The Chicken That Changed Its Sex

Have you heard the one about the chicken that got a sex change? 

Not the most pervasive of suburban legends, granted, but it seems to keep popping up. It goes something like this...

Confused Farmer finds Hen is now Cock
The mature hen, Gertie, who had laid eggs
the previous year, suddenly stopped, grew

chin wattles and started to crow. 

So, can chickens really change sex? - Short answer: NO. 

A male and female pair of domestic
Chicken (Gallus gallus domesticus)

There are a few different explanations for stories such as these, but the important point of all of them is that these apparent sex changes are merely superficial - the hen might be visually and behaviourally male, but she is still unable to fertilise another hen's eggs. So why does the hen suddenly start crowing? It is thought to occur due to a hormone imbalance, either stemming from a heritable condition or from an environmental source (e.g. food). Fungi sometimes found in animal feed produce mycotoxins, some of which can influence sexuality through their interaction with the hormone oestrogen.

Chickens might not be capable of changing sex at will, but there are other creatures that can. Birds and mammals have a fairly strict system of sex determination: sex chromosomes. Sex is therefore determined at the point of fertilisation. To change from one sex to another after sexual maturation is pretty much unheard of. Whilst in mammals, the sex chromosome is Y and denotes the male sex (females XX males XY), in birds the system is reversed, with the sex chromosome W denoting the female sex (females ZW males ZZ). This has some interesting effects, but doesn't make it any easier to change sex. Some insects use a different, but also relatively inflexible method of sex determination: fertilisation. In bees, ants and wasps, males develop from unfertilised eggs and females from fertilised ones. 

Jacky Dragon Amphibolurus muricatus

Sex determination in other species, however, is more flexible, and may allow for a more maleable outcome. For example, many reptiles lack a sex chromosome and instead use an environmental cue; temperature, to determine the sex of their offspring. This mechanism is found in adaptation to temperature-dependent sex biases in fitness; where males and females fare differently at different temperatures. In some parasitic nematodes, host overcrowding is a big problem, but males tend to fare better in dense populations than females do. So sex is determined after eggs are laid in the host; males will tend to develop in crowded hosts and females in more sparsely populated hosts. 

In some reptiles, including many sea turtles, males are produced at low temperatures and females at higher temperatures. Other reptiles exhibit a more complex system with females being produced at all temperatures but males only at intermediate ones. The latter system is used by the Jacky dragon (Amphibolurus muricatus), and research has shown a 3-fold benefit to being male at intermediate (normal) temperatures, but a substantial benefit to females at temperatures below or above average. 

Being sexually flexible pays when environments are variable. Some species can't even make up their mind if they want to have sex or not; aphids are capable of both asexual (clonal) and sexual reproduction. Aphids are generally female and reproduce asexually throughout the year, until autumn when they start producing sexual males and females who lay eggs that will overwinter. Many other species are hermaphroditic, with all individuals capable of taking on a male or female role, although self-fertilisation is rare. Some snails, slugs, a few fish, earthworms and almost all plants have both male and female reproductive parts. 

Clown Fish or Anemone Fish (Amphiprion ocellaris)

A few species take sexual plasticity to a whole new level. Known as sequential hermaphrodites, several species of fish are capable of changing their sex according to circumstance. Nemo is a sequential hermaphrodite, in fact; clownfish (Species name) change sex during their lives according to social cues. Living hidden amongst an anemone, clownfish have a strict social hierarchy, in which the only female and the largest male get to reproduce. When the present female dies, the largest male (her partner) becomes a female and begins reproducing with the next largest male. This bizarre mating system has been favoured at least in part due to the isolated, close-knit communities that form in the safety of the anemones (dispersal is very, very dangerous!).

Coral gobies exhibit an equally weird reproductive system. They live and reproduce in monogamous pairs, but when one member of the pair dies, the widower must leave home and find a new mate. If the first available mate they find is the same sex as them, well, no worries, they'll just switch sex. It's so dangerous to go out and look for a new partner, that it's better just to change sex than try to search for another partner of the opposite sex! 

Gobies from the genus Lythrypnus produce both eggs and sperm all the time, although they operate as just one sex at a time. When they choose to switch is informative as to the underlying evolutionary explanation for this unusual reproductive strategy; it's all about size. Large males enjoy a disproportionate amount of mating success, and so a large individual is best off being a male to maximise it's reproduction. Female reproductive output is less variable than males  and so a small female does better than a small male. In support of this, large gobies tend to chose to become males, and enjoy a reproductive benefit when they do. This pattern is predicted by the size-advantage model, however situations in which the opposite pattern (better to be a large female or a small male) exists are also possible, and exist in nature. 

