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I've heard of claims that aphids can be born pregnant, like tribbles in ''Star Trek''.

However, doing a web search didn't provide very convincing evidence.

The Straight Dope refers to being pregnant before birth as paedogenesis. The Wikipedia article on paedogenesis has no citations on pregnancy before birth (it does, however, have a citation on male offspring eating its mother!). It also states that the term also refers to animals that get pregnant before they're fully sexually mature.

Wikipedia's article on Aphids refers to the phenomenon as Telescoping generations, which is a single paragraph stub article that has no citations.

Searching for paedogenesis in pubmed only got 8 hits. (Is pubmed an appropriate tool for such a search?)

Are animals being born pregnant well-established but little discussed, or is is yet to be well-established?

(Note: I'm referring to organisms being pregnant to their own offspring, rather than accidentally being pregnant with their own siblings)

  • en.wikipedia.org/wiki/Parthenogenesis Happens in quite a few organisms, vertebrates as well. Gynogenesis is similar, but I don't think it can occer in the womb/egg etc. – user7473 Jun 9 '12 at 11:40
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    @user7473 parthenogenesis doesn't usually count. While these creatures may have a child without mating they are usually not actually pregnant at time of birth. Most pathogenic creatures grow through their normal life cycle and only once full grown will they conceive and begin their pregnancy. There are also creatures that are born pregnant that are not an example of parthenogenesis (see my comment describing the fascinating adactylidium on ESultanik's post; that example actually involves sexual reproduction so it's not parthenogenesis) – dsollen Aug 22 at 13:46
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A good tool to use for this is Google Scholar. The first hit revealed this article which claims

Most aphids are born pregnant and beget females without wastrel males. ... Embryos complete development within the mother’s ovary one after another, in assembly line fashion. These developing embryos contain developing embryos of the third generation within them, like Russian dolls.

The second hit is an interesting set of slides on the subject, called "Aphid biology and life history: Implications for management.", and says "Live birth, born pregnant"

  • a more interesting/horrible example is Adactylidium. The mother will have one son and multuple daughters. The son will mate with his sisters while still in the womb! The daughters will eventually eat their way out of the mother, killing her in the process, and leave their brother/baby-daddy to die in the carcass of their mother while waiting for their own incestuous (and already pregnant) daughters to one day kill them via cannibalism/birth. en.wikipedia.org/wiki/Adactylidium – dsollen Aug 22 at 13:42
  • Actually looking at your example more closely aphids are not actually pregnant. the young does not develop in the womb and are laid as eggs, so technically this does not qualify as 'pregnancy' by the official definition. – dsollen Aug 22 at 13:52
  • This answer doesn't really present any evidence that these sources are authoritative, rather than just summaries repeating the claim. Do either of these have references to detailed scholarly research? Without that, this feels like a good answer for a biology Q&A, but not a strong one for a community of Skeptics. – IMSoP Aug 23 at 8:53
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In some gall midges, the larvae develop inside the mother even before she is mature (pupal paedogenesis). Stephen Jay Gould wrote about them in 'Ever Since Darwin,' which you can find from google books at this large link:

http://books.google.com/books?id=_VCnI02FwHAC&pg=PA91&lpg=PA91&dq=gall+midges+stephen+jay+gould&source=bl&ots=KOHnKuFWFt&sig=irJkYgdR23KARbO8MGXh-8Qe_f0&hl=en&ei=eySyTdi-Eojh0QHH_-SbBg&sa=X&oi=book_result&ct=result&resnum=1&sqi=2&ved=0CBgQ6AEwAA#v=onepage&q&f=false

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You mention a number of closely related, but different, concepts in your question. The answer is still yes, because every concept you listed does exist in nature; Evolution is pretty good at coming up with any example you can think of. However, they are distinct and separate concepts, so let's address each of them individually.

Parthenogenesis

You didn't mention this yourself, but it was the first comment, and is relevant to the rest of the topics so we'll touch on it first.

