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Category: Zoology

Sex Reversal – When Males Grew Ovaries Instead of Testes

Summary: Sex reversal is not unusual in some animals, especially in invertebrates. As for the vertebrates, there are reptiles and fish that can have their sex reversed under certain circumstances. Sex reversal in these animals is often driven by environmental and social factors. But how about mammals and humans whose gonads are fully differentiated and thereby permanent by the time they reach adulthood…? Can they, too, undergo sex reversal without going through any artificial intrusions? If so, up to what extent…?




Sex reversal occurring in nature

In a biological context, sex reversal pertains to the phenomenon in which the sex (gonadal and secondary sexual characteristics) of an organism is altered from one gender to another. This is fairly common among invertebrates, like a number of free-living nematodes. Within a given population, they can alter their gender – from male to female and vice versa. Less-favourable environmental conditions drive them to their sex reversal.1 Slipper-shell snails (Crepidula, sessile molluscan species) males are also capable of turning into females. Initially as a male when young, it changes into a female when a male sits close.2  Certain vertebrates are capable of sex reversal, too. For example, in a group of goby fish, the loss of an alpha male causes the largest female in the group to assume the role.3




Genetic factors in mammalian sex reversal

Sex reversal: sex-reversed male XY mouse (left) and female XX mouse (right). Credit: Greta Keenan, Francis Crick Institute.


In mammals, natural sex reversal is plausible, albeit genetically and during early gonadal development. It can occur even in humans but only among those with genetic tendencies.  Our gonadal plasticity is so limited that our gonadal sex is determined not by social or environmental factors but essentially by the activity of the existing sex chromosomes.  Typically, a male has one Y chromosome and one X chromosome; a female has two X chromosomes. Previously, people mistook the X chromosome as the sex-determiner. Proofs from subsequent studies, such as the discovery of the sex-determining region Y (SRY) gene normally located on the Y chromosome, debunked the notion. The SRY gene carries the code responsible for the synthesis of a protein that can initiate the development of the testes. If the SRY gene is dysfunctional or nonexistent the testes will not form. Hence, the protein it encodes for was named testis-determining factor (TDF).  In 1991, scientists successfully incited sex reversal on a mouse when introduced with SRY gene while still an embryo. Although it was chromosomally female to begin with, it eventually grew male gonads.4 Thus, in spite of the minuscule size and having fewer genes than the X chromosome, the Y chromosome is considered the key to determining the chromosomal sex owing to its SRY gene.


(Read a related article on Y chromosome: Men could go extinct? Y chromosome is slowly disappearing)



Junk DNA Enh13 in sex reversal

A research team from the Francis Crick Institute found yet another chromosomal piece that resulted in gonadal sex reversal in mice. It contained genes that were regarded as junk. Junk DNAs are called as such because they are noncoding genetic material. In human genome, a meager 2% codes for the building blocks of proteins crucial to life. The remaining percentage, which comprises the bulk, is deemed unnecessary, and therefore, dubbed as junk. Apparently, they do not code for proteins. Nevertheless, recent findings suggest that they, too, play an important role. The enhancer 13 (Enh13) exemplifies it. The researchers found that the lack of Enh13 in male mice led to their sex reversal. Instead of testes, the mice grew ovaries and female genitalia. 4 This could mean that the junk DNA Enh13 takes a crucial role in early gonadal development. Enh13 supposedly enhanced the production of SOX9 protein. SOX9 gene encodes for the SOX9 protein. The SOX9 gene does so when TDF proteins bind to the enhancer sequence upstream of the SOX9 gene. The more SOX9 proteins produced the more that the embryo will commit to developing into a male.




Impact of Enh13 findings on sex reversal

Dr. Nitzan Gonen, one of the researchers on the team, talked about the impact of their study on mammalian sex reversal. He said that so far they identified four enhancer regions and they were surprised how a single enhancer could control “something as significant as sex”.4 Their Enh13 findings could be used as a genetic basis to understanding sex reversals in humans and other mammals.




