DISCOVERY IN THE WEEDS: EXPLOSIVE SEED DISPERSAL IN HAIRY BITTERSWEET

May 2024. 

This winter we spent several months in Florida. Our beach-fronting residence had no enclosed garden for our Jack Russel terrier to roam freely, so I had to provide “comfort” walks several times a day. On these jaunts I intentionally sought out “waste places” to accommodate the natural needs of my canine companion. Besides this humane and practical purpose, the walks offered the opportunity to observe, identify and photograph wild plants, forbs and herbs, i.e. “weeds”..a long-practiced hobby of mine.

Spring  (for weeds) occurs early in February in northern Florida. Many flowering plants are in bloom. One of the earliest and most common species is a tiny plant with white flowers arising from a basal rosette of heart shaped leaves, known as HAIRY BITTERCRESS (Cardamine hirsuta).  This form of bittercress is a native of Europe, Asia and Africa and is today found widely distributed in the USA, and many other places around the world.  

Bittercress was common bordering north Florida walkways, in untended lawns, waste places, disturbed areas and weed patches. If seemed to prefer places also preferred by my dog.  So by late February to early March, I had seen much of it when the weed had grown to about ten to 12 inches (30 cm) high.   At this time, it was topped with tiny clusters of white flowers, each with four petals in a “cross pattern”. This “cross pattern” revealed it to be a member of the “Cruciferae” or Mustard Family (Brassicaceae). The plant arises from a basal rosette of low growing leaves which are round (or “liver”shaped). These appear first, often in the fall of the year.  By early spring (or February in the south) the plant produces 6 to 10 inch long “flower” stems.  These are toppex with a cluster of white cruciform flowers.  The flower shoots sprout compound leaves composed of four or six leaflets, each ending in a single terminal leaflet. 

Each of the tiny flowers, when pollinated, mature into a long, narrow, purple-colored seed-pod (like a tiny, straight and elongate pea pod). The pods grow vertically to stand upright at the top of the plant like a spike. The numerous seed-pod spikes (called siliques) each about 1-2mm thick and about one inch (2.5cm) long, are clustered at the top of the plant and give the species its “hairy” (or in Latin, hirsuta) look. The spiky siliques are the source its name: “Hairy” Bittercress.  Later in the season, when the plant has died and turned brown, the hairy look  intensifies and persists to provide even greater justification for its name.

 

The Mustard, crucifera or Brassicaceae family to which this plant belongs includes many common food crops, such as mustard greens, broccoli, cabbage, turnips, bok choy, rape (canola) and even radishes.   Like the other plants in this group, the leaves, and flowers, of Hairy Bittercress are edible. Though I make this statement without personal knowledge, having made no taste tests (given the source of my observations in waste places) .  The name “Bitter cress” is claimed a misnomer since its taste is reported as not bitter but more peppery-like. It is harvested and used as a peppery salad green or cooked with other pot herbs. (I assume for this purpose it is picked from areas where canines, pesticide applications, and other contaminant sources are excluded)

But in truth none of these interesting attributes motivated me to write this piece. What did spark my interest was the way its seeds were dispersed…by ballistics.  This surprising observation became apparent as my dog, Finn nosed around in the waste places he preferred, which were  dominated by specimens of Hairy Bittercress.  When he did the tiny Bittercress seed pods exploded around him launching hundreds of the tiny pale or cream colored mustard seeds two or more feet above the surface and a good three to four feet away. Many of the tiny seeds settled on his coarse furry coat. 

Though floating down stream or on an ocean currents (cocoa nuts) carried by wind (Dandelion), rolling down hill by gravity (Black walnuts) , whirligiging Maple “polynoses”(samaras) and many other dispersal systems are well known, actually firing seeds like a Roman ballista into the air is quite uncommon. I wondered about the mechanism. We often think of plants as immobile. The ability to discharge seeds by a mechanical means like that of a Roman ballista has given this process its name ..” ballistic” seed dispersal. It is an unusual seed dispersal mechanism though not unknown phenomenon in the plant world. The pods are hair-triggered. Any slight touch or breeze and certainly the snuffling nose of a dog immediately elicit a violent explosive eruption too fast to see what was actually occurring. 

