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?
The short answer – No.
The Long Answer
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.
Sex determination in other species, however, is more flexible, and may allow for a more malleable 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.
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 ( Amphiprion ocellaris) 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 genusLythrypnus 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.
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, simultaneous 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 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
Featured image is used under a creative commons licence from Wikimedia commons. Original image by Andrei!