Sex reversal in mammals occurs but is not as extensive as that in other animals. Gonadal plasticity is confined during the time of embryonic development. Upon reaching adulthood, the gonads are already formed and will not change from one type to another. When both male and female gonads develop (in the case of intersexuality), usually, only one of them, or none, will be functional. Sex reversal in humans has also been reported. One such example is the case of two brothers and a paternal uncle from the U.K. They are outwardly and anatomically males but genetically females (with 46, XX karyotype).5 Genetic mutations might have been the underlying cause.  More studies on the molecular genetics of sex reversal could help provide insight as to the gender ambiguities in humans, which, unfortunately up to this day, have no clear genetic explanation.



— written by Maria Victoria Gonzaga



1 Kent, G. C. (2018). Animal reproductive system: sponges, coelenterates, flatworms, and aschelminths. Encyclopædia Britannica. Retrieved from
2 Smith, N. G. (1999). Reproductive behavior. Encyclopædia Britannica. Retrieved from
3 Wilson, M. (2013). Sex-reversal in adult fish. The Company of Biologists. Retrieved from
4 The Francis Crick Institute. (2018). Non-coding DNA changes the genitals you’re born with. ScienceDaily. Retrieved from
5 Cox, J. J., Willatt, L., Homfray, T., & Woods, C. G. (2011). A SOX9 Duplication and Familial 46, XX Developmental Testicular Disorder. N Engl J Med. 364 (1):91-93. doi: 10.1056/NEJMc1010311. Retrieved from

Tracking the migration of Whip-poor-will in Americas

Eastern whip-poor-will (Antrostomus vociferous) is continuously declining due to habitat loss and unavailability of insects for food. Little is known about whip-poor-will migration because of their nocturnal quite habit during non-breeding season. At high latitude 80% avian species are migratory wherein factors affecting migration includes predators, anthropogenic threats and pathogens. Migratory strategies allows individual to track seasonal changes mostly for temperate breeding aerial insectivores. However, population declines among temperate insectivore birds due to extreme weather condition, cost of migration and reliance on sensitive prey. In addition it is important to determine the migratory routes, year round habitat requirement and temporal constraints of threatened species.


Geolocator deployment of Whip-poor will

There were 20 males and 2 females of whip-poor-will have been tracked using geolocators in four regions of Canada. The study shows that this species breed more in northern part than southern breeding population and experienced different wintering conditions. Also a high migratory cost happens such as novel threats, energy expenditure and the ability to adjust time in tracking breeding ground condition.  In contrast, both eastern and western breeding individuals wintered together wherein mostly concentrated in Guatemala and some provinces of Mexico. However, male often have higher benefits of early arrival on the breeding grounds thus accept higher cost of wintering further. Additionally, early arrival on breeding grounds is more advantageous on whip-poor-will males allowing occupation on higher quality territories.


On the other hand female whip-poor-will forced to migrate further on lower latitude with less competition and more abundant resources. Most of this species travel overland through Mexico and Central America. However, only two individuals flights across portion of the Gulf of Mexico during autumn and spring. It just shows that this pattern is the response to prevailing winds and availability of resources along different route. Also more species migrating along Eastern North America, South and Central America over ocean flights during autumn. While in spring more species taking longer over land route around western side of the Gulf of Mexico.


Therefore, geolocators is helpful in identifying wintering areas, stopovers and migratory route of whip-poor-will. These migratory stopovers in the southeastern and central United States as well as in southern Mexico and Central America are both important for the whip-poor-will species. Finally, habitat protection and insect population might increase the number of these species despite pressures of long migrations and climate changes.


Source: Prepared by Joan Tura from Springer BMC Zoology

Volume 2:5, 2017

The Amazing Spider Dads

Humans are not the only fathers capable of giving it all for the sake of their progeny. The animal kingdom is teeming with fathers that are downright heroes.  For instance, male redbacks and dark fishing spiders would voluntarily throw themselves into the clutches of their mated female and eaten… never again to procreate or see the light of the ensuing days. This seems disturbing. How could mating be evolutionary costly for these unfortunate yet amazing spidey dads?




Sexual cannibalism of spider fathers

Sexual cannibalism. (Credit: Kumon, Flickr)


The mating of spiders in which the female eventually devours the male is one the most fascinating animal couplings. In this case, male spiders that engage in the rituals of courtship and copulation are likely dead fathers. Literally hungry for more, the females consume their mates in an act called sexual cannibalism. The devouring of another individual of the same species could occur before, during, or after copulation. The female spiders are usually the sexual cannibal, perhaps, because they are often larger than their male counterparts, and so, more physically dominating. Cannibal female spiders are often hostile and unenthused to mate. Thus, male spiders ought to be valiant to approach and fancy them with their moves. An impending death may be too much of a price to pay but these hopeless romantics are willing just so they can be fathers to their mate’s soon-to-be spiderlings.