Try as I might, I could not find an unfired seed pod that I could examine, prior to explosion. It was immediately apparent that the pods exploded on the slightest disturbance. After being triggered, and the seeds launched into the air,  all that remained of the seed pod was the empty, blade-like, central-wall of the narrow silique. On close inspection, the interior surface was marked here and there with a few stem portions of the launched seeds which remained attached to this inner central wall of the seed pod.  Apparently, seeds formed on both sides of the central pod wall. I counted what appeared to be up to about 20 seed locations on each side. Thus each pod might contain as many as 40 tiny seeds. On a plant with a single flowering stem, I counted ten purple colored siliques. That plant might have dispersed 400 or more seeds when triggered to explode. Most plants must have had many dozens of pods. So the number of seeds dispersed must be very numerous and thus very effective.  It may well explain the ubiquitousness distribution of this weed plant.

I did observe  as well that the movements of my dog nosing through the weedy patches readily discharged seeds from the siliques and the seeds showered down on the furry 20 pound canine. Many hundreds of seeds must have lodged in his furry coat, dor at least a short time.  His movements at setting off the explosive siliques would be similar to small native denizens of this area ,where raccoons, rabbits, squirrels, opossum, armadillos as well as other small rodents might move and cause discharges. Small animals may increase the distances that seeds are dispersed, by first causing the explosive discharge, then become an agent of plant seed dispersal themselves by transporting seeds that fall on their fur of cling to their bodies. This animal transport effect may account for much further and effective seed dispersal than that observed by the immediate explosive aerial discharge of a meter or more.  

I was fascinated by this most unusual and effective means of seed dispersal of this tiny inconspicuous plant. But was frustrated that while in Florida I could not determine the mechanism of just how the explosion worked. 

My winter season in Florida ended, and in early April I traveled  back to  New York. En route I found flowering Bittersweet all along my route, attesting to the wide ranfe of this very common weed. In South Carolina, where I continued our necessary dog walks, I observed numerous Hairy Bittersweet specimens on the lawns and waste places and weedy paths of that state.  Further north, in Salisbury, Maryland, Hairy Bittercress was also in abundant evidence. Arriving in New York State, Finn and I were greeted with Hairy Bittercress in his favorite weedy places and in our own untended garden. So it seemed clear that Hairy Bittercress is widely dispersed up and down the temperate zone of the east coast of North America. It was in my own garden where I finally had the opportunity to closely examine a silique as it exploded and  immediately afterward. 

I picked up a plant with several unexploded siliques. As I touched one of them, the pod exploded. It was still to fast to see, but after the seeds launched away. l found several tiny rolled up fragments of the outer walls of the seed pod on my hand. These fragments appeared to be  composed of the coiled up outer-wall of the silique. So I tentatively concluded that it was likely the rapid detachment and coiling up of the outer walls of the silique which was the likely ballistic mecahanism.   I theorized that on explosive discharge the outer walls rapidly coil up, and as  they do, they detach the seeds from the central wall while they remain partly connected to the rapidly coiling outer wall. In this motive force they are launched into the air. In the process the silique wall, fragments into several parts. 

An examination of the silique wall cylindrical fragments I recovered suggest that the  outer silique walls detach from the base (?) and coil upwards in such a way that the lighter colored interior of the silique wall was exposed and formed the outside of the cylinder fragment, while the outer purple colored wall wound up on the interior by the coiling process. Apparently, the two outer walls detach from the base and coil up violently. As they do, the rapidly coiling  pod walls pull away the attached seeds, launch them into the surroundings and in the violent process—the silique walls fragment and fall away as small barrel shaped, coiled up wall-fragments. The central wall of the silique remains behind, stiff and upright, and remains attached to the plant, but with the purplish outer walls and all or mo#t seeds gone, the central, light-tan-colored wall remains intact and upright.

A short time after this happy discovery, I came across a published article from more than a decade ago which actually explained the full mechanical and chemical mechanism of this explosive process of the Cardamine hirsuta.   See: Mechanism for explosive seed dispersal in Cardamine hirsuta, August 8, 2011 by K. Vaughn, A. Bowling, K. Rue, in: Amer. Jour Botany, 8-01-2011, Vol 98, Issue 8, p1276-1285. The full story is even more intriguing…I highly recommend  reading through it…though it is technical. 