Blue-banded Goby (Lythrypnus dalli)

Sometimes, it pays to be flexible. But there are major benefits to having fixed sexes as well - this allows for divergence of males and females to be better adapted to their particular role. In maintaining lifelong plasticity, simmultaneous and sequential hermaphrodites forgo the opportunity to adapt to just one job. They prefer to be a jack of all trades. But in a dangerous and unpredictable environment, flexibility can be very rewarding. 

Want to Know More?
Warner (1988) Sex Change and the Size-Advantage Model. TREE 3(6): 133 - 136
Munday (2002) Bi-directional sex change: testing the growth-rate advantage model. Behav Ecol Sociobiol 52: 247 - 254
Munday, Buston and Warner (2006) Diversity and flexibiliy of sex-change strategies in animals. TREE 21(1): 89 - 95
Bennett and Klitch (2003) Mycotoxins. Clinical Microbiology Reviews 16(3): 497 - 516
St Mary (1993) Sex allocation in a simultaneous hermaphrodite the blue-banded goby (Lythrypnus dalli): the effects of body size and behavioral gender and the consequences for reproduction. Behavioural Ecology 5(3): 304 - 313
Charnov and Bull (1977) When is sex environmentally determined? Nature 266: 828 - 830
Warner and Shine (2008) The adaptive significance of temperature-dependent sex determination in a reptile. Nature 451: 566 - 569

Dengue: The Neglected, Neglected Tropical Disease

Modern medicine can boast a number of triumphs against infectious disease over the past century, but it comes as no surprise that those diseases against which we have had the greatest success are those that affect developed nations. Increasing attention is now being paid to neglected tropical diseases.

read more >>>> (Article in Experimentation)

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Seeing into the Future

The Bionic Contact Lens

(Photo credit: Wikipedia)

Around 125 million people wear contact lenses world wide, generally to correct vision. But soon even those of us with 20-20 vision might be wearing them, as scientists have made early successes in incorporating computers into contact lenses.

Yes, the future is officially here. A US team has released the details of their latest development – a contact lens containing an antenna that allows it to wirelessly gain power to light a single LED (in the same way your oyster card uses radio waves to gain energy and register you passing through the barrier). One of the biggest challenges in developing these lenses has been allowing the eye to focus on the image in the lens; normally we cannot focus on objects closer than about 10cm from our eyes. Their trick to deal with this is to place several microlenses on the surface of the contact lens, which have the effect of making the LED appear further away, and thus allowing the eye to focus on it.

All we have so far is a single pixel, which isn’t much, although even something this simple could be of use to the hearing impaired, for example. Second Sight hope to soon incorporate more LEDs into their lens, eventually enough to create words and images in front of your eyes. The majority of the contact lens remains transparent, allowing the user to interact with and navigate the real world whist receiving real-time information through their lens. This kind of technology could be applied to almost everything, from enhancing computer games to providing real time subtitles in any language, to navigational assistance, and, well the mind boggles really!

Although still in, the early phases of development and testing, researchers are optimistic about the future of these technologies, and animal tests have shown no adverse reactions to wearing the bionic lenses. Contact lens technologies that can monitor pressure changes and detect early signs of glaucoma are already available on the market, and others which monitor glucose levels in diabetes sufferers have shown promising results in early testing. Researchers at the University of Washington have even been experimenting with incorporating solar power into their contact lens designs, enabling the devices to run without the need for

Want to Know More?

Lingley et al (2012) A contact lens with integrated micro solar cells. Microsystem Technology 18: 453 - 458

Yao et al  (2011) A contact lens with embedded sensor for monitoring tear glucose level. Biosensors and Bioelectronics 26: 3290 – 3296

Lingley  et al (2011) A single-pixel wireless contact lens display. Journal of Micromechanics and Microengineering 21: 125014

Parviz (2009) Augmented reality in a contact lens. IEEE Spectrum

Sensimed

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Planet Ant

Check out the fantastic new BBC documentary, Planet Ant, which I had the pleasure of being a part of. Wonderful documentary revealing the intricacies of ant life, and how this relates to our own lives. Thanks to everyone who helped make this project the fantastic success that is has become. 

