The Weirdest Animals on Earth: 12 Amazing Facts About Seahorses

A seahorse in all its glory. Photo by Gustavo Gerdel at Wikimedia Commons.

1. Seahorses are fish. They include about 54 different species of fish and are closely related to sea dragons and pipefish. But seahorses are not your typical fish! A baby seahorse is called a fry (like in other fish), but a group of seahorses is called a herd (like in horses).

2. Seahorses have skeletons unlike any other fish. Unlike other bony fish, seahorses have a neck, an exoskeleton, and a prehensile tail. Seahorses do not have pelvic fins, ribs or scales. Instead, their skin is stretched over a series of bony plates arranged in rings.

3. Seahorses are terrible swimmers and can die of exhaustion if the sea is rough or the current is too strong. The only fin they have to get around with it the tiny one in the middle of their back (the dorsal fin). They use even smaller pectoral fins on the sides of their head to steer. Seahorses and razorfish are the only fish to swim upright, because it is horribly inefficient. It is a good thing they have a prehensile tail to hang on to whatever is nearby.

A pygmy seahorse in camouflage.
Photo by prilfish at Wikimedia.

4. Seahorses are experts at camouflage and can change color. They are even able to grow fleshy appendages (called cirri) that help them with camouflage by giving them a weed-like appearance.

5. Seahorses have terrible smell but amazing vision. They have the fewest genes for olfactory receptors (used in other animals for smell and taste) of any ray-finned fish species known. But seahorses have excellent vision and their eyes can work independently, meaning they can look forward and backward at the same time!

6. Seahorses eat weird. They have a toothless, tubular snout, which they use to suck up small fish and crustaceans. They swallow them whole. Seahorses do not have stomachs and don't digest very well, so they have to eat constantly.

7. Seahorses are one of the ocean's deadliest predators, with a 90% kill rate. Because of the shape of their head and their slow, finless method of movement, seahorses move with near hydrodynamic silence, barely moving the water as their stealthily sneak up on their prey. Once they are within striking distance, they snap their heads and suck up their prey. 

8. Seahorses click when they're courting and growl when their stressed. 

9. Seahorses are monogamous and pair for life. Their courtship begins with a daily dance between the couple that they do for days. The final courtship dance can last eight hours before the female "impregnates" her partner.

10. Male seahorses get "pregnant". They are the only males that take on the full responsibility of pregnancy, carrying up to 2,000 babies at a time! Although they don’t have a mammalian womb and placenta, they do have an enclosed abdominal pouch specifically for the purpose of incubating the babies. The female deposits her eggs in his brood pouch, in which he fertilizes them and incubates them for 10-45 days (depending on the species). During this time, his body undergoes a number of hormonal and physiological changes. When the babies are ready to emerge as fully developed little seahorses, seahorse dads even experience contractions as they give birth! 

11. Seahorses are evolving faster than any other group of bony fishes. Scientists have sequenced the entire genome of a tiger tail seahorse, a threatened tropical seahorse species.

12. Seahorses are under threat because of the traditional Chinese medicine trade, the pet trade, and the curio trade, all of which capture seahorses from the wild, and because of habitat depletion and pollution.

A New Key to the Story of How the Sexes Have Come to Be

In the beginning, we are all male and female… More specifically, we are all in between male and female. So what makes our embryonic selves choose and follow a developmental path to becoming the sex that we are today? New research has dramatically changed our understanding of this process.

During early embryonic development, all mammals develop a single pair of gonads that are neither testes nor ovaries, but have the potential to become either. Likewise, the external genitalia at this early stage has the potential to become either a penis and scrotum or a clitoris, vagina, and labia. Two pairs of ducts develop to connect the gonads to the undifferentiated external genitalia: One set of ducts, the Wolffian ducts, would become the epididymis, vas deferens, and seminal vesicles if this animal becomes male. The other set of ducts, the Müllerian ducts, would become the oviducts, uterus and innermost part of the vagina if this animal becomes female. So what determines if a given animal will develop male or female reproductive anatomy?

Early in development, mammalian embryos have one set of gonads that has the potential to become either testes or ovaries (here labeled as "bipotential gonad"). These gonads are connected the the developing external genitalia by two sets of tubes: The Wolffian ducts become the reproductive tracts in males and the Müllerian ducts become the reproductive tracts in females. The ducts that do not become reproductive tracts typically disintegrate. However, XX female embryos that lack the COUP-TFII protein do not dismantle their male-like reproductive tracts. Figure from Swain, 2017.