 

INFLATION IS AN INSIDIOUS BURDEN ON THE NATION

INFLATION

Inflation describes an economy in which prices of goods and services tends to go up, resulting in the purchasing power of the dollars you hold to go down. (A better term might be dollar deflation which accurately describes the circumstance when the value of your dollars are shrinking.) 

We see the insidious effects of inflation every day in the market or at the gas pump at our local station. Why is gasoline so high? Why is that loaf of bread, dozen eggs or cut of beef almost two times what they were the last time I purchased them? Costs continue to rise almost daily.  The rising costs limit our choices, frustrate and anger us. We cannot buy the special brands or quality “stuff” we once did. We feel cheated. We have to make compromise purchases that do not satisfy. We question the functioning of government that permits this problem. Why does it occur? 

LAW OF SUPPLY AND DEMAND

The law of supply and demand states that when supply of goods or services is greater than demand, prices fall, and when demand of goods or services outstrips supply, prices rise. 

INFLATION DEFINED

The term “inflation” which means: “to fill a bladder with air or other gas”  was first used in 1838 to describe monetary inflation, an economy in which the general trend is one of increased prices of goods and services . The name was coined in 1837-1848 during the US economic crisis of that period.

Monetary inflation is caused by government. Government and only government is responsible for providing its citizens with a stable-valued paper money supply. By printing up more paper money and dumping “dollars” into the economy faster than industry and business can produce goods and services, government can cause money to be less valuable than the goods and services it is chasing.  In other words inflation most often occurs when government increases the supply of money while the demand for goods or services remains constant or falls. The end result of this government policy is cheap money and costly products and services.  We call this economic phenomenon  “inflation”, the “inflating or blowing up” of prices of all goods and services,  or conversely the deflation of money, i.e. the reduction in “value” of the government-issued money we hold. 

As noted above, inflation may also be caused by scarcity of goods and services.  Scarcity of products  may result of an imbalance in which results in more money in circulation relative to the goods and services available to purchasers. (Rf the Law of Supply and Demand).  Again government may be responsible for this phenomenon by as a result of its misuse of its regulatory and legislative functions. In a most recent and egregious 2021 example of government policy, arbitrary government regulations designed to promote a “Green Agenda” intentionally reduced the supply of fuel oil, gasoline, and natural gas by government regulation. The immediate result was that  the cost of these now scarce products rose dramatically. Sadly this was the result of a foolish doctrinaire government policy which put a theoretical and political agenda ahead of sound economic protocols.  Since the price of these products—gas and oil— was and remains universally embedded in almost every aspect of a modern economy-the  effects of its cost increases were immediate and devastating. Prices of fuel ballooned, causing a massive, generalized price increases for goods and services nationally. Government unwise policy created inflation!   

Furthermore, this government policy was imposed at the worst possible time—when the world was just emerging from a devastating pandemic which had caused interruptions in a global supply chain. 

Shortages and unavailable products generated scarcity and rising  prices. The result of these shortages was massive inflation—lots of money chasing scarce goods—caused prices to rise by 9% or more in a very short time. This 9% rate of inflation was the highest recorded  in almost forty years, or since the 1979-1980 period when inflation hit 12%.  (In one example of shortages generating infaltion: in the UK, only a few companies made cardboard egg cartons, needed to package eggs for market. The pandemic disease outbreak at one of these producers cut UK production of egg cartons to 50% of what was its normal supply.  UK farmers were producing eggs, but they could not be sold without cartons. Huge price rise in eggs resulted.)  

INFLATION ACCUMULATES

Inflation has cost you (purchasers) 9% of your buying power in 2021, then 6% of your buying power in 2022, then 4% of your buying power in 2023—or totally by about @ 19% of your buying power year over year so far! If you had a savings account in 2020 of $1,000 dollars by 2023 its buying power shrunk to $810 dollars.   Inflation rate has fallen from its 9% peak, but its insidious effect continues to eat away at your salary and saving year after year. This year (2024) we suffer from more than 3.4% inflation so far.  Inflation accumulates annually…its rate may go down and up..it  but its terrible effects do not go away as it keeps gnawing away at your salary and your savings. 

