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Size growth after birth seems to be the norm in the animal kingdom (as well as among plants). This makes sense. It's evidently physically challenging for an animal to produce an offspring of its equal size, both if born through gestation or eggs.
But, is this always the case? Are there exceptions, i.e. animals that are born at their "normal", adult size? I don't seem to find any (nor that I am a biologist or expert on the topic).
Are some species of animals born pregnant?
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)
How Old is My Adopted Dog?
If you have decided to adopt a dog from an animal shelter or rescue GOOD FOR YOU! If you have already adopted or acquired your dog as a stray or from a friend THAT’S GREAT TOO!
Chances are, you know the approximate age of your adopted dog based on what you were told when you got him, but it is often difficult even for a veterinarian to tell the exact age of your pup. We can make an educated guess based on a few factors like teeth and fur color. You know your dog better than anyone. Learn the canine signs of aging and it will help you make more appropriate choices in the type of care you give your dog.
First let’s talk a little about aging profiles of dogs. Small dogs can live up to 16 years, medium size dogs 10 to 14 years and large dogs, like a Great Dane size, typically live 7 to 8 years. Large breed dogs stay “puppylike” for 24 months or more, compared to the usual 12 to 15 months for medium and small dogs.
Puppies: It is easier to tell how old a puppy is than an older dog based on their baby teeth:
- At one month of age, milk teeth start pushing through the gums
- Permanent Canine teeth come in around 5 months of age
- The last permanent teeth to come in will be the back molars, those come in between 5 and 7 months of age
Ridges and Unevenness on the Front Teeth: At about 1 year of age, a dog will have ridges or bumps along the tops of their 4 front incisors, top and bottom jaw. Front incisors are the teeth that your dog uses for that nibbling type of grooming. As he ages, the bumps will wear down. At 3 to 4 years of age, the ridges should be halfway worn away and at about 7 years of age, the tops of these incisors should be completely smooth.
Tartar Buildup: Tartar generally starts to form around the teeth at about 4 years of age and gets darker and thicker the older your dog gets.
Just like with people, tooth condition depends on genetics and dental care or lack of care. Tooth condition is not an exact indication of age, just a guide.
Fur: Fur color is not a great way to gauge age. Fur around the muzzle or under the chin can start turning grey as early as 2 years old. Premature greying in dogs does exist. Stress is considered a factor, and dogs that had a rougher start in life may go grey earlier. Genetics are mainly responsible for when a dog goes grey, just like in people.
If you notice greying everywhere, like on the chest and face, behind the legs, ears and on the paws, your dog may be approaching senior status.
At what age is my dog considered a senior?
To answer this question most accurately, we have to refer to the Aging Profile factor. A dog is considered a senior in the last 25% of their expected lifespan.
- A Great Dane has a life expectancy of 8 years, so they will be considered a senior at 6 years old
- A Chihuahua has a life expectancy of 16 years, so they will be considered a senior at 12 years old
Young Senior Dog Indicators:
- Senior dogs start developing lumps on their bodies called lipomas – these are fatty lumps and usually nothing to worry about
- General slowing down and tiring out quicker after play
- Behavioral changes: some not so great like new found fear of thunderstorms, others can be positive like attentiveness and patience
- Muscle tone tends to decrease in senior dogs
Super Senior Dog Indicators:
- Having accidents: Physical examinations should always be done if a housebroken dog starts having accidents indoors. If your older female dog has gone incontinent, your veterinarian can prescribe medication for this, and for males, there’s always a belly band. Belly bands are available for all sizes of dogs and come in both a disposable option or a washable, reusable band – as a bonus, many of the reusable belly bands come in fun colors and designs. Perfect for matching your dog’s personality.
- Lenticular Sclerosis is a bluish grey haze that covers your dog’s eyes when they get older. It does not affect your dog’s vision, but will look cloudy to you. Your veterinarian should be able to tell you if your dog has this when he shines a light into them. Lenticular Sclerosis is different than Keratitis (cloudy eye), the latter being very serious with a host of signs that should indicate vet attention is necessary.
Every dog is an individual, and if you spend time with them and tune into them you can probably guess pretty accurately how old they are. If you are planning a trip to the animal shelter, you can take along these tips if age is a factor in picking out your new best pal.
The Michelson Found Animals Foundation’s mission of saving pets and enriching lives is made possible by the generous contributions of Dr. Gary Michelson and Alya Michelson.
