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Today I read a BBC Report about how Pablo Escobar had once imported 4 hippos (1 male, 3 female) into his estate in Colombia for his private zoo. After his downfall, while other species were shipped out, hippos were considered too big to move and expected to not survive.

However, to the surprise of all the hippos are thriving and are so numerous that there have been calls to cull them. From the report:

Numbers are projected to only get bigger. [Colombian biologist Nataly] Castelblanco and her peers say the population will reach over 1,400 specimens as early as 2034 without a cull - all of them descended from the original group of a male and three females. In the study, they envisaged an ideal scenario in which 30 animals need to be culled or castrated every year to stop that happening.

My understanding is that since there was only 1 male, the gene pool would be limited and lead to lot of inbreeding in the descendants. This would cause population to not explode because some individuals would be unfit to survive.

Why has this not happened in case of hippos? Is it because there are 3 females (probably unrelated to each other) which keeps gene pool large enough? Or can mutations explain this phenomenon? Would results have been different if originally 2 females had been moved and only 1 retained?

EDIT - I have just found that current population is maybe less than 100 individuals which though big is not massive.

EDIT2 - I have edited the question title to keep focus on the hippos although a general answer would be welcome.

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    $\begingroup$ It is definitely not the case of all invasive species but in sunflowers at least, there is a tendency of invasive lineages to reproduce clonally (Bock et al, 2018) $\endgroup$ – Remi.b Feb 11 at 8:16
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    $\begingroup$ It happens in other species. Hamsters, for example. Every golden hamster alive today is descended from 1 mother and a litter found in Syria in the 1930's, and there's millions (maybe billions?) of them due to their popularity as pets. And hamsters don't live nearly as long as hippos, so we're talking far more generations, and a longer timespan, and that genetic bottleneck hasn't been a problem yet. $\endgroup$ – Darrel Hoffman Feb 11 at 16:12
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    $\begingroup$ Keep in mind, it usually takes several generations for inbreeding to show its effects. The first couple of generations won't be that inbred at all, but as the inbreeding coefficient rises and the average individual is more and more related to one another they are more likely to show ill effects. Hippos only reach sexual maturity at about age five to six so there have only been a handful of generations born in Colombia as of now. $\endgroup$ – user2352714 Feb 11 at 18:01
  • $\begingroup$ @DarrelHoffman at least according to wikipedia, that's not quite true about golden hamsters - only domestic ones (maybe only north american? wording unclear) and it also has a glaring "citation needed" $\endgroup$ – llama Feb 11 at 18:33
  • $\begingroup$ @llama Well, domestic hamsters are by far the majority of them - apparently a few have been found in the wild since, but nowhere near the numbers of domestic ones. Either way, even if it is only North America, we're still talking about millions of hamsters descended from one litter. $\endgroup$ – Darrel Hoffman Feb 11 at 18:37
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I think one of the important things to understand in thinking about this case is that it just hasn't been that long, generationally.

Escobar imported the hippos in the late 1980s. Hippos reach sexual maturity at an average of about 7.5 years old for males and 9.5 years for females, space births about 2 years apart, and live for 40-50 years. Thirty years out, this means that only about 3 generations have passed for females. Moreover, the dominant males are highly territorial, which means that much of the breeding might even now be still coming from the initial male and not his descendants. Still, the initial male breeding with his own daughters would indeed produce a significant inbreeding coefficient.

The impact of inbreeding, however, is also affected by the density of deleterious alleles. In geographically restricted species, the higher natural level of inbreeding can result in purging selection that leads to a much lower frequency of accumulated deleterious alleles than in highly social and gregarious species like humans and dogs. Hippos appear likely to be such a species, as well as having overall lower genetic variation than other large African mammals, suggesting a fairly recent expansion.

In short: it hasn't been that many generations, and if the inbreeding sensitivity of hippos is indeed fairly low to begin with, it may indeed be that there simply isn't any significant impact from inbreeding at this point in time.

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    $\begingroup$ Its also important to understand inbreeding does not guarantee a die out, it increases the chances of it. Its like being a heavy smoker makes lung cancer much more likely but does not guarantee you will get lung cancer. $\endgroup$ – John Feb 12 at 15:22
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    $\begingroup$ Note that at least according to the BBC report "Indeed, conditions in their South American home seem so ideal for the hippos that studies show they start reproducing at earlier ages, she said." where "she" refers to Nataly Castelblanco - co-author of the article referenced in MikeSerfas' answer. So they may well be beyond the 3 generations suggested in this answer. $\endgroup$ – Dragonel Feb 12 at 18:02
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They often do. You hear a lot about all of the times invasive species succeed in invading a new habitat, because there is a surviving population around for researchers to observe. There are lots of cases where new species are introduced to an environment in a single event but fail to survive. Rabbits and foxes were repeatedly introduced to Australia, only for most of the events to fail. The fact that there are rabbits and foxes there now is a consequence of the sheer number of times Europeans tried to introduce them. There are also many cases where captive animals escape captivity but fail to establish a breeding population, everything from "alien" big cats to coatimundis in Oklahoma.

This has actually been suggested to be one reason you don't see a lot of carnivores on islands. Carnivores might be able to disperse to islands, but they do so infrequently and the islands can't support their population that they end up dying out and we never know there was a dispersal event in the first place.