Parthenogenesis is "is a natural form of asexual reproduction in which growth and development of embryos occur without fertilization." This means that the creature can have young without having sex, but this does not mean it is born pregnant. In most cases, creatures capable of parthenogenesis will still develop through their normal life cycle and only start the development of their young once they reach maturity. In other words, they may have an egg but they don't yet have an embryo in them at birth. Some species, such as sharks, usually reproduce sexually and only resort to parthenogenesis in the absence of males. Others need triggers to start their pregnancy, such as the commonly quoted example of the whiptail lizard which produces more young if mounted by another lizard to emulate sexual reproduction. Despite technically being defined as asexual, some pathogenic females actually need to mate with a male, using the sperm to trigger the development of young. This is called Gynogenesis

Usually these species are not pregnant at birth, even if they are able to conceive without mating.

Paedogenesis

I'm using the definition Straight Dope was referring to here, I go more into other definition below.

Paedogenesis is not an example of being pregnant from birth. Instead paedogenesis is "reproduction by sexually mature larvae, usually without fertilization". The key part of this definition is that it's a sexually mature larvae. Larvae are usually considered the early, usually not sexually mature, stage in a life cycle. Usually a larvae needs to go through metamorphosis into an 'adult' stage before they are able to reproduce. Thus a larvae being able to reproduce on its own is noteworthy enough to have a special term to describe it.

Usually paedogenesic species start as having a larval form and adult form, as most insects have. At some point a genetic mutation makes it possible for the larval form to reproduce on its own, usually via parthenogenesis. Paedogenesic species often will eventually stop developing into their adult phase, though they can be triggered to develop into them sometimes. Less commonly, paedogenesis can also involve sexual reproduction.

As with the distinction I made with parthenogenesis, these species are not necessarily born pregnant. Usually they don't develop embryos until a certain stage in life. That stage is usually early due to their never developing out of larval phase, but still embryos usually don't develop pre-birth. As pointed out, a small number will still reproduce sexually and may require mating before the embryo will develop.

Neoteny

Your Wikipedia link for paedogenesis redirects to neoteny, which is a different concept. Neoteny is "the delaying or slowing of the physiological (or somatic) development of an organism, typically an animal". This can be any sort of retention of immature characteristics. Domestic dogs are an example of neoteny because they have been breed to retain many puppy traits into adulthood, mostly because humans find those traits adorable and prefer dogs with those traits.

Since most examples of paedogenesis involve the creature never metamorphosing out of their larval phase, or delaying the metamorphosis much longer, this is an example of neoteny, but most cases of neoteny are not paedogenesis.

The reason Wikipedia redirects to neoteny is because they are working with a slightly different definition of paedogenesis. To quote Wikipedia's neoteny definition:

In progenesis (also called paedogenesis), sexual development is accelerated.

This is an alternate, but less common, interpretation of paedogenesis. Unfortunately many of these terms in biology have a few slightly different meanings, as you will see again shortly. Wikipedia is using a different one to me.

Telescoping Generations and Aphids

Now we're getting somewhere. As you saw, Wikipedia defines telescoping generations as:

characterized by a viviparous female having a daughter growing inside her that is also parthenogenetically pregnant with a daughter cell.

This definition explicitly says that the daughter is already pregnant. So, yes, aphids are born pregnant!

These aphids are clonal. That means that their daughters are identical copies of them, and reproduce via parthenogenesis. They don't have to have sex. Considering this, there is no little need for them to wait. In fact, while a daughter is still growing in her mother, her own eggs will hatch inside of her, so the original mother has her granddaughter developing inside of her.

The advantage of this technique is that it speeds up birthing the next generation, since some of the development time of the third generation overlaps with the development of the second generation. The main downside is that it means the aphids need to quickly find and eat food after their birth, as their own daughters are already well developed and are dedicating additional resources to developing granddaughters. The newly born aphid is not just eating for one, but for many dozens. The other downside is that they are clonal, and all clonal species lack genetic variation which makes them vulnerable to disease and less capable of adapting to changes in environments.

Incest and Cannibalism

Despite the definition of telescopic generations, Wikipedia (and most other dictionaries) use, there is usually another species that is considered as a telescopic generation: Adactylidium.