How are humans different from other animals?
There are many similarities between humans and other animals that you may have noticed. Humans and animals both eat, sleep, think, and communicate. We are also similar in a lot of the ways our bodies work. But we also have a lot of differences. Are there any differences that set humans apart, uniquely, from all other animals?
Some people think that the main differences between humans other animal species is our ability of complex reasoning, our use of complex language, our ability to solve difficult problems, and introspection (this means describing your own thoughts and feelings). Others also feel that the ability for creativity or the feeling of joy or sorrow is uniquely human. Humans have a highly developed brain that allows us to do many of these things. But are these things uniquely human? First, let’s get into the fuzzy part of that question.
A baboon is being given a mirror test. Image by Moshe Blank via Wikimedia Commons.
There are a lot of things that humans think are true about animals and animal behavior, but some of these ideas are problematic. Sometimes, when we do tests on animal behavior, we use tests that apply to animals like humans, and we expect animals to perform in a similar way if they have similar abilities. For example, the mirror test is used to see if animals have awareness of themselves as the image that they see in a mirror. If a mark is placed on the animal, they should show signs of knowing that the mark is on their body. Maybe they try to rub it off with their hands or, if they can’t use their limbs that way, they may move their body a bit to see the mark better. But what if an animal doesn't have the best vision? Do we just say that, because they can't perform the test in that way, they wouldn't pass? Expecting all other animals to perform similarly to humans on tests can be problematic. This makes learning about some parts of animal behavior difficult.
But, what we have learned is pretty exciting. As we keep learning more and more about animal behavior, we are continually surprised.
Gunnison's prairie dogs seem to have a fairly complex language. rather than just sounding a basic alarm call, researchers have found that their alarm calls can describe specific predator speed, color, shape, and size. So when is this communication complex enough for us to call it a language? Elephants have been found to communicate across miles of land through subsonic sound. And when researchers slow a hummingbird's chirp down, it seems the song may be as complex as a song from some other birds, though more studies need to be done to understand this. Do we view animal "language" as limited just because we have trouble understanding it?
This Caledonian crow is solving a water level problem. It adds small blocks into columns of water to raise the water level, allowing it access to food. The crow also had to realize that one column was too wide, so the limited blocks wouldn't raise the water enough. Image from video by Logan C, Jelbert S, Breen A, Gray R, Taylor A via Wikimedia Commons.
Caledonian crows can solve problems and build tools, and can solve multiple-step puzzles that require a plan. Are these examples of difficult problems? Where do we draw the line to say something is "difficult" enough, or that we've given an animal proper motivation to want to even solve one of these problems?
Gorillas and chimpanzees have painted pictures of birds, describing (through sign language ) that that is what they were trying to create. If they had a goal in mind and then made it, is that a sign that they had introspection? That they are describing their own thoughts? And that they are doing it by using their own creativity? Seems like it might be.
And animals do appear to feel joy and sorrow. There are videos out there showing a raven using a piece of plastic to sled down part of a snowy roof. The raven picks it up and slides down over and over again… they aren’t playing with another bird, they are enjoying sledding and having fun, perhaps feeling joy. And we continue to learn of more and more species that show sorrow, especially at the loss of members of their family or other loved ones. Animals that grieve include elephants, wolves, sea lions, magpies, and many more. A recent video of javelinas (peccaries that live in the American southwest) show that they mourn their dead. But we didn’t realize this, until it was captured by a field camera.
So maybe there isn’t that much that makes us uniquely human. Maybe we need to pay more attention to what animals are doing, and try to view the world through their eyes. And, perhaps our ability to consider animal's feelings and hope for the well-being of these other amazing creatures is our best, and most uniquely human ability.
Are there any homosexual animals?
During the winter mating season, competition is fierce for access to female Japanese macaques. But it's not for the reason you might think. Males don't just have to compete with other males for access to females: they have to compete with females too.
That's because in some populations, homosexual behaviour among females is not only common, it's the norm. One female will mount another, then stimulate her genitals by rubbing them against the other female. Some hold onto each other with their limbs using a "double foot clasp mount", while others sit on top of their mates in a sort of jockey-style position, says Paul Vasey of the University of Lethbridge in Alberta, Canada, who has been studying these macaques for over 20 years.
