I’ll Fly Home—Or Not

The snowy owl is sedentary, meaning it does not

migrate. The males are almost perfectly white,

while the females and juveniles can have black

barring. They sit, look, and listen for their prey,

which includes small rodents and even other birds.

Their hearing is good enough to let them target a

mouse under the snow from hundreds of yards away.

The Arctic tern travels from north of the Arctic Circle to Antarctica and back again every year. On the other hand, the Snowy Owl lives in the arctic region year-round; it doesn't migrate at all.

The Question of the Day:

Why do some birds migrate while other birds stay in one place?

The possible explanations are many. Maybe the type of food they eat is present only part of the year, or maybe they can’t stand the cold temperature. They might need to have their babies in a place away from predators, or perhaps migration is an evolutionary holdover that had a reason in the past, but no longer is necessary.

How could you start to determine the reason for migration in only some bird species? I would start by looking at how closely related the migratory and non-migratory species are. Maybe all the birds that migrate are more closely related to one another than to the birds that don’t move around during the year. This would suggest that there is a genetic basis for why only some species migrate.

One research group did just this in 2007. The looked at 379 species of flycatchers, a closely related group of birds. They found that almost equal numbers of species were migratory or resident, so it doesn’t appear that genetic relatedness is the answer.

The painted bunting is among the most colorful

birds in North America. This male has several colors,

while the female is a brilliant green. They breed in

south central US or on the southern east coast, but

winter in Central America; therefore, they are

seasonally migratory.

Maybe the need to migrate has to do with the geographic region. About 90% of birds in the arctic migrate, some are present there only in the mid-summer months. The arctic tern is a good example. Arctic terns move with the summer, breeding in the arctic in May-July, moving down along continental coasts to arrive in Antarctica for the months of December to February. The entire distance traveled could be as much as 32,000 km (20,000 miles) in a single year.

Similar to the arctic region, the east coast of North America has species that migrate and species that are sedentary. About 80% of the birds species of the east coast move south during the colder months, but on the Pacific coast, almost all the bird species are non-migratory.

So migration is not due to the type of geography around the birds. However, the east coast of North America does have larger temperature swings than the west coast, so maybe it is just that some birds can’t deal with the cold. 

Some non-migratory birds can control the amount

of blood that travels to the legs in order to conserve

body heat. This works even better if they can reduce

the amount of contact with the cold surfaces, so some

of these birds perch on one leg at a time.

Many birds that do not migrate have special adaptations to deal with the cold. Trying to keep a constant body temperature (endothermy) takes a lot of energy, and birds live right on the edge of having enough energy anyway. Flying requires a huge amount of energy and they must eat almost constantly just to keep enough carbohydrates in their system to be able to move around to find more food.

Burning more energy to keep warm might tip them over the edge into starvation. To alleviate this problem, many birds can allow parts of their bodies cool down to freezing or near freezing, while keeping their internal organs at a temperature that will preserve their function. Blood flow is a major way to keep parts of the body warm, a duck standing on the ice can reduce the blood flow to its feet and reduce the amount of heat lost to the cold ice. The duck’s chest may be 40˚C, but its feet could be just one degree above freezing.

But let us look again at the arctic tern. It migrates from the north polar region to the Antarctic region in such a way that it sees two summers each year. But these are summers in name only. The arctic summer has an average temperature from -10˚C to 10˚C, so much of the time the tern is there, the temperatures are near zero.

The arctic tern has a ghastly commute each year.

The trip is even more amazing when you consider

that during its yearly molting, the tern flies very

little.  So, all that distance must be fit in to just a part

of the year, not the entire 365 days. I guess they

vacation by NOT traveling.

Then when they reach the Antarctic, the summer there has an average temperature of -2˚C to 2˚C. This is hardly a balmy vacation destination for the tern. The temperatures in both its breeding grounds and wintering grounds would require it to have elaborate temperature control and energy-saving adaptations. Therefore, inability to tolerate cold temperatures is not the reason for migration, at least not for many birds.