 If you were fortunate enough to get a raise of 3% in 2023, your salary of $1000 per week rose to $1,030.00. But remember that previous inflation had cut your buying power to $810. After your “raise” you now are earning a nominal $1030. Don’t think you gained anything. Your new income has increased your buying power over 2020 (810+30=840) to only $840*. Thus you earnings are $160 dollars less in buying power than you had in 2020.  Do you now understand what “inflation is insidious” means? (*Note that I am not counting the 3.4% inflation we experienced in 2024.)

Today, as a result of accumulated inflation, inflation is 20% and in effect the US dollar has been deflated to @ 20% of what it could purchase in 2019.  That is because all prices we encounter in the US are inflated —as a result  of US government regulations and unwise policy. Today, our dollar buys almost 20% less than what it did pre-2020. If you scrimped and saved away one thousand dollars in your kitchen cookie jar in 2020, today that same rubber-band-tied-roll of bills is worth only $800. If your salary was $1000 per week in 2020–today that same salary will buy you only $800 dollars worth of goods and services. Because of inflation you are poorer by about 20%.  That person with a $1,000 per week income has lost 200 dollars a month in purchasing power or  $200 X12 months= $2,400 dollars annually. 

BIDENFLATION

When President Joe Biden was elected in 2020 annual inflation rate was less than 2% (or actually about 1.5%).  That meant that the dollars you held in 2020 would lose only a little less than 2 dollars worth of buying power for each $100 dollar bill you held.  But as a result of the Biden “Green Agenda”, the administrations free-for-all borrowing and spending policies (which added billions of  borrowed and printed dollars into the economy) as well as post pandemic supply chain problems, our accumulated inflation burden today is ten times what it was prior to Biden entering office—or a devaluation of purcahing power equivalent to about $20 dollars per each $100 dollars you hold.  As a result that $100 dollar Benjamin you saved in 2019 is today is worth only $80 dollars! That change in value should wipe the smirk off of Old Ben’s greenish-gray printed face.

Government is responsible for inflation. Free wheeling spending, carefree dumping dollars into the economy cheapens the dollars citizens hold. Government “give-aways” —like government subsidies, or government subsidies to generate lower prices for some goods, or loan forgiveness for particular groups,  all add to the burden of paper dollars circulating in the economy which compete with the dollars you earn or save with such difficulty. Some few of us may gain a small advantage in their personal economy—like students with forgiven loans, or seniors with slightly cheaper medicines, but, overall inflation bites back by imposing the harsh burden of higher prices for all necessities like food, fuel, gasoline and essential services that overwhelm the small savings government largess has offered. 

Policies which dump dollars into the economy cause inflation.  Policies which create scarcity of essential goods like home heating fuel and gasoline cause massive inflation. Inflation hurts everyone, but it hurts the working class and poor the most. 

CONVERGENT EVOLUTION OR SPECIATION AT THE BACKYARD BIRDFEEDER

 Our backyard bird feeder has had some interesting visitors in Spring of 2024. Among the common visitors were a pair of Tufted Titmice, Goldfinches, a pair of Mourning Doves, Cardinals, several Song Sparrows, several Common Whitethroat Sparrows, a flock of Brown-headed Cowbirds, a White Breasted Nuthatch, and perhaps just passing through to more northern climes: a Baltimore Oriole, and several Rose Breasted Grosbeaks.    Three species of woodpecker were common visitors. The large Red Bellied Woodpecker and two smaller woodpeckers, the Hairy and Downy woodpeckers. These two latter woodpecker species were unusual in that they look almost identical, only differing in size. This is unusual in nature…different species most often look different. Similar looking species are most often are very closely related. 

THE WOODPECKERS

One of these, the more common Downy Woodpecker, (Picoides pubsecense) is  about the size of a large sparrow, while its look-alike, the Hairy Woodpecker (Leuconotopicus villosus) is closer in size to a Cardinal or Robin. These two distinct species puzzled me, as well as other observers, because they are similar in appearance, are found in almost identical ranges over North  America, and occupy very similar habitats.  In the natural order of things a species is an interbreeding group of similar appearing organisms which interbreed to produce viable young.  They are genetically almost identical and thus have similar morphology.  Different species should “look different” and those differences are generally the result of their adaptation to some specific ecological “niche” in which they are best adapted. One must wonder why has Mother Nature generated two near identical (excepting for size), yet distinct species inhabiting similar habitats?  