To our eyes these encounters look startlingly intimate. The females stare into each other's eyes while mating, which macaques hardly ever do outside of sexual contexts. The pairings can even last a whole week, mounting hundreds of times. When they're not mating, the females stay close together to sleep and groom, and defend each other from possible rivals.
That many humans are homosexual is well known but we also know the behaviour is extremely common across the animal kingdom, from insects to mammals. So what's really going on? Can these animals actually be called homosexual?
Animals have been observed engaging in same-sex matings for decades. But for most of that time, the documented cases were largely seen as anomalies or curiosities.
The turning point was Bruce Bagemihl's 1999 book Biological Exuberance, which outlined so many examples, from so many different species, that the topic moved to centre stage. Since then, scientists have studied these behaviours systematically.
On the face of it, homosexual behaviour by animals looks like a really bad idea
Despite Bagemihl's roster of examples, homosexual behaviour still seems to be a rarity. We have probably missed some examples, as in many species males and females look pretty much alike. But while hundreds of species have been documented doing it on isolated occasions, only a handful have made it a habitual part of their lives, says Vasey.
To many, that isn't surprising. On the face of it, homosexual behaviour by animals looks like a really bad idea. Darwin's theory of evolution by natural selection implies that genes have to get themselves passed on to the next generation, or they will die out. Any genes that make an animal more likely to engage in same-sex matings would be less likely to get passed on than genes pushing for heterosexual pairings, so homosexuality ought to quickly die out.
But that evidently isn't what's happening. For some animals, homosexual behaviour isn't an occasional event &ndash which we might put down to simple mistakes &ndash but a regular thing.
Take the macaques. When Vasey first observed the females mounting each other, he was "blown away" by how often they did it.
The females were simply seeking sexual pleasure
"So many females of the group are engaging in this behaviour and there are males sitting around twiddling their thumbs," he says. "There's got to be a reason for this. There is no way the behaviour can be evolutionarily irrelevant."
Vasey's team has found that females use a greater variety of positions and movements than males do. In a 2006 study, they proposed that the females were simply seeking sexual pleasure, and were using different movements to maximise the genital sensations. "She can do so in a homosexual context just as easily as in a heterosexual context, so the behaviour spills over," says Vasey.
But for all the homosexual pairings the females indulge in, Vasey is clear that they are not truly homosexual. A female may engage in female-female mounting, but that doesn't mean she isn't interested in males. Females often mount males, apparently to encourage them to mate more. Once they had evolved this behaviour, it was easy for them to apply it to other females as well.
In some cases, there is a fairly straightforward evolutionary reason why animals engage in homosexual behaviour.
Take male fruit flies. In their first 30 minutes of life, they will try to copulate with any other fly, male or female. After a while, they learn to recognise the smell of virgin females, and focus on them.
The males are using homosexual behaviour as a roundabout way to fertilise more females
This trial-and-error approach may look rather inefficient, but actually it is a good strategy, says David Featherstone of the University of Illinois at Chicago, US. In the wild, flies in different habitats may have slightly different pheromone blends. "A male could be passing up an opportunity to have viable offspring if they are hardwired to only go for a certain smell," says Featherstone.
Male flour beetles use a distinctly sneaky trick. They often mount each other, and go so far as depositing sperm. If the male carrying this sperm mates with a female later, the sperm might get transferred &ndash so the male who produced it has fertilised a female without having to court her.
In both cases, the males are using homosexual behaviour as a roundabout way to fertilise more females. So it's clear how these behaviours could be favoured by evolution. But it's also clear that fruit flies and flour beetles are a long way from strictly homosexual.
Other animals really do seem to be lifelong homosexuals. One such species is the Laysan albatross, which nests in Hawaii, US.
Among these huge birds, pairs are usually "married" for life. It takes two parents working together to rear a chick successfully, and doing so repeatedly means that the parents can hone their skills together. But in one population on the island of Oahu, 31% of the pairings are made up of two unrelated females. What's more, they rear chicks, fathered by males that are already in a committed pair but which sneak matings with one or both of the females. Like male-female pairs, these female-female pairs can only rear one chick in a season.
Same-sex coupling is a response to a shortage of males
The female-female pairs are not as good at rearing chicks as female-male pairs, but are better than females that go it alone. So it makes sense for a female to pair up with another female, says Marlene Zuk of the University of Minnesota in Saint Paul, US. If she did not, she might manage to mate but would struggle to incubate her egg and find food. And once a female forms a pair-bond, the species' tendency towards monogamy means it becomes life-long.