The group who carried out the 2007 study concluded that the main reason that only some flycatcher species migrate is not due to what they eat, or when they breed, or what is trying to et them, but to how available their food source is. Whether they are fruit eaters, or insect eaters, or seed eaters, how easy it is to find their food is the most common reason that migration has evolved for a specific species in a specific location.

The food availability hypothesis is supported by certain types of migration that are common in North America. Irruptive migration is characterized by a population moving to another place, but there is no yearly, seasonal, or geographic pattern. The birds may migrate one year, then not again for a dozen years, or they might go for several years in a row. North American seed-eating birds are famous for these migrations. The distance and number of individuals that migrate are also not very predictable, and this all makes it sound like the movement is linked to food availability. However, it could also be to escape some population explosion in a predator species or for some other reason.

It isn’t only birds that might undergo partial migration.

Some crab species will migrate for breeding purposes,

like these Christmas Island Red Crabs.  Individuals

that won't breed just don’t make the trip. They may be

too young, too old, too lazy. In other cases, when

populations migrate away from the breeding grounds,

some individuals may remain there the year round.

Another type of migration is partial migration, a pattern wherein not all birds of a species in a certain location will migrate, only some of them leave in non-breeding times, while others stick around year-round. Food may be available for some, but not all, or the environment may be unsuitable for some weaker individuals to have enough time to forage for a sufficient amount of food. These (and other reasons) might explain why partial migration exists, but one question remains, who stays and who goes? The choices could be based on age, altruism, suitability, dominance… laziness?

As an aside, it isn’t just birds that migrate. Mammals move from place to place, sometimes with a defined pattern during the year, but sometimes they just follow the food, a process called random migration. And some insects migrate as well.

For many years, the migration of the Monarch butterfly was believed to be the longest insect migration. But this is not the typical migration we think of, where an individual moves from one place to another and then back again. The migration of the monarch butterfly takes four generations to complete. Some generations are born and fly a long distance to lay their eggs, while others are born, live, and reproduce in a small area. But altogether, this butterfly moves from as far north as Canada to the high mountains of Mexico and back each year, about 7000 km (4400 miles).

Globe skimmer dragonflies breed in freshwater pools,

so they migrate from India’s monsoon season to the rainy

season in East Africa, all in search of a place to lay

eggs. They make stopovers on the Indian Ocean islands,

but only to rest, because there are very few

pools of freshwater on these coral cay islands.

A few years ago, a biologist in the Maldive Islands started to wonder about the movement of globe skimmer dragonflies where he lived. They seemed to be plentiful in some periods and absent in others. He started to track them, and found that they have an even larger migration pattern than the monarch butterfly. What is more, they fly long distances over the ocean with no place to stop and rest.

Over a series of generations, the dragonflies move from India to the Maldives, some 600-800 km across the open sea. Then they move to east Africa, from Uganda to Kenya and Mozambique. In January, they start back toward India, and complete their migration of more than 18,000 km (>11,000 miles).

So birds may get most of the publicity, but insects hold their own in the migration game. Of course, it does take four generations of dragonflies or butterflies to make their complete journey, where a single arctic tern may make its entire 20,000 mile trip thirty times in its lifetime. O.K., they are both pretty impressive when you consider most people need a car to go down the street to the grocery store and back.



Davenport LC, Goodenough KS, & Haugaasen T (2016). Birds of Two Oceans? Trans-Andean and Divergent Migration of Black Skimmers (Rynchops niger cinerascens) from the Peruvian Amazon. PloS one, 11 (1) PMID: 26760301

Ahola MP, Laaksonen T, Eeva T, & Lehikoinen E (2007). Climate change can alter competitive relationships between resident and migratory birds. The Journal of animal ecology, 76 (6), 1045-52 PMID: 17922701

Hobson KA, Anderson RC, Soto DX, & Wassenaar LI (2012). Isotopic evidence that dragonflies (Pantala flavescens) migrating through the Maldives come from the northern Indian subcontinent. PloS one, 7 (12) PMID: 23285106

The Best Cure for Insomnia Is To Get A Lot Of Sleep

Biology concepts – theories of sleep, REM sleep, circadian rhythms, neural plasticity

You open the door to your house and find your roommate sprawled out on the couch. Is he sleeping, unconscious, or dead? Knowing your roommate, you figure it could be any of them – you stop yourself short of naming a preference.