This is a type of mite that manages telescopic generations without using parthenogenesis. This is a reason I stress parthenogenesis is different from telescopic generations. With this species, the mother will be pregnant with one son and multiple daughters. The son will mate with his sisters while still in the mother, impregnating them before they are even born. The daughters will eventually eat their way out of the mother, killing her in the process, and leave their brother/baby-daddy to die in the carcass of their mother. The newly born daughters are already pregnant and will spend 4 days looking for food (a specific type of egg) while waiting for their own incestuous daughters/nieces (who are in turn already pregnant with the hosts grandchildren) to one day kill them via their cannibalism/birth.

Believe it or no,t this can be seen as advantageous for diversity compared to the aphids. It still retains some form of sexual reproduction. By mating with her brother, the mother has ensured each daughter will get different sperm and thus will have slightly different genes. The difference isn't large, since there isn't much genetic variety left after numerous generations of obligatory incest, but any degree of genetic variety will allow some degree of protection against disease or changing environmental pressures. The mere presence of sexual reproduction increases the odds of mutation, which does increase the odds of being able to adapt to different situations. So the Adactylidium gets the evolutionary thumbs up of approval for incest and cannibalism.

Other evolutionary fun

This is just a start of all the interesting types of evolution we have. A quick list of other interesting things that come up.

  • Sharks also commit cannibalism in the womb

  • Bed bugs have an even ickier means of pregnancy, with their aptly-named traumatic insemination, a technique that numerous separate species all evolved independently.

  • Fruit Flies are one of the very tiny number of species that preferentially prefer incest with their brothers over other options, as opposed to begrudgingly allowing it because its too hard to avoid. The advantage of the female's children having 3/4 of her genetics instead of 1/2 is just enough to compensate for the harm incest causes to the success of the kids.

  • Male ants have no father, only a grandfather.

  • Some frogs will keep half as much male DNA, resulting in all kinds off odd hybrid situations

I could go on, but were leave it there for now. Evolution can lead to some interesting things.

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sexual reproduction would exclude the possibility (as there'd be no way to impregnate them prenatally, obviously, unless you want to have offspring impregnated by their own fathers by encapsulting some of their semen into their bodies during gestation), so you should limit your search to those creatures reproducing a-sexually.

The closest I could find quickly was the budding of jellyfish, but that's not really the same. Here a single polyp splits into multiple young jellyfish over time. This is asexual reproduction of a polyp which is itself the result of sexual reproduction.

Nice description (with a diagram) on wikipedia: http://en.wikipedia.org/wiki/Jellyfish More info confirming this on http://www.wildsingapore.com/wildfacts/cnidaria/others/jellyfish/jellyfish.htm http://www.nature.com/scitable/blog/creature-cast/more_budding_jelly_babies http://fusionanomaly.net/jellyfish.html

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    Sexual reproduction wouldn't be theoretically impossible - they could be impregnated by their siblings. (Weirder stuff has happened, such as embryos eating each other) – Andrew Grimm Apr 22 '11 at 13:45
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    you are excluding the possibility of parthenogenesis which can happen in either an adult or in utero, even if that particular species typically breeds by sexual reproduction. Parthenogenesis is documented in aphids and is mentioned in the citation provided by Esultanik. – Monkey Tuesday Apr 22 '11 at 19:57
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    impregnations by siblings might theoretically happen, but would have no benefit over asexual reproduction as there'd be no genetic mixing. Makes it evolving rather unlikely. – jwenting Apr 22 '11 at 20:22
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    Check @ESultanik's Aphid link - in the case they write about, sexual reproduction can take place once a year. The rest of the year the aphids are born pregnant with exact genetic clones of the mother. – FoleyIsGood Apr 23 '11 at 9:54
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    @jwenting There is a viviparous mite (Acarophenax tribolii) where the male siblings impregnate the female siblings in the mother's body (the males then die and the mother bursts open and dies). This is an interesting edge case for sex ratio theory as it's one of the few cases where you find a skewed sex ratio (fewer males because it's local mate competition it doesn't make sense to have the "excess" males). Here's a blogpost on the subject. – Ruben Jun 10 '12 at 16:05

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