There is even a subtle advantage for the females. The system means that they can get their eggs fertilised by the fittest male of the group, and pass his desirable traits on to her offspring, even if he is already paired with another female.
But once again, the female albatrosses are not inherently homosexual. The Oahu population has a surplus of females as a result of immigration, so some females cannot find males to pair with. Studies of other birds suggest that same-sex coupling is a response to a shortage of males, and is much rarer if the sex ratio is equal. In other words, the female Laysan albatrosses probably wouldn't choose to pair with other females if there were enough males to go round.
So perhaps we've been looking in the wrong place for examples of homosexual animals. Given that human beings are known to be homosexual, maybe we should look at our closest relatives, the apes.
Bonobo sex also cements social bonds
Bonobos are often described as our "over-sexed" relatives. They engage in an enormous amount of sex, so much so that it's often referred to as a "bonobo handshake", and that includes homosexual behaviour among both males and females.
Like the macaques, they seem to enjoy it, according to Frans de Waal of Emory University in Atlanta, Georgia, US. Writing in Scientific American in 1995, he described pairs of female bonobos rubbing their genitals together, and "emitting grins and squeals that probably reflect orgasmic experiences".
But bonobo sex also plays a deeper role: it cements social bonds. Junior bonobos may use sex to bond with more dominant group members, allowing them to climb the social ladder. Males that have had a fight sometimes perform genital-to-genital touching, known as "penis fencing", as a way of reducing tension. More rarely, they also kiss, perform fellatio and massage each other's genitals. Even the young comfort each other with hugs and sex.
Bonobos show that "sexual behaviour" can be about more than reproduction, says Zuk, and that includes homosexual behaviour. "There's a whole range of behaviours that fit in well with how evolution happens that now include homosexual behaviour." In fact, female bonobos still have sex when they are outside their reproductive period and can't get pregnant.
They don't show a consistent sexual orientation
Just like humans can use sex to gain all sorts of advantages, so can animals. For instance, among bottlenose dolphins, both females and males display homosexual behaviour. This helps members of the group form strong social bonds. But ultimately, all concerned will go on to have offspring with the opposite sex.
All these species might be best described as "bisexual". Like the Japanese macaques and the fruit flies, they switch easily between same-sex and opposite-sex behaviours. They don't show a consistent sexual orientation.
Only two species have been observed showing a same-sex preference for life, even when partners of the opposite sex are available. One is, of course, humans. The other is domestic sheep.
In flocks of sheep, up to 8% of the males prefer other males even when fertile females are around. In 1994, neuroscientists found that these males had slightly different brains to the rest. A part of their brain called the hypothalamus, which is known to control the release of sex hormones, was smaller in the homosexual males than in the heterosexual males.
That is in line with a much-discussed study by the neuroscientist Simon LeVay. In 1991, he described a similar difference in brain structure between gay and straight men.
How could this preference for other males be passed on to offspring?
This seems quite different from all the other cases of homosexual behaviour, because it is hard to see how it could possibly benefit the males. How could this preference for other males be passed on to offspring, if the males do not reproduce?
The short answer is that it probably doesn't benefit the homosexual males themselves, but it might benefit their relatives, who may well carry the same genes and could pass them on. For that to happen, the genes that make some males homosexual would have to have another, useful effect in other sheep.
LeVay suggests that the same gene that promotes homosexual behaviour in male sheep could also make females more fertile, or increase their desire to mate. The female siblings of homosexual sheep could even produce more offspring than average. "If these genes are having such a beneficial effect in females, they outweigh the effect in males and then the gene is going to persist," says LeVay.
While male sheep do show lifelong homosexual preferences, this has only been seen in domesticated sheep. It's not clear whether the same thing happens in wild sheep, and if LeVay's explanation is right it probably doesn't. Domestic sheep have been carefully bred by farmers to produce females that reproduce as often as possible, which might have given rise to the homosexual males.
So LeVay and Vasey still say that humans are the only documented case of "true" homosexuality in wild animals. "It is not the case that you have lesbian bonobos or gay male bonobos," says Vasey. "What's been described is that many animals are happy to engage in sex with partners of either sex."
Homosexual behaviour doesn't challenge Darwin's ideas
The funny thing is, biologists should have predicted this. When Darwin was developing his theory of natural selection, one of the things that inspired him was the realisation that animals tend to have far more offspring than they seem to need. In theory a pair of animals need only have two offspring to replace themselves, but in practice they have as many as they possibly can &ndash because so many of their young will die before they manage to reproduce.