You find your roommate passed out in his underwear,

and can’t decide if he is sleeping, unconscious or dead.

If you have chosen Homer as a roommate, you have

already clued us in to your decision-making abilities.

The live/dead question is easy; hold a mirror under his nose and see if it fogs up. If he’s not breathing, there’s only one thing to do – go through his pockets and look for loose change (with a nod to “The Princess Bride”). But if you do see condensation, how do you decide if he is passed out or just sleeping - or are you considered unconscious when sleeping?

Sleep is voluntary, at least most of the time. I try to stay awake at the ballet, but I don’t always succeed. But besides drinking yourself into a stupor, going unconscious is usually not voluntary. Unfortunately (or fortunately), you weren’t there to see what preceded the crease marks on your roommate’s face or his drooling on the couch pillow, so how can you identify his state?

Sleeping implies that one has a diminished ability to respond to external stimuli with reduced sensory perception. However, unconsciousness appears much the same. The difference lies in the degree of diminished capacity; you can be roused more easily from sleep and perhaps not at all from deeper unconsciousness. Some people I know must pass out every night, because they are tough to wake up. You might parse the difference and just say that sleep is more easily rousable unconsciousness.

Sleep has stages and these stages have cycles. If deprived

of a particular stage the night before, your body will

change your cycles so that you make up the lost time in

that stage on the next night. Source for image: http://xavier 

appsychology.wikispaces.com/Chapter+5,+ Period+6

A more profound difference between the two exists, but you won’t be able to detect it in your roommate without monitoring his brain waves. In sleep, you go through different phases, each with characteristic brain wave patterns. In 2007, a revised set of sleep stages was published, identifying 4 distinct phases, although stages 2 and 4 are repeated more than twice. Stage 4 is REM sleep, in which many many animals dream.

In general, the safer an animal is, the more it dreams. Predators dream more than prey and big species dream more than small species, though there are several exceptions to this rule. For example, ruminants (cows, deer, goats, and buffalo) dream very little (about 5 minutes/night), and cetaceans (whales, dolphins, porpoises) may not dream at all.

In contrast, animals that are born immature (not able to live on their own) tend to experience lots of REM sleep. These altricial (meaning “requiring nourishment") animals, including marsupials, cats, dogs, and most rodents, may have 6-8 hours of REM sleep a night. What is more, as adults they continue to dream heavily – about what, I have no idea.

Do you know the differences between dolphins and porpoises? Dolphins have longer bodies and snouts, and porpoises have a straight front edge on their dorsal fin. But, they are both cetaceans and have the same sleep patterns. Opposums, the only marsupials in North America, have immature young, and for some reason they dream much more. They are probably dreaming about the day their kids will get off their back.

 But even this exception has an exception. Many birds are born very immature. They have no feathers, they can’t fly, they usually have their huge eyes closed, so they are definitely altricial species. But, birds have extremely short cycles of non-REM and REM sleep. Avian REM cycles might total only 5 minutes in a night, and each episode might be only 9 seconds long. What can you get done in a 9 second dream?

Brain waves recorded on an electroencephalogram

(EEG) show that dolphins and birds have normal

activity in one hemisphere while the other is at rest.

The heartbeat is constant showing that there is

normal body rhythm. This is unihemispheric sleep.

Image is taken from: Ridgeway, S. et. al. J. Exp. Med.

209:2902-2910, 2006.

REM sleep is deeper and harder to be roused from compared to non-REM sleep, the short cycles might be related to birds’ sleep pattern, which is unihemispheric (one half of the brain) in non-REM sleep, and is probably related to their need to keep watch for predators. Birds don’t lose muscle tone when they sleep; often they have to remain on a perch while they sleep. How embarrassing it would be for a bird to fall asleep and then fall off their branch- they would deserve to be eaten.