The sex of a mammal is determined by the combination of sex chromosomes it has. If the mammal has two X chromosomes, it will likely become female, and if it has an X chromosome and a Y chromosome, it will likely become male. The story physiologists have been telling for decades is that there is a single gene located on the Y chromosome, called the SRY gene, that single-handedly makes an embryo become a male. When expressed, the SRY gene produces a protein, called testes-determining factor, which interacts with the cells of the undifferentiated gonads to turn them into testicular cells. These newly formed testicular cells produce two key hormones: testosterone, which causes the Wolffian ducts to become the epididymis, vas deferens, and seminal vesicles, and anti-Müllerian hormone (AMH), which causes the Müllerian ducts to degenerate. In other words, if an animal has a Y chromosome, it will typically have an SRY gene that will trigger the sequence of events that causes the animal to develop into a male. If the animal does not have the Y chromosome, it will typically become female. However, it is not just the lack of a Y chromosome that can make a female; Any disruption of this pathway (such as an SRY gene that is not expressed, or the lack of testicular hormones) typically causes the animal to develop into a female. For this reason, females in mammals have been called the default sex. The scientific understanding since the 1950s has been that, in mammals, the development of a male reproductive system is an active process and the development of a female reproductive system is a passive process. However, a new study reveals that the process of becoming female mammal is not as passive as we have thought.

Fei Zhao, Humphrey Yao and their research team at the National Institute of Environmental Health Sciences and Baylor College of Medicine discovered a critical role for a specific protein, called COUP-TFII, in the active process of becoming a mammalian female. The research team examined female mouse embryos (which lack a Y chromosome, and hence an SRY gene) that had been genetically modified to lack a particular protein called the COUP-TFII protein. They compared these genetically modified XX embryos to genetically typical XX mouse embryos. When the unmodified XX embryos had developed to have only Müllerian ducts (the “typical” female reproductive pathway), the XX embryos without COUP-TFII protein retained both Müllerian and Wolffian ducts! Unfortunately, these XX mice that lacked the COUP-TFII protein died shortly after birth, so it was difficult to tell if this developmental process would have continued. The research team cultured reproductive organs of XX mice with and without COUP-TFII protein and found that this developmental trajectory likely would have continued after birth.

Images A and B show the reproductive tract from the side (A) and as a cross-section (B) in a "typical" XX female mouse embryo. Images D and E show that XX females that lack the COUP-TFII protein retain both Müllerian (pink arrows) and Wolffian (blue arrows) ducts. Figure from Zhao et al., 2017.

We know that testosterone helps promote the development of Wolffian ducts in XY males, so the most likely explanation of what they witnessed is that the lack of COUP-TFII protein somehow increased action of testosterone in these genetically modified XX embryos. The researchers ran a number of tests to explore this possibility. Testosterone is mostly produced by the gonads, so they compared the gene expression and enzymes of ovaries of unmodified XX mice with the ovaries of XX mice that lacked the COUP-TFII protein, and they found no differences that pointed to differences in testosterone production. They then considered the possibility that testosterone was produced somewhere else in the body, but the XX mice that lacked the COUP-TFII protein did not have more masculine body features compared to the unmodified XX mice. Finally, the researchers gave extra testosterone to the mother mice that were pregnant with unmodified XX mice and XX mice that lacked the COUP-TFII protein. The extra testosterone did not affect any of the mouse pups; it did not cause the Wolffian ducts of the XX mice that lacked the COUP-TFII protein to regress. Together, the researchers found that no, XX embryos that lack COUP-TFII protein do not have any more testosterone-like activity than their non-genetically modified XX sisters. This means that testosterone alone is not enough to keep Wolffian ducts.

This research has shown us that for the Wolfian ducts to go away during the reproductive development of a mammalian female, they need to be actively dismantled using a biochemical process (similar to how AMH dismantles Müllerian ducts during male reproductive development). COUP-TFII protein appears to be the chemical in charge of triggering this process. Female mammals are not the passive result of simply not becoming male, as has been taught in physiology classes for decades. Becoming a female mammal requires a process all its own, and we are only now starting to learn what that is.