The Downy Woodpecker is about 6 inches long, with a 10 in wingspan, and weighs between 3/4 of an ounce to as much as one ounce.  It has a white breast and a typical woodpecker black and white patterned back. Males are distinguished by a red spot on the back of their head. (See data from: Shenandoah National Park Svs).  

The Hairy Woodpecker is a medium sized woodpecker of about nine (9) inches long with a wingspan of @ 15 in and weighs in at about 2.5 ounces, or about two and a half times that of its presumable close relative.  The distinct size differences around a backyard feeder make it easy to determine which is which. In the field the fact that the bill of the Hairy Woodpecker is about as long as its head is wide, while that of the downy is shorter than its head-width help to make a species determination. There are also several relatively minor and difficult to discern differences in color patterns.

These two woodpeckers, are often seen at the same bird-feeder, they both inhabit the local woodlands, and a literature review of their national range maps indicates that they have almost identical ranges, Both occupy almost all of North America, except where trees and woodlands are absent.  The larger Hairy woodpecker’s range includes a bit farther north in Canada and Alaska, and it ranges a bit further south into mountains of central America, as far south as Panama. While the Downy is confined to the forested areas of most of North America. I did consider mass vs surface area factor, such as the fact that the larger bird would be better adapted to a colder climate  than the smaller one. But a closer examination of the range maps tended to obviate that reasoning. The Hairy woodpecker range does extend relatively further north—but also further south. 

The Hairy Woodpecker’s diet consist of more than 75% larvae of wood boring beetles, bark beetles, ants, moth pupae, insect cocoons, bees, wasps, caterpillars, spiders millipedes and lesser numbers of crickets and grasshoppers

The natural prey diet of Downy woodpeckers is similar with more than 75% is insects and grubs found under or in tree bark, on plant stems and in tree galls. The diet of the the two species are almost identical.  So why are there two size ranges?

WHAT IS A SPECIES?

A “species” is defined in biology is an interbreeding group of morphologically and genetically similar individuals which can produce viable, and fertile offspring. Most often (and in obviously in paleontology) scientists often have only the morphology of specimens as a method to establish a “new  or different species” from another. In paleontology, scientists  have only skeletal or shell remains(or other preservable hard parts or evidences) and thus are confined to  base their species determinations on strictly morphological similarity. 

Paleontologists thus often have to decide if similar “appearing” species are indeed distinct “species”.   Scientists who operate in the field of science nomenclature and who make these distinctions are often divided onto two classes themselves, often being grouped as either “lumpers” or “splitters”. The  former are more likely to combine similar specimens into one species, and the latter tend to use even minor variations as a reason to create a “new” species.   In more modern times genetic analysis of DNA of living species has often provided data to confirm or deny these species relationships.

Species may evolve from one species into another as a result of geographic separation or other factors which act to prevent interbreeding of the pool of genetically similar organisms (species) and thus over time genetic isolation and changes occur resulting in morphological variations. Thus on the physical margins of the gene pool, or when geological or other phenomena separate species so they can not continue to interbreed and exchange gene material—a new species may come into existence.   

WHY THEIR MORPHOLOGY IS SO SIMILAR

I was puzzled by the fact that these two very similar appearing woodpecker species, with almost identical diets and which occupy overlapping ranges and almost identical habitats—as far as is known. Based on observations of the species appearance, one should conclude they were very likely closely related species. A first assumption I made this morphological  similarity was that this phenomenon was a manifestation of the process of “speciation” or the formation of a new species. 

I reasoned, that perhaps, the size differences  (six inches long for the Downy and closer to nine inches long for the Hairy) were related to differences in exploitative behavior.  Since both species have a similar diet, foraging among tree trunks and branches for grubs, caterpillars, and wood boring insects for their caloric intake. This led to my original hypothesis: the Downy was likely adapted to exploiting the more numerous smaller branches and limbs, while the Hairy exploited larger branches tree trunks and perhaps favored more mature forests.

ADVANTAGE/DISADVANTAGE OF SIZE?