It seems obvious that this built-in need to keep reproducing would manifest itself in a powerful sex drive, one that might well spill over into mating while females are infertile, or same-sex matings. Victorian scientists saw animals having more offspring than seemed necessary: today we see animals having more sex than seems necessary.
"Homosexual behaviour doesn't challenge Darwin's ideas," says Zuk. Instead there are many ways it can evolve and be beneficial.
We may never find a wild animal that is strictly homosexual in the way some humans are. But we can expect to find many more animals that don't conform to traditional categories of sexual orientation. They are using sex to satisfy all sorts of needs, from simple pleasure to social advancement, and that means being flexible.
Introduction to LS1.B
The characteristic structures, functions, and behaviors of organisms change in predictable ways as they progress from birth to old age. For example, upon reaching adulthood, organisms can reproduce and transfer their genetic information to their offspring. Animals engage in behaviors that increase their chances for reproduction, and plants may develop specialized structures and/or depend on animal behavior to accomplish reproduction.
Understanding how a single cell can give rise to a complex, multi-cellular organism builds on the concepts of cell division and gene expression. In multi-cellular organisms, cell division is an essential component of growth, development, and repair. Cell division occurs via a process called mitosis: when a cell divides in two, it passes identical genetic material to two daughter cells. Successive divisions produce many cells. Although the genetic material in each of the cells is identical, small differences in the immediate environments activate or inactivate different genes, which can cause the cells to develop slightly differently. This process of differentiation allows the body to form specialized cells that perform diverse functions, even though they are all descended from a single cell, the fertilized egg. Cell growth and differentiation are the mechanisms by which a fertilized egg develops into a complex organism. In sexual reproduction, a specialized type of cell division called meiosis occurs and results in the production of sex cells, such as gametes (sperm and eggs) or spores, which contain only one member from each chromosome pair in the parent cell.
By weight, which animal has the largest baby relative to body size?
Human childbirth is hard enough – imagine giving birth to a baby half your weight!
Asked by: Emily Cane, Newcastle
Despite a kiwi being about the size of a chicken, the female lays an egg that is about half her weight! It’s so big because it has an enormous yolk, which sustains the chick for the first week of its life. Here you can see some other animals that have enormous babies, as well as those that have teeny tiny offspring (with humans thrown in for good measure).
Kiwi egg (1/2)
Beluga whale (1/17)
Giant clam (1/500,000,000)
Ocean sunfish (1/1,500,000)
Red kangaroo (1/100,000)
Honey possum (1/2,400)
Giant panda (1/900)
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Behaviors that are closely controlled by genes with little or no environmental influence are called innate behaviors. These are behaviors that occur naturally in all members of a species whenever they are exposed to a certain stimulus. Innate behaviors do not have to be learned or practiced. They are also called instinctive behaviors. An instinct is the ability of an animal to perform a behavior the first time it is exposed to the proper stimulus. For example, a dog will drool the first time&mdashand every time&mdashit is exposed to food.
Significance of Innate Behavior
Innate behaviors are rigid and predictable. All members of the species perform the behaviors in the same way. Innate behaviors usually involve basic life functions, such as finding food or caring for offspring. Several examples are shown in Figure below. If an animal were to perform such important behaviors incorrectly, it would be less likely to survive or reproduce.
Examples of Innate Behavior. These innate behaviors are necessary for survival or reproduction. Can you explain why each behavior is important?
Intelligence and Innate Behavior
Innate behaviors occur in all animals. However, they are less common in species with higher levels of intelligence. Humans are the most intelligent species, and they have very few innate behaviors. The only innate behaviors in humans are reflexes. A reflex is a response that always occurs when a certain stimulus is present. For example, a human infant will grasp an object, such as a finger, that is placed in its palm. The infant has no control over this reaction because it is innate. Other than reflexes such as this, human behaviors are learned&ndashor at least influenced by experience&mdashrather than being innate.
Innate Behavior in Human Beings
All animals have innate behaviors, even human beings. Can you think of human behaviors that do not have to be learned? Chances are, you will have a hard time thinking of any. The only truly innate behaviors in humans are called reflex behaviors. They occur mainly in babies. Like innate behaviors in other animals, reflex behaviors in human babies may help them survive.