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• Insect beauty was clearly in the eye of the beholder in Planet Ant

Selfish Strategies in Cooperative Societiy

Social insects show extraordinarily high levels of cooperation, giving up their reproductive opportunities and even sacrificing their own lives to save the colony. The social insect colony is a well-oiled machine, each part has its own key role to play, together forming an intelligent and adaptive society. Most people are familiar with the highly advanced social insects, such as honeybees and leaf cutter ants. Their societies are huge and intricate, and we have gained many fascinating insights from them. However, they tell us very little about how these societies evolved, or what it meant to be cooperative in a more primitive sense. 

Advanced society in the honeybee (Apis mellifera) (Photo credit: Wikipedia)

The most striking characteristic of advanced social insects such as these is that almost everyone in the colony is physically incapable of mating and reproducing. One or a few individuals (queens) retain this ability whilst the others are committed to sacrifice themselves for the good of the queen. This seemingly counter-intuitive self-sacrifice is favoured by evolution because the altruistic workers are closely related to the reproductive queen. When they help her to produce offspring, she passes on genes which they share with her. But again, sterility in the worker caste is a feature of highly advanced social insect societies, and tells us little about the early evolution of social behaviour. 

Some social insects alive today still have a simpler society, similar to what might have occurred when sociality first evolved. In these societies, although workers generally refrain from reproduction, they are still capable of it. This gives them many more options  compared to their sterile cousins, and the line between queen and worker in these societies is often blurred. Subordinates in these simple societies can choose to work, they can choose to leave found their own nest, or they can even try to sneakily lay eggs when their queen isn't looking. What they choose to do often depends on a delicate balance of costs and benefits, and this varies from species to species. 

In the tropics, where colonies are not limited by the necessity to hibernate over winter, and thus nests can be potentially immortal, it is common for the queen to die before the colony does. In circumstances such as these, when the royal position opens up, the workers have an opportunity to take over and reproduce. This opportunity is much better than leaving to found your own nest, as it comes complete with an inheritance - subordinate workers. In some species, the subordinates left on the nest after their mother dies will fight it out to decide who is next in line, while in others a prearranged hierarchy decides the heir to the throne. These hierarchies are generally quite dynamic and can change rapidly, with high-ranked workers being displaced and losing their bid for the throne. For an individual sitting in a high-rank position, the key is to try and ensure that you take over the throne before you lose your rank or die.

Dinosaur Ant (Dinoponera quadriceps)
Photo copyright Claire Asher 2011

A Royal Strategy
What can an individual ant do to increase it's chances of inheriting the throne? There are a number of options:

1) Be Lazy
Working hard tends to make you die younger. So, if you want to be sure you out live your royal parent and inherit the reproductive role, your best bet is to stay put and do as little as possible. In the socially simple Dinosaur ant (Dinoponera quadriceps), high-ranking workers spend more time sitting around the nest doing nothing than other colony members. And in simple wasps societies, such as those of the paper wasp Polistes dominulus and the hover wasp Liostenogaster flavolineata, individuals closer to inheriting the throne tend to work less hard. 


2) Don't Take Risks
One of the most dangerous jobs a worker can perform is foraging. This requires them to leave the safety of the nest and venture out into a world filled with predators and the very real possibility of getting lost. High-ranking dinosaur ants refuse to leave the nest for foraging or taking out the rubbish, and recent research has shown that they also refuse to defend the nest against attack, letting the lower-ranking subordinates take that serious risk. Likewise, it is the low-ranking colony members in the social Damaraland mole rat (Cryptomys damarensis) who defend the nest against attack.

Damaraland Mole Rats (Cryptomys damarensis)
Photo credit: LiveScience

3) Monitor the Situation Closely
Figuring out when the time has come to overthrow the current queen is also crucial to success. Social insects don't always wait for her to die gracefully; in Dinosaur ants, subordinates will pin her down and force overthrow when her fertility drops too far. And high-ranking workers spend an awful lot of their time inspecting and generally hanging around her eggs - this may be a way of assessing her fertility, enabling workers to pick their moment for a coup. 

These three tactics may enable a high-ranking worker to hold onto her position just long enough, and sense when the time is right to take over. And should she succeed, she inherits a well established nest, a substantial workforce and the opportunity to mate and produce her own young. Should she fail, then she's either dead or forced into a low-rank position for the rest of her life. And as a low-ranker, her best bet is to work as hard as she possibly can and take any necessary risks in order to protect the colony, as it contains her relatives and the only opportunity for her to pass genes to the next generation. So, it may be that these colonies reach a form of homeostatis, high-rankers prefer to be lazy, and low-rankers prefer to work hard, and everybody is happy, or as happy as they possibly can be. 