Other species of bird can sleep while flying, the arctic tern for example, whose migration can be as long as 22,000 miles one way. During flight, the eye connected the active half of the brain will remain open to navigate, but the bird will not dream, since both hemispheres are required in all animals for REM sleep.

Dreaming less doesn’t necessarily correlate with sleeping less. Animals that dream little may still sleep a considerable amount. For prey animals, sleep may represent a dicey time when they must be on the lookout. But it might also represent a way to stay motionless, blend in, and avoid predators. Either theory is practical, since predators seem to take the old, young, and diseased, whether sleeping or not.

Indisputably, every animal needs to sleep to survive, but why? It is interesting that science hasn’t quite figured this out yet. It is known that many beneficial events occur during sleep, but just being good for you doesn’t make them vital. But it must be vital, since even hibernating animals will cycle from hibernation to sleep in order to reap the benefits. Several theories exist for the necessity of sleep:

Energy conservation theory of sleep. Smaller animals carry less fat than large animals, which means they have a smaller margin of error in energy usage – they must conserve energy or feed more often. By sleeping longer, smaller animals keep their metabolic rate low and conserve more energy for when they need it, like for finding more food.

Related to this, animals with fewer predators seem to sleep longer than animals who may be hunted by many other species (as discussed above). However, since resting saves 90% as much energy as sleeping and that animals could watch for predators while resting, there clearly must be additional reasons to sleep instead of just rest.

Repair theory of sleep. This theory contends that non-REM sleep is important for repairing the physical body. Indeed, cell division and protein synthesis increase during non-REM sleep. On the other hand, REM sleep is necessary for restoring mental function, but we will leave the reasons for why we dream for another discussion.

Information packaging theory of sleep. You may sleep in order to provide the brain with time to process all that occurred the previous day, and be ready to take in more the next day. This relates to something called neural plasticity (new connections, ie. learning) and memory consolidation. Recent evidence shows that sleep deprivation harms recall, so sleep may help move information from short-term to long-term memory.

One theory of sleep is that your brain returns to a set

point so you can learn things the next day. Learning

means making new connections between neurons;

these connections are reinforced by neurotransmitters

being released to stimulate the next neuron in series. If

the neuron isn’t fired, it will not release neurotransmitters

to stimulate the next neuron in the path. If they are not

repeatedly fired, the pathway will no longer exist, and

new connections can be made.

In terms of plasticity, a 2007 study indicated that the slow brain waves in non-REM sleep are linked to our ability to learn new information. Dr. Guioli Tononi stated that neural connections become progressively weaker during slow wave sleep, so that by morning, the connections are ready to record new information, but still strong enough to hold the old memories. 


An extension of this study, published in 2011 by the same group showed that in some groups of neurons, synapse size and number was affected by the amount of sleep that fly and the amount of experience that the fly had. More experience required more sleep in order to prune the connections and strengthen those that were used repeatedly. After a few hours of wake, synapse size and number increased, and sleep was required to reduce those that are weak and strengthen the remaining circuits.

If sleep provides all these benefits, and higher animals can’t survive without it (even insects and worms have periods of inactivity that look a lot like sleep), then how is it that the giraffe sleeps only 2-4 hours per day? As prey, nature may have deemed it more important to stay alert; or maybe they just can’t find a long enough blanket.

Cetaceans, like birds, only let half their brain sleep at a time, so they probably don’t dream either. Being mammals, they still have to be able to surface to inhale and exhale while sleeping, called conscious breathing. This might require that some part of their brain be active at all times.

I say might because most cetacean sleep studies have been done in captive animals (the smaller species). But in 2008, the boat of a cetacean research team accidentally floated into the middle of a pod of inactive sperm whales. The whales were unresponsive to the researchers and had both eyes closed. This agreed with another observation that electronically tagged sperm whales spent about 7% (1.68 hr/day) of their drifting with the tide. If this is true sleep (not unihemispheric), it would be a new finding in cetaceans and would indicate that that sperm whales sleep less than any other mammal.