 One might reason that the smaller, lighter Downy had more “recently” evolved,  possibly as it adapted to exploit the more prevalent in modern times second or third growth forests. Thus, this closely related smaller species was better adapted to exploit grubs and boring insects in the younger, smaller diameter branches of trees and perhaps also more likely to find appropriate sized nesting sites in a less mature forests, or human managed forests with fewer old, dead or dying trees of late maturity. This was an example of “speciation in process”. The process of a new species evolving from an older species in response to alterations in environment-such as biological, environmental and physical changes.

On the other hand, the Hairy was better adapted to forage for bugs and wood boring insects on older, more mature and often more common larger diameter trunks, boles and branches of trees in more mature perhaps climax forests—which in present times are more restricted in range. Thus, I reasoned,  perhaps we are seeing the development of a new species better adapted to the “altered, less mature more likely managed forests” and suburban wooded environments of more modern times.  

Woodpeckers often feed clinging precariously to a near vertical tree trunk or branch. Their feet, armed with sharp claws  are well adapted for this purpose. Most species have four toes with two forward and two rearward toes which enable a secure grasping grip on rough tree bark. This secure footing coupled with its rigid tail feathers, which are used to press into the tree or limb at an angle, and in this way act as strong brace against their feet.  The tail brace insures a secure perch for feeding, pecking holes to excavate grubs, wood borers and larvae and for excavating nest holes in trees.  

In this position, with their sharp pointed beak, they can actively extract boring insects from tough bark, excavate nest holes, and —in the face of a threat—sidle horizontally left of right around the circumference of the trunk or limb, thus placing the mass of the branch or tree trunk they are exploiting  between them and a threat from a predator,  such as owls, hawks or even arboreal snakes. It is not uncommon to see woodpeckers sidling horizontally around the circumference of a branch to keep a threat such as a human observer or predator at bay. 

I reasoned, that the size variation between the Hairy (larger) and Downy (smaller) versions of these two closely related species (I assumed) were possibly an adaptation of one of these two  species to changes in forest maturity, assuming less mature forests would have greater opportunity for smaller woodpecker species while more mature climax forests might favor the larger Hairy species. Thus the reduction in area of old growth, climax forests might have favored the development of smaller woodpecker species better able to exploit smaller diameter limbs, branches and tree trunks.  

The larger Hairy woodpecker would be less likely to exploit smaller branches for food, which would not offer adequate protection from predators, while the smaller Downy could effectively do so. In addition, a similar reason may be proposed for finding appropriate nest hole sites for the two species. The smaller Downy might have an advantage in having more potential areas for food exploitation, and for nest sites in less mature forests.  

My hypothesis was that what we are observing in these two species of almost identical appearance, except for size, was the process of evolutionary adaptation of an earlier, larger species (Hairy)—which had adapted to the old growth primeval mature, climax  forests of North America, was evolving into a more recent variant species (the Downy) better adapted to the more recent forests of North America…where over the last several hundred years mature old growth climax forests have been cut down giving way to second and third growth forests and urbanized environments.   Perhaps the Downy was an example of “speciation” or the origin of a new species more closely adapted to life in the present day forests.  

NEW DNA EVIDENCE UPENDS A HAPPY  HYPOTHESIS

 

Then an uninvited fact came to my knowledge and obviated my  hypothesis. In 2015 biologists studying the genetics of these two woodpeckers concluded that the Downy and Hairy Woodpeckers —were NOT closely related at all!   In fact, they were so different genetically that they would most likely be, in time, assigned to completely different genera. This threw a monkey wrench into the workings of my, “speciation hypothesis.  So what is happening? Perhaps these new data on woodpecker genetics reveal an even more interesting story. 

A NEW HYPOTHESIS: MIMICRY A FORM OF CONVERGENT EVOLUTION

There are many examples in nature of convergent evolution. A process in which different species, genera, even classes and orders of animals evolve similar morphology, structures, or color patterns in response to predation, to a changing environment, to competition, or adaptation to similar habitats.  One classical example is this form of evolution is that of fossil ichthyosaurs and modern marine dolphins.  Ichthyosaurs which lived in the Mesozoic Era (200 Mya) and the modern day Dolphin are an example of convergent evolution.   Both species were adapted to similar marine habitats and method of predation. Reconstructions of fossil ichthyosaurs indicate that they would have looked very similar to living dolphins.  Both evolved long toothed jaws, similar shaped flippers and shaped tails.  Their morphology was similar because they inhabited similar environments and had a similar niche in their environment.   There are several forms of convergent evolution.