Spider Fathers avoiding sexual cannibalism

Sexual cannibalism in spiders is real. However, not all spider dads end up harmed after mating.  Many male spiders, in fact, do not end up in the females’ gut. Not all female black widow spiders consume the fathers of their prospective spiderlings. Thus, the notion that all black widow females are sexual cannibals (hence, the “widow” on their name) is a misconception.1 There are also male spiders that came up with their own tricks. One fantabulous example is to play dead. By appearing stiff dead, male wolf spiders avoid ending up as a palatable snack after copulation.2  The apparent death trick is called thanatosis.

(Read: Thanatosis – faking death to escape doom)



Altruistic Spider Fathers

While certain spiders dodge sexual cannibalism, there are those that do not just welcome it but also incite it. These male spiders are the quintessential altruistic spider fathers. Male redbacks (Latrodectus hasseltii), for instance, encourage adult females to engage in sexual cannibalism. After inseminating the adult female, the male somersaults to bring his body close to her mouthparts like a cue saying “eat me now”. The female spider spits gut juice, and then feeds on him. If lucky enough to live after that, he returns to her, filling her with more sperm, plus a nutritious “meal”.  Eventually, he dies by succumbing to his injuries from slow cannibalism. Another example is the male dark fishing spider (Dolomedes tenebrosus). As if a befitting sacrifice to his female, the spider curls up with no hesitation. Thus, one may wonder: “Why would these male spiders sacrifice their life for a one-time sex?” It seems unsound and evolutionary counter-productive. Research3 on dark fishing spiders implicated that cannibalism improved offspring survival. Females that ate their spiderlings’ fathers had more surviving offspring than those that did not. The spider dads seem to possess unknown components that significantly boosted their offspring size, fitness, and survival.

Redback spider. (Credit: Ryan Wick, Flickr)

Dark fishing spider. (Credit: Charles de Mille-Isles, Flickr)





In essence, the self-sacrificing behavior of these spider fathers is a manifestation of how ready they are to die for the sake of their progeny. With the assurance that their genes are passed on to their offspring, they served a remarkable purpose as befitting fathers to their spiderlings even if it means acceding to sexual cannibalism.




— written by Maria Victoria Gonzaga




1 Crawford, R. (2015). Myth: Black widows eat their mates. Retrieved from
2 Seriously Science. (2016). Male spiders play dead to avoid “sexual cannibalism.” Retrieved from
3 Choi, C. (2016). Journal Club: Self-sacrificing male spiders assist in their own cannibalism to aid offspring. Retrieved from

Muscles of the Apes – a thwack on human muscle evolution

Summary: A recent finding suggests that apes do have the muscles for bipedalism, vocal communication, and facial expressions that were largely held exclusive to humans.




When Planet of the Apes hit the big screen, many of us thought it was a far-fetched display of what could have been if apes were able to act, speak, and think like humans do. Of course, it was a downright fictional movie and was great at that. What comes first to mind is the concept’s plausibility. In essence, humans possess the distinctive capabilities for vocal communication, facial expressions, and prolonged bipedalism that closely-related apes do not display. It seems that the apes lack the muscles purportedly “unique” to humans. However, this notion may be completely overturned. A recent finding suggests that apes do have the muscles for bipedalism, vocal communication, and facial expressions that were largely held exclusive to humans.




Human muscles lacking in apes?