Simple eusocial societies, such as those of Dinosaur Ants, Paper wasps and mole rats, can reveal subtly in social behaviour that are not so evident in more advanced societies, and suggest mechanisms by which social order was maintained during the early stages of eusocial evolution. 

Want to know more?
Asher et al (2013) Division of labour and risk taking in the Dinosaur ant, Dinoponera quadriceps (Hymenoptera: Formicidae). Myrmecol.News 18: 121 - 129
Field (2009) Social stability and helping in small animal societies. Phil.Trans.Roy.Soc. B 364: 3181
Cant & Field (2001) Helping effort and future fitness in cooperative animal societies. ProcB 268: 1959 - 1964
Monnin & Peeters (1999) Dominance hierarchy and reproductive conflicts among subordinates in a monogynous queenless ant. Behav. Ecol 10: 323 - 332

Rethinking Mammalian Evolution

The bedtime story of mammalian evolution begins humbly, with small shrew-like creatures diversify into over 5000 species after the extinction of the dinosaurs. This story has held for decades, however recent research is causing scientists to rethink the conventional tale.

Read More >>> (Article in Experimentation)

Five Programs That Will Change the Way You Work

As I draw towards the end of my PhD, I have recently been introduced to a number of applications and websites which I really wish I'd found sooner - these tools have revolutionised the way I read papers, make notes, organise my thoughts and my time, and write essays. For the majority of them, their use extends well beyond the field of science, and almost anyone could find benefits from incorporating them into the way they work.

1. Organise Journal Articles with Papers
One of the only recommendations that is probably fairly science-specific: Papers. This application allows you to organise all your PDF files in one convenient location, making notes and tagging different articles with terms you can later search for. You can group PDFs together for different purposes and create smart folders containing all PDFS on certain topics. This is a far cry from the mish-mash folder organisation system I used as an undergraduate, and almost entirely eliminates the "I know I read that somewhere" problem that so frequently delays essay and article writing. It's £50 (or $80) but students get a 40% discount and it's well worth the cost in terms of time saved. If you're not prepared to pay, Mendeley is a fairly good (free) alternative.

2. Track and Manage Time with Toggle
Track your work day with Toggle. Record different activities as you do them and group them into categories. You can even group tasks according to client (for the non-scientists!). Summary stats on your week's work show the total working time (good for feeling smug about how hard you've worked) and a break down of time dedicated to different tasks. A great way to keep track of what you've been doing, with the potential to reveal insights into your working habits.

3. Make Lists with Trello
For all those list-makers out there, there's trello. A flexible and easy to use site which allows you to create and label different tasks according to categories of your choosing, easily move tasks between lists and keep track of what you need to do. Filter tasks according to category or due date. You can even make lists within lists and check them off as you go. It's function is very open to moulding into whatever way you want to work - keep a list for each different aspect of your life, or lists for urgent, to do and done, or whatever else you can think of! Get the satisfying feeling of moving or archiving a task once it's complete.

4. Map Your Thoughts with FreeMind
This software is completely free and although the look isn't totally polished, it's a great piece of software for organising your thoughts. Different 'bubbles' can have notes attached which can in turn be formatted in whatever way you like - I use it to keep track of references for different aspects of an essay or article, but in theory it's applications are as big as your imagination. Another nice online alternative is Bubbl, although this doesn't allow for extensive notes.

5. Store and Access Files Anywhere with DropBox
This is a really obvious one, but for someone who works both at home and in the office, this free online storage facility has almost eliminated the "but I left that data at home" problem which I must confess I've been rather prone to. It also leaves you with the peace of mind that your most important files are stored somewhere externally. And it's easy to share files with friends and colleagues (although personally, I prefer Google Drive for that).

The Magic of Medicine: Gene Therapy

The Magic of Medicine
Gene Therapy

In the 60 years since Watson and Crick’s landmark discovery of the structure of DNA, our understanding of how genes influence disease has increased exponentially. For some conditions, an exciting therapeutic prospect exists: gene therapy. Gene therapy attempts to repair faulty genes instead of simply treating symptoms. 

Read More >>>> (article in Experimentation)