The American bullfrog is fully alert when inactive, so is it

asleep? Scientists think that the bullfrog is so territorial and

is such a good parent that it will not let its guard down until

it dies.

An even more amazing exception to the sleep rule is the American bullfrog (Rana catesbeiana). Brain wave studies (electroencephalography) of the nocturnally active bullfrog did show signs of rest during the day, but bullfrogs had no loss of sensory perception. They could react to stimuli just as if awake. Other frogs show similar brain waves, but are much harder to arouse. The bullfrog might be the only animal to pull a lifetime all-nighter. He should really be ready for that math test.

Nobody yet knows exactly how sleep restores the brain or why bullfrogs and giraffes need so little, but we do know that people who are deprived of sleep suffer physically, emotionally, and intellectually – or worse. How would you like to be condemned to death for not taking a nap? We’ll talk about this in a few weeks. But we've got winter and Christmas things to discuss first.

Daniel Bushey, Giulio Tononi, Chiara Cirelli (2011). Sleep and Synaptic Homeostasis: Structural Evidence in Drosophila Science DOI: 10.1126/science.1202839

For more information, classroom activities, and laboratories on theories of sleep, and sleep in animals, see:

sleep –

http://www.ninds.nih.gov/disorders/brain_basics/understanding_sleep.htm

http://faculty.washington.edu/chudler/sleep.html

http://www.emedicinehealth.com/sleep_understanding_the_basics/article_em.htm

http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060216

http://www.whatislife.com/issues/issues-sleep.htm

http://www.pbs.org/wgbh/nova/teachers/activities/3410_01_nsn.html

http://faculty.washington.edu/chudler/experi.html

http://science.education.nih.gov/supplements/nih3/sleep/guide/using-lessons.htm

http://www.bioedonline.org/resources/nsbri.cfm

http://busyteacher.org/classroom_activities-vocabulary/sleep_and_dreaming-worksheets/

www.pbs.org/wgbh/nova/teachers/activities/pdf/3410_01_nsn.pdf

http://www.njdrivereducation.com/lessonplans

theories of sleep – http://psychology.about.com/od/statesofconsciousness/p/TheoriesofSleep.htm

http://www.macalester.edu/academics/psychology/whathap/ubnrp/dreaming/py4.html

http://www.livescience.com/10579-theory-questions-sleep.html

http://www.camden.rutgers.edu/~bwhitlow/Courses/Consciousness/POCdreamtheory/index.htm

http://cognitiveconvolutions.blogspot.com/2011/05/energetic-theories-of-sleep-function.html

stages of sleep and REM sleep –

http://www.webmd.com/sleep-disorders/excessive-sleepiness-10/sleep-101

http://www.ninds.nih.gov/disorders/brain_basics/understanding_sleep.htm

http://library.thinkquest.org/C005545/english/sleep/REM.htm

http://academic.pgcc.edu/~mhspear/sleep/stages/nrsleep.html

http://psychology.about.com/od/statesofconsciousness/a/SleepStages.htm

http://www.sleep.com/content/rem-sleep

http://web.mst.edu/~../userweb/psyworld/sleep_stages.htm

http://longevity.about.com/od/sleep/a/sleep_stages.htm

http://webspace.ship.edu/cgboer/genpsysleep.html

http://web.mst.edu/~../userweb/psyworld/sleep_stages.htm

sleep in animals –

http://library.thinkquest.org/25553/english/animals/index.shtml

http://faculty.washington.edu/chudler/chasleep.html

http://www.sleepfoundation.org/article/how-sleep-works/animals-sleep-there-human-connection

http://marinelife.about.com/od/marinelife101/tp/SleepInMarineAnimals.htm

http://www.madsci.org/posts/archives/2000-08/965504574.Zo.r.html

http://www.sleepeducation.com/howdoanimalssleep.aspx

http://www.newscientist.com/article/dn14164-why-brainy-animals-need-more-rem-sleep-after-all.html

http://www.apa.org/monitor/jan06/wild.aspx

http://thebrain.mcgill.ca/flash/capsules/outil_jaune07.html

Ovaries March To A Different Drummer

Biology concepts – neuroendocrine, ovary, bilateral asymmetry, internal asymmetry, absence asymmetry, hormones, ovulation

Many things we are taught in school just aren’t so.