An example of a reflex behavior in babies is the sucking reflex. Newborns instinctively suck on a nipple that is placed in their mouth. It is easy to see how this behavior evolved. It increases the chances of a baby feeding and surviving. Another example of a reflex behavior in babies is the grasp reflex (Figure below). Babies instinctively grasp an object placed in the palm of their hand. Their grip may be surprisingly strong. How do you think this behavior might increase a baby&rsquos chances of surviving?
One of the few innate behaviors in human beings is the grasp reflex. It occurs only in babies.
7 gender-bending animals in the animal kingdom
After images of five Botswana lionesses with manes were released earlier this week, it got us thinking.
What other animals in the animals kingdom have gender-bending traits, or engage in sexual mimicry as it’s called in biology?
Below are seven such animals. Some of them you may already know about, but some of them are likely to surprise you.
1. Marsh harrier
Male marsh harrier with female colouration. (via AAP)
While the majority of male marsh harriers are covered in a grey down of feather and have yellow eyes, 40 per cent of these males resemble their female counterparts. Females have brown feathers, white eyes, and are much larger than male birds.
These gender-bending males adopt a female-like down, with the change occurring in the second year of their life. Their eyes and small build stay the same, however.
Scientists believe these males assume a female appearance to prevent them from being attacked by other males.
2. Giant Australian cuttlefish
Two male giant Australian cuttlefish, jostling for the right to mate with a female (via AAP)
Male cuttlefish outnumber females 11 to 1. So, if you’re a male cuttlefish, your competition to find a mate is stiff. But there's one advantageous tool at your disposal - like other cephalopods (such as squids and octopi), cuttlefish can camouflage.
That’s why some smaller males within the species will camouflage into the muted brown tones of their female counterparts while swimming through male-dominated areas. Their camouflage protects them from being attacked by other males, while giving them easier access to a female cuttlefish.
3. Red and olive colobus monkey
Colobus monkey sitting on a tree branch in the East African archipelago of Zanzibar. (via AAP)
When a male colobus monkey comes of age, they are shooed away from their pod to seek other single males with whom to form a coalition. But olive and red colobus use a peculiar evolutionary trait to hold off that rude departure.
Just as they reach puberty, the area around their anus swells, mimicking that of a female colobus in heat. Though this doesn’t confuse other males within their own and related species, it does stop them from being kicked out of their pod.
This swelling stops for the olive colobus once they reach adulthood, while the red colobus retain this feature for life.
4. Spotted hyena
Spotted Hyena standing on savannah. (via AAP)
Both female and male hyenas have testes and a penis. And for both sexes, the penis goes erect around female hyena. For males, the erection is a function of arousal for females, it’s due to familiarity and safety.
In actual fact, the female ‘penis’ is actually an elongated clitoris, so large that in biology it is referred to as a pseudo-penis. Female hyenas urinate, mate, and even give birth through the appendage. When it comes to having intercourse, females roll up their clitoris to give permitted males access to their vagina, which is internal.
Even though the female penis isn’t a true sex organ, it’s longer than the male one.
A clownfish swimming through coral near the Quicksilver platform on the Outer Barrier Reef. (via AAP)
Clownfish live within a strict hierarchy, where each school is headed by a female and seconded by a submissive male with whom she mates. Other fish in the school are all male, which doesn’t pose too much of a threat since clownfish are born hermaphrodites, though eventually become all male.
When the dominant female dies, her mate takes her place and changes his sex to female.
The hierarchy is also maintained within the school through body mass. The female is the largest of the school, followed by her second and so on. Once the second takes the deceased female’s role, he expands to her size. The other fish in the school also grow according to their new hierarchal positions.
The animal kingdom
Animals evolved from unicellular eukaryotes. The presence of a nuclear membrane in eukaryotes permits separation of the two phases of protein synthesis: transcription (copying) of deoxyribonucleic acid (DNA) in the nucleus and translation (decoding) of the message into protein in the cytoplasm. Compared to the structure of the bacterial cell, this gives greater control over which proteins are produced. Such control permits specialization of cells, each with identical DNA but with the ability to control finely which genes successfully send copies into the cytoplasm. Tissues and organs can thus evolve. The semirigid cell walls found in plants and fungi, which constrain the shape and hence the diversity of possible cell types, are absent in animals. If they were present, nerve and muscle cells, the focal point of animal mobility, would not be possible.