographical features, those communities might die off, if the population is too low. Most of that research assumes a normal ratio of M to F, for a reason.


Compared to the others, this is minor on earth, but sessile species, already near their lower population density limits would probably be totally screwed. So I particularly wouldn't want to be a barnacle or mussel, if this were to occur!


It is important to understand the mechanism for why gendered sexual reproduction evolved in the first place. Essentially the idea was that the base requirement for a new offspring is a genetic pairing, from this evolved two evolutionary strategies. Also referred to as evolutionarily stable strategies (ESS.) These two were: if I pack food with the genes it will have more to eat and be more likely to survive (female), and if I put less food in I'll be able to make more sex cells and I'll have more chances to succeed.

From this evolves behavioural ESS. The several stable varieties in which a gender can be very dedicated to their partner or can have several sexual partners. Varies species tend to end up in one quadrant but there is strong evidence that given a significant upset it will change.

I would imagine wiping out 90% of males would count. The result would be essentially every male would have many female sexual partners, in every species. As this would be a uniform blow to all species I would imagine that most extinctions would be avoided.

To address the concern for genetic diversity, you would be only losing around 45% of the population. This is not nearly enough to cause (most) species to go extinct, especially considering genetics, other than sexual characteristics, are distributed in a uniform fashion. If you kill 90% of human males would hair colour genetic diversity really be that affected?

To address the concern of food levels, all species are being effected equally. As a result any predators population would be by ratio decreased the same amount as any prey population.

The main issue with removing that percent of the population would in caste based species. In a social species where males perform a particular task that females don't/can't perform there would be severe damage likely leading to extinction. Offhand I cannot think of any such species.


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