The Salem witch trials, for example, did not result in

women being burned at the stake. Sure, some were

imprisoned and a couple dozen were hanged, but

none burned at the stake.

A lot of the things we think we know just aren’t so. I’ll give you a few examples. Vincent van Gogh cut off his ear in an insane rage, right? Nope, his “friend” Paul Gauguin cut it off as he drew his sword in a drunken fight with van Gogh. They made up the story so Gauguin could avoid jail. Not nearly so tragic, but it has the ring of truth if you knew Gauguin.

Engineering professors and even physicists at university will teach you that glass is an amorphous liquid. The reason that windows in extremely old buildings are thicker at the bottom is because the glass has had time to flow. Nope.

Glass doesn’t have a crystalline lattice when solid, but it doesn’t flow. The reason old windows are thicker at the bottom is because they used to make glass panes by pouring molten glass on a wheel and spinning it. The force would spread it out, but it would be thicker on the outside edges. When the panels were cut for panes, they installed the thick side at the bottom for stability. So there.

Finally, there were 13 original American colonies… or maybe not. Delaware was swapped back and forth between Maryland and Pennsylvania. Delaware didn’t come into existence as its own colony until the Revolutionary War. It was known as the ”Three Lower Counties” from 1664 until 1776 and shared a governor with Pennsylvania for the last 75 years of its existence.

Speaking of things that just aren't so, there are couple of things in the neuroendocrine system that most people think they know. There are asymmetries in every part of the endocrine system, ant the others are no exception to having exceptions.

The ovary produces hormones and releases ova. Notice

that the oviduct (fallopian tube) doesn’t connect directly

to the ovary. When the egg is ready to be released, the

estrogen and progesterone cause the fimbria at the near end

of the oviduct to swell and come closer to the ovary. When

the egg is released, the cilia on the fimbriae cells sweep

it into the oviduct.

The ovaries are the source of eggs to be fertilized; those eggs might become small people with wrinkly skin.  But they are so much more. As part of the neuroendocrine system, they are stimulated by hormones and neural impulses and respond by releasing hormones of their own. Depending on the time in a woman’s fertility cycle, they release various amount of estrogen, progesterone and even testosterone.

The ovaries are paired organs like the testes of males, but not all animals have two functioning ovaries. In the Greater Horseshoe Bat (Rhinolophus ferrumequinum), there is only one functioning ovary, the right one, and it's 3-4x the size of the non-functional left ovary. On the other hand, the Natal Clinging Bat bat (Miniopterus natalensis) has only a left functioning ovary and it is several times the size of the non-functional right ovary.

Single ovary examples also exist in the primitive fishes. Lampreys have only one ovary as result of fusion of the two gonadal primordial into a single functioning gland. Hagfishes have a single ovary simply because the other one doesn’t develop.

The hagfish is a primitive fish. The females only have one

ovary, but that isn’t the weirdest part. They produce

proteins and mucins that mix with water and form a slime

when they are disturbed. The filaments are 100x thinner

than hairs, but 10x stronger than nylon, so they are a

subject of much research.

Interestingly, sometimes it’s the right that doesn’t develop and sometimes it’s the left. On the other hand, sharks start out with two ovaries, but the left one atrophies over time, leaving one ovary but two oviducts.

Many birds have one ovary – almost always the left one. A study from 2013 made use of very rare early bird fossils that preserved the ovary tissue; preservation of soft tissue elements is indeed rare. They found that these early fliers had already donated one ovary to the cause of flying.  The hypothesis is that dinosaurs laid many eggs because they had two ovaries, but early birds sacrificed an ovary to reduce weight and make it easier to fly.

The survival advantage afforded by flight offset the disadvantage of fewer eggs, so it was basically a reproductive no harm, no foul. These basal birds had already moved away from the reproductive mechanisms of dinosaurs and present crocodilians toward more bird-like strategies. What this doesn’t explain is why many raptors – like hawks and eagles, have two functioning ovaries. A 2014 paper showed that the right ovaries were functional and capable of responding to, and producing, estrogen and progesterone.