Want to know more? Check these out:

F. Zhao et al. Elimination of the male reproductive tract in the female embryo is promoted by COUP-TFII in mice. Science. Vol. 357, August 18, 2017, p. 717. doi: 10.1126/science.aai9136

A. Swain. Ductal sex determination. Science. Vol. 357, August 18, 2017, p. 648. doi: 10.1126/science.aao2630

The Best Moms in the Animal Kingdom

Moms are really important to life on Earth, as evidenced by the fact that maternal care is fairly common across the animal kingdom. In most species, females produce fewer, larger, and costlier eggs than males do sperm. Therefore, it is usually beneficial to females to maximize the possible success of each one, sometimes by gestating them inside their own bodies (as mammals do), or incubating the eggs until they are ready to hatch (as birds do), or by providing prolonged protection, food and training until they are ready to take on the world for themselves. But there is still a lot of variation in how and how much each mother gives to her offspring. Here are some of the best moms in the animal kingdom:

1. The Endurance Prize goes to orangutans: Orangutan infants cling to their moms’ bellies non-stop for the first four months of their life and they continue to completely depend on their moms for the first two years for food and transport. Their moms will continue to carry them often until they are five and will sometimes continue to breastfeed them until they are eight! After that, the young still stay close to mom, learning from her and helping her until they are sometimes in their teens.

An orangutan mama and toddler. Photo by Mistvan at Wikimedia Commons.

2. The Provider Prize goes to the crab spider, Diaea ergandros: These crab spider moms create a brood chamber and nest out of eucalyptus leaves. They guard their eggs and then spiderlings, providing protection and prey for food. These moms also make extra eggs, just for their babies to eat, and finally, they give themselves… quite literally! The babies eat their mothers completely in a rare behavior called matriphagy.

3. The Pregnancy Prize goes to elephants: Elephants are pregnant for about 22 months… nearly two years! And a baby elephant is not light to carry around… by the time it is born, it will weigh nearly 250 pounds! Just the thought of it makes me uncomfortable. So why would elephant moms need to gestate their young for such an incredibly long time? It is thought that the long gestation is needed for the proper development of their brains, so they are born with the complex cognitive and social skills needed to survive in their herd.

An African elephant family plays in the hot sun. Photo by Bernard Dupont at Wikimedia Commons.

4. The Brooding Prize goes to a deep-sea octopus: Two years is a very long time to carry a developing baby inside your body, but some animals care for their developing empryos ouside of their bodies with a behavior called brooding. Although brooding may sound easier than pregnancy, it is not for the faint of heart. A deep-sea octopus was observed brooding her eggs in the Monterey Submarine Canyon off central California for 53 months… That is nearly 4 and a half years! And that whole time she did not eat, but instead guarded and aerated the water around her precious eggs.

A deep-sea octopus. Photo by NOAA at Wikimedia Commons.

5. The Multi-Generational Prize goes to humans: Many human moms are not only good mothers, but also good grandmothers. Grandparenting is extremely rare in the animal kingdom (the first documented case of grandparenting in non-humans was as recent as 2008) and human females excel at it. They provide care, advice, resources, lessons and hugs to increase the success of their offspring and grand-offspring… It’s amazing other species haven’t picked up on this amazing secret yet!

The best moms in the animal kingdom. Photos by Sarah Jane Alger.

The Weirdest Animals on Earth: 12 Amazing Facts About Platypuses

What IS that? A photo by Stefan Kraft at Wikimedia Commons.

1. Platypuses are so strange, that when British scientists first encountered one, they thought it was a joke: A Governor of New South Wales, Australia, sent a platypus pelt and sketch to British scientists in 1798. Even in their first published scientific description of the species, biologists thought that this duck-beaked, beaver-bodied, web-footed specimen may be some Frankenstein-like creation stitched together as a hoax. But this is only the beginning of their oddities…

2. Platypuses are egg-laying mammals. Mammals are animals that have a backbone, are warm-blooded, and females produce milk for their young. Most females that nurse their young also carry their developing babies in their bodies and give birth to live young… But platypuses don’t play by those rules. Platypuses are monotremes, egg-laying mammals that include the platypus and four species of echidna. Most female mammals have two functional ovaries, but female platypuses, like most female birds, only have a functional left ovary. Once a year, a female platypus may produce a clutch of two or three small, leathery eggs (similar to reptile eggs), that develop in her uterus for 28 days. Because female platypuses don’t even have a vagina, when the eggs are ready, she lays them through her cloaca, an opening that serves for reproduction, peeing and pooping. (In fact, monotreme comes from the Greek for “one hole”). She then curls around them and incubates them for another 10 days until they hatch.