It's worth mentioning as well that many invasive species events probably aren't a "Noah's ark" type situation where you have only a couple of surviving adults. Many of the fishes that invaded the Great Lakes either did so in large numbers through the Welland Canal or were introduced in large number as eggs (smelt, salmon, some suspect alewives could have been introduced this way). Ballast ships can contain thousands of genetically distinct larvae and propagules of algae, small fishes, and invertebrates. Most deliberate introductions (e.g., rabbits and foxes in Australia) usually try to introduce enough individuals that inbreeding won't be a problem. Many invasive plants or feral domesticates (Asian carp, wild boar) were bred in captivity in large numbers and merely became wild.

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    $\begingroup$ The links you have posted are interesting! If I understood correctly, the second and third paras of the answer deal with overpopulation and multiple introduction events. As the question is specifically about dying out due to inbreeding (not over population) and from a small gene pool (rather than multiple individuals), maybe you would like to tackle those aspects as well? $\endgroup$ – RedBaron Feb 11 at 11:07
  • $\begingroup$ I have edited the question title to make it clear that I am more interested in why the hippos thrived. $\endgroup$ – RedBaron Feb 11 at 11:13
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    $\begingroup$ This is answering the opposite question. The question presumes that they should have died out like the examples you give, and asks why they didn't. $\endgroup$ – Barmar Feb 11 at 15:51
  • $\begingroup$ I'm almost tempted to purchase the rabbit article (just not $35 worth of tempted), as I was under the impression that a single release was the source of mainland rabbits. $\endgroup$ – Michael Richardson Feb 11 at 16:20
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    $\begingroup$ I don't think he's thinking they represent the rule, he seems surprised about it. And he's wondering how they achieved this miracle. $\endgroup$ – Barmar Feb 11 at 17:59
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There are some references on this: Castelblanco-Martinez, 2021 is a recent one. There doesn't seem to be much doubt at least that hippopotami are thriving as an invasive species.

I didn't see anything on the genetics, but I will give in to the temptation to speculate even if perhaps I shouldn't. There are some advantages that are likely working against the effect of inbreeding depression:

  • being an invasive species with perhaps fewer diseases and predators or more assured access to resources that in their native range.
  • artificial selection to avoid deleterious alleles, if particularly healthy specimens were chosen for transport to South America by people who really did not want to leave their customer dissatisfied.
  • outbreeding enhancement ("hybrid vigor"), perhaps, if the founders were brought in from widely separated populations as a collector might tend to prefer. Since we care about genetic diversity, not the literal number of founders (nor their sex, much) this directly opposes the bottleneck circumstance.
  • mundane issues: perhaps the females were imported pregnant, or an escaped hippo was surreptitiously replaced by a terrified lieutenant at his own expense.

Given these advantages, the hippos might do at least as well as the wisent, which was already facing extinction (suggesting harsher circumstances) before being reduced to 12 individuals, but has since begun to recover.

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To focus on the inbreeding:

  • Inbreeding in itself is not a factor that would make species die out. Rather, inbreeding increases the risk of genetic diseases, if the deleterious alleles are already present.
  • Three females do not represent such a large genetic pool, so that inbreeding would still occur.
  • Given that there have been only few generations since the introduction event, there were obviously no outright deleterious alleles present, while it is too early to talk about dicrease in overall fitness.
  • Restoration of populations of nearly extinct species from small populations has been the focus of the conservation movement for over several decades. In many of the cases where a species was brought back to life we are talking about severely inbred populations. Notably, the modern population of Przhevalski's horses is claimed to descent from one ancestor.
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Animals don't die out from any causes related to themselves, their breeding, or anything like that. They die out either because their ecological niche goes away (climate change, etc.), because something else outcompetes them in that ecological niche (introduced fish outcompeting cichlids, for example), or because an external force exterminates them (dinosaurs and a damn great rock).

Inbreeding doesn't stop breeding - it just means they're more prone to certain developmental problems. If you look at pure-breed dogs for example, all breeds are inbred because that's the very definition of a "pure-breed". The result is that they're all very prone to issues with joints, eyes, arthritis, and more obscure issues like "cocker spaniel rage syndrome" (yes, that's a real thing). None of this stops dogs from breeding though.

In the case of the hippos, they've been dropped into an ecological niche which is perfect for them, and which has no predators capable of limiting their numbers. If given time and limited to that area, their numbers will naturally become limited by overgrazing and starvation. But before that, they'll have made a complete mess of an environment which hasn't evolved to deal with hippos in it. Of course inbreeding means they'll develop some genetic issues which cause many individuals to be unhealthy in various ways, but it won't stop the herd from continuing.

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Some individuals being unfit to survive, doesn’t necessarily lead to extinction, it leads to removing the traits that make them unfit to survive from the breeding pool. From a species POV this is a good thing.

This leading to extinction happens when so many individuals have the trait, that those without it can’t can’t find a breeding partner to have children that don’t have the trait.

Critical in that happening is the trait being present in the breeding pool AND it making them unfit in the their current environment. For instance, a trait that made them unfit to face off against lions or hyenas wouldn’t matter, if they aren’t fighting something that is at least similar to them.

Take snakes in Guam, lots of them probably came from similar small lots, but without predators, it doesn’t have so much of an immediate impact. Of course that doesn’t mean they will be successful in the long term either, as they are likely to run out of prey and starve to death.

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