Some birds of prey have two ovaries and some have one. In some, the two are both functional and in others the right is vestigial. This makes me wonder about the evolution of birds. Did the loss of an ovary occur independently several times in different lineages? Or did it occur once in the progenitor of all birds, but some of the descendants evolved the second on again?

The mountain viscacha is a rodent, but looks like a cross

between a rabbit and a chinchilla. They have short forelegs

and long fluffy tail to go along with the rabbit-like ears.

They live in dry places, so they almost never drink. They get

all their water from the plants they eat.

There are even a couple of mammals with a single functioning ovary. The waterbuck (Kobus ellipsiprymnus) lives in sub-Saharan Africa. It is related to antelopes, but differs in that the females of waterbucks have only a left functional ovary.

The mountain viscacha (Lagidium viscacia) is a rodent that lives in the rocky, high altitude, mountainous regions of South America. It starts out with two functional ovaries, but about the time of their first breeding season, the right ovary overgrows, the left shrinks a bit; only the right becomes functional. The exception in this exceptional animal is that if the right ovary is injured or diseased, the left will grow and take over its functions. This doesn’t occur in our other examples.

Similar to testes, there is also a functional asymmetry in ovaries, and this is where we get into the major fallacy that people are taught about the female reproductive system. We are taught that ovaries are good sharers, they take turns ovulating, right-left-right-left, one each month. No……it just ain’t so.

They can take turns, but they usually don’t. And women who want to have children should be glad that they don’t split the load equally. For humans (women mostly), there is slightly less than 50% chance that the opposite ovary will release an egg in the next cycle, according to a 2000 study. This means that side of ovulation is basically random for any given month, but this doesn’t mean that every ovulation has an equal chance of producing an embryo.

Notice that after ovulation, the fertilization of the egg takes

 place in the oviduct, not the uterus. The embryo already

has 32-128 cells by the time it hits the uterine wall. This is

why it is important for the ovary to be producing estrogen

and progesterone the whole time. The uterus must be made

ready for the incoming embryo.

Another 2000 study showed just how unequal ovulations can be. In thousands of ovulations tracked in fertile and infertile women, 64% of pregnancies occurred after right ovary ovulations. In infertile women treated with intrauterine insemination or in vitro fertilization, pregnancy rates were low, as they always are. But if coordinated with right-sided ovulation, they were twice as likely to produce pregnancies as when compared to left-sided ovulations.

However, it isn’t just a right-sided ovulation that produces the best odds of pregnancy; the series of previous ovulations matters as well. If you were to monitor which ovary ovulated over three cycles, there would be eight possible sequences: left-left-left, left-left-right, etc. all the way to right-right-right. This is exactly what a 2011 study tracked, along with pregnancy rates.

The researchers found that the pattern most likely to produce a pregnancy was left-left-right. These results would need to be repeated several times, especially since significant results are difficult to assess when there are eight variables, but their numbers were very convincing. So asymmetry in the function of the ovaries can have a very real affect on hormone levels and pregnancy rates.

Ovulation of a single follicle might be a 50/50 shot each month, but over time the right side of the reproductive system in women seems to be dominant. If the follicles were counted in each ovary (sort of a permanent record of ovulations) of a woman late in her reproductive years, about 62% of them will be on the right side.

Likewise, progesterone and estrogen blood levels are higher during a right-sided ovulation cycle. This data, along with the pregnancy data, indicate the female reproductive system is really right-side dominated. Why is the right side dominant?

The top image shows the corpus luteum that develops from

the follicle that released the egg. This is a huge source of

hormones. It also shows the follicle atresia, where primary

follicles degenerate before releasing eggs. The bottom image

is the timetable of the happenings in the ovary. Notice that the

degenerating corpus luteum is still bigger than the primary

follicle, so over time, the ovary does get bigger. Then as she

gets close to menopause, they get smaller.