3. Platypuses sweat milk! Not only do female platypuses not have vaginas, they don’t have nipples either! Instead, lactating mothers ooze milk from pores in their skin, which pools in grooves on their bellies so the babies can lap it up. …And they’re not even embarrassed about it!

4. Adult platypuses are toothless. Baby platypuses (that is the actual technical term for them, by the way… not “puggles”, which would be way more fun) are born with teeth but they lose them around the time that they leave the breeding burrow. In their place are rigid-edged keratinized pads that they use as grinding plates. When they catch their prey (worms, bugs, shrimp, and even crayfish), they store it in their cheek pouches and carry it to the surface, where they use gravel to crush it in their toothless maw.

5. The platypus “duck bill” is a sensory organ used to detect electric fields. Muscles and neurons use electrical impulses to function, and these impulses can be detected by electroreceptors. Although common in shark and ray species, electroreception is rare in mammals, only having been discovered in monotremes and the Guiana dolphin. Platypuses have rows of around 40,000 electroreceptors on their highly sensitive bill, which they wave back and forth in the water, much like a hammerhead shark, to determine the location of their prey. It’s a good thing this sense is so sensitive, since they close their eyes, nose and ears every time they dive.

6. Platypuses don’t use their tails like beavers do. Whereas beavers use their large, flat, leathery tails for swimming and slapping the water to send signals, platypuses don’t use their tails for any of that. Platypuses have large, flat tails for storing fat in case of a food shortage. Unlike beaver tails, platypus tails are covered in fur, which the mothers use to snuggle with their incubating eggs.

A platypus ankle spur. Photo by E.Lonnon at Wikimedia Commons.

7. Male platypuses have venomous ankle spurs. Their venom is strong enough to kill small animals and to create excruciating pain in humans. Since only males have it and they produce more venom during the breeding season, we think its main function may be to compete for mates and breeding territories.

8. Platypuses are knuckle-walkers with a reptilian gait. Although they are well-built for swimming with their webbed feet and legs on the sides of their bodies, these traits make it quite awkward to get around on dry land. To walk, they pull in their webbing and walk on their knuckles, exposing their claws. Like reptiles and salamanders, platypuses flex their spines from side-to-side, supported by their sprawling legs.

9. Platypuses have unusually low body temperatures. As unusual as they are, platypuses are still mammals, which are defined, in part, by their ability to generate most of their own body heat with their metabolism. Platypuses do this as well, but whereas most mammals maintain body temperatures between 37-40 degrees C (99-104 degrees F), platypuses are happy with a body temperature of 32 degrees C (90 degrees F). This lower metabolism reduces the amount of calories they need to eat.

10. They have no stomach. Stomachs are specialized protein-digesting chambers of digestive tracts that contain protein-digesting enzymes and acids to activate them. Not all animals have them, but most carnivores do. The most common exceptions to this rule are fish… and platypuses. Why? We don’t know for sure, but many of these animals consume diets high in calcium carbonate, which is a natural antacid. If their own diet would constantly neutralize their stomach acid, then the stomach really isn’t going to do them any good anyway.

11. They have 10 sex chromosomes! Most mammals have two sex chromosomes, one from each parent. An individual that has two X chromosomes is usually female and an individual that has one X and one Y chromosome is usually male. Thus, female mammals pass along an X chromosome to each offspring and males can pass along an X or a Y. But platypuses are not content to be normal in any way…They have 10 sex chromosomes: 5 from mom and 5 from dad. All 5 chromosomes from mom are Xs, whereas a male sperm either contains 5 Xs or 5 Ys. Birds also have two sex chromosomes, but in birds, individuals with two of the same type are usually male and individuals with different chromosomes are usually female. Their system is called ZW, where the mammalian system is XY. The platypus X chromosome is more similar than the X chromosome of other mammals to the bird Z chromosome.

12. The platypus genome is as much of a hodgepodge as its body. Only 80% of the platypus’ genes are like other mammals. Some of their genes have only previously been found in birds, reptiles, fish, or amphibians.

To learn about more weird animals, go here.

References:

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