How is it possible that humans can talk and walk but apes cannot? Is it because the apes do not have the necessary muscles? A long-held theory elucidates that apes cannot speak and walk like we do because they lack the muscles essential for vocal communication and full-time bipedalism. These muscles are dubbed as “uniquely human” since nobody has them but humans. Examples of these muscles are fibularis tertius (also called peroneus tertius, a lower limb muscle linked to effeicient bipedalism), musculus arytenoideus obliquus (or oblique arytenoid, a laryngeal muscle used for vocal communication), and risorius (a facial muscle involved in facial expression).1

The exclusively-human muscles helped, particularly, the modern human species, to veer away from an arboreal way of life. Our ancestors used to live above the ground on the trees just so they could evade predators and at the same time have access to food, such as fruits and insects. Eventually, they developed new habits. They expanded their food choices. They became skilled hunters. They enriched their social interactions. The pressure of a risky bipedal habit on the ground teeming with predators perhaps pushed our ancestors to living in groups. Soon, they acquired features that made them very well adapted to living life on the ground. One of them is muscle adaptations. For instance, humans evolved an opposable thumb that enabled multitasking and tool use. A full-time bipedal would also need to balance and center the force of gravity on two feet. Thus, a thigh bone with an inward slope down the knee eventually developed. This enabled the gluteal abductors to adapt efficiently to the stress. Apparently, humans lost certain features (such as the flexible plantaris muscle in foot essential for grabbing and gripping) but gained important attributes that helped thrive on the ground.


Muscles that were believed to be exclusive to humans but were also recently found in apes:
A. Risorius, a facial muscle involved in facial expression (in bright red)
B. Musculus arytenoideus obliquus, a laryngeal muscle used for vocal communication (see arrows)
C. Fibularis tertius also called peroneus tertius, a lower limb muscle linked to efficient bipedalism (in bright red)



Exclusively-human muscles found in apes

A study on ape anatomy1 published in Frontiers in Ecology and Evolution posits that the seven muscles linked to sophisticated vocal communication, facial expressions, bipedalism, and tool use once thought as “unique” to humans were present in apes as well. The muscles found in certain ape species even included fibularis tertius, musculus arytenoideus obliquus, and risorius.

The author, Rui Diogo, an Associate Professor at Howard University, USA refuted the long-held theory that such muscles are unique to humans. His findings contradicted this perception on human muscle evolution. He explicates, “Our detailed analysis shows that in fact, every muscle that has long been accepted as ‘uniquely human’ and providing ‘crucial singular functional adaptations’ for our bipedalism, tool use and vocal and facial communications is actually present in the same or similar form in bonobos and other apes, such as common chimpanzees and gorillas.”2

Due to the anatomical studies on apes are scarce, the long-held theory on exclusively-human muscles has not been defied with substantial empirical data but now.

In another study3 published in the Proceedings of the National Academy of Sciences, a team of researchers looked at the molecular features of the muscle fibers of the thigh and calf muscles of an ape group, the chimpanzees (Pan spp.). They found out that the chimpanzees had twice more myosin heavy chain II (MHC II, also referred to as the fast-twitch fibers) in those muscle areas than humans. In contrast, humans had higher myosin heavy chain I (MHC I, also referred to as the slow-twitch fibers) content than the chimpanzees. Their findings indicate that our human ancestors seemingly traded off strength for endurance, which was more crucial in a life of roaming and foraging.




Many people believed that humans are far more superior than our ape relatives. Our sophisticated vocal communication, facial expression, and full-time bipedalism are simply unmatched. However, these studies point out that humans were not that grander. While we could be better in terms of endurance, some of the apes are indeed physically sturdier than us. They even have muscles that were previously thought as exclusive to us. In essence, we did lose certain physical attributes that our closely-related apes retained for their worth in an arboreal way of life. Even so, we gained a leeway to living life on the ground, which we could meekly consider a human milestone.




— written by Maria Victoria Gonzaga





1 Diogo, R. (2018). First Detailed Anatomical Study of Bonobos Reveals Intra-Specific Variations and Exposes Just-So Stories of Human Evolution, Bipedalism, and Tool Use. Frontiers in Ecology and Evolution, 6. DOI: 10.3389/fevo.2018.00053
2 Frontiers. (2018, May 23). ‘Uniquely human’ muscles have been discovered in apes: Apes also have muscles long-believed to be only present in humans and used for walking on two legs, using complex tools, and sophisticated facial and vocal communication. ScienceDaily. Retrieved from
3 O’Neill, M.C., Umberger, B.R., Holowka, N.B., Larson, S.G., and Reiser, P.J. (2017). Chimpanzee super strength and human skeletal muscle evolution. PNAS , 114 (28): 7343-7348. Retrieved from