A good explanation comes from the fact that the drainage of blood for each ovary is different. The left ovary is drained by the left renal vein, but the right drains in to the inferior vena cava (like the adrenals, see this post).  There tends to be higher venous pressure in the left renal vein and so this side drains slower.

If the blood moves out slower, then the corpus luteum (the leftover follicle of ovulated egg) stays around longer, and this makes it less likely that the left ovary will be ready to ovulate again the following month. Over time, the right will have more follicles from ovulations. The hormone levels would also be higher if carried out of the ovary faster, so this is probably why plasma hormone levels are higher after a right-sided ovulation.

The right side dominance is likely to switch to the left side later in a woman’s reproductive years because of the relatively lower numbers of ova left on the right side. This may be why it is less likely that a woman will become pregnant in her later years.

In most mammals, right and left ovaries are about the same size (given the exceptions of mountain viscacha and waterbuck we talked about above). But in humans, this is merely how they start out. Later in the reproductive years, there is often an acquired size asymmetry.

A dichotic listening test is for attention and picking out one

noise precisely. Most people have a right ear advantage (REA)

for speech because the speech centers are on the left side of the

brain. Research shows that women have a reduced REA compared

 to men and it is affected strongly by estrogen and progesterone

(2011). This may be so that they will focus in less on one sound

and might then be able to pick up on distress from their baby.

After ovulation, the follicle expands and becomes the corpus luteum. This structure produces hormones that would stabilize the system to prolong the pregnancy. If the egg is not fertilized or the pregnancy is not carried for a long time, the corpus luteum reduces in size and hormone production falls. However, the leftover follicle is larger than the pre-ovulatory situation. 

Over time, the number of follicles increases and the size of the ovary increases. More right-sided ovulations (since the right side drains faster and therefore means it can be ready to ovulate again the next month) means that it enlarges more than the left, and a size asymmetry develops.

Hormones, number of ovulations, pregnancy rate – there isn’t anything about the ovaries that isn’t asymmetric – and yet people are taught that they have a symmetric size and function. Right-left-right-left.... yeah sure.

Next week, asymmetries abound in the human body, but they're usually a little here a little there. But what if every organ in you body turned out to be on the wrong side?

Cowell, P., Ledger, W., Wadnerkar, M., Skilling, F., & Whiteside, S. (2011). Hormones and dichotic listening: Evidence from the study of menstrual cycle effects Brain and Cognition, 76 (2), 256-262 DOI: 10.1016/j.bandc.2011.03.010

Rodler D, Stein K, & Korbel R (2015). Observations on the right ovary of birds of prey: a histological and immunohistochemical study. Anatomia, histologia, embryologia, 44 (3), 168-77 PMID: 24895012

Zheng, X., O’Connor, J., Huchzermeyer, F., Wang, X., Wang, Y., Wang, M., & Zhou, Z. (2013). Preservation of ovarian follicles reveals early evolution of avian reproductive behaviour Nature, 495 (7442), 507-511 DOI: 10.1038/nature11985

Fukuda, M. (2000). Right-sided ovulation favours pregnancy more than left-sided ovulation Human Reproduction, 15 (9), 1921-1926 DOI: 10.1093/humrep/15.9.1921

Ecochard, R. (2000). Side of ovulation and cycle characteristics in normally fertile women Human Reproduction, 15 (4), 752-755 DOI: 10.1093/humrep/15.4.752

Fukuda, M., Fukuda, K., Tatsumi, K., Shimizu, T., Nobunaga, M., Byskov, A., & Yding Andersen, C. (2011). The ovulation pattern during three consecutive menstrual cycles has a significant impact on pregnancy rate and sex of the offspring Fertility and Sterility, 95 (8), 2545-2547 DOI: 10.1016/j.fertnstert.2011.02.010

For more information or classroom activities, see

Not too many classroom activities for ovaries and hormones, but here's a link to a series suggested by Matthew Knoepke:

http://nubio.northwestern.edu/labs/28-days-later



 I am also impressed by the idea of using a pomegranate as a model for the human ovary.