MIMICRY: OR “COPYCAT” LIFE INSURANCE.

Mimicry is a special form of convergent evolution, in which a species evolves a morphology and appearance similar to another usually unrelated species. In this form of convergent evolution copying the appearance of another species provides  a level of survival advantage for the mimic.  

LOOKING TOUGH OR DANGEROUS MAY SAVE YOUR LIFE

In one form of mimicry, a species with no chemical defense mechanism evolves to converge on the appearance of another species which has developed a poison or unpalatable chemical defense system. One well-documented example of this form of convergent evolution is that of  the harmless King Snake which has evolved a color pattern which resembles the poisonous Coral Snake. This form of mimicry is called Batesian Mimicry. (Though recent investigations reveal a more complex relationship which  includes several look alike species which resemble each other..not all of which are poisonous)

Or in another similar case, the Viceroy butterfly evolved to closely resemble the Monarch butterfly.  The assumed advantage for the Viceroy is that by looking like the mildly poisonous or foul tasting Monarch it gains a survival advantage.  The Viceroy improves its survivability by being less likely to be attacked if it looked like the (unpalatable ) Monarch.  But recent experimental accounts reveal that both species are either poisonous or unpalatable. A more recent explanation is that the two species appear to be mimicking each other.  In this form of mimicry both species gain a survival advantage. Predators are more likely (and quickly)  to learn to avoid the general orange and black color patterns displayed by Viceroys and Monarchs (both unpalatable) than they would if these two species each had to depend only on their own species deaths or unpalatability by predation to develop a warning to predators.  Thus this mutual form of mimicry is an advantage to both species. 

In the case of the Hairy and Downy Woodpeckers we may have an exceptional example of convergent evolution. In the same environment or ecological niche two different species have evolved to have almost exactly identical colors, male /female identity markings, black and white ladder striped dorsal patterns and other anatomical features. Why?  We must assume that these patterns, colors, etc. must have evolved as a result of the fact that they somehow provide some form of adaptive advantage to the species. 

MOST WOODPECKER SPECIES LOOK ALIKE, WHY?

Given this we might wonder, why are almost all woodpeckers black and white? Why do most have ladder-like black and white dorsal patterns, almost all are dark dorsally and have white underparts, almost all have some form of red-colored patches on their heads.

Protective coloration is often claimed as the reason. Specifically a form of coloration called disruptive coloration—patterns which help to break up the shape of the animal (woodpecker) in the eye of a predator. In the environment of tree trunks and tree limbs this contrasting “color” pattern, though making the species more visible to us, tends to hide the bird in plain sight of predators by disrupting the predator eye in such as way that the outline of the bird is obscured.

Take for instance a man or woman dressed all in black, at night against a dark background, this person would be nearly invisible. But in a forest during the day or an open field, black would do nothing to conceal the outline of this human.  However, suppose this person wore a black costume with one white leg and a wide white sash across its breast. In the dark one would see the white parts clearly, but the one white leg and sash would not signify “human” to an observer. In the light of day, the same disruptive patterning would also serve to confuse the eye of the beholder, making the identifiable “external form” of the costume wearer as “human” less likely to be identified rapidly, and more difficult to identify as human. This effect would remain were the costume wearer to move into a wide diversity of background colors. And also any dark patches in the background would tend to greatly enhance the camouflage effect. 

This form of camouflage known as “disruptive coloration” may be the underlying reason that Hairy and Downy woodpeckers who inhabit almost identical niches in the forest…have near identical color pattens. If one ignores their differences in size which can be ascribed to exploiting different parts of the forest tree -trunk-branch and limb continuum, but their almost identical camouflage patterns are likely a result of very effective protective disruptive coloration pattern that probably protects them from predation. This pattern appears to have some selected for advantage in the North American forests where they are found. 

PROTECTION IN A GANG: MULLERIAN MIMICRY

But this does not seem to explain why so many woodpeckers species have very similar markings and feather patterns. Perhaps this phenomenon is a fine example of Mullerian mimicry. With many woodpecker species having similar patterns, predators may quickly learn to avoid attacks on species which have black and white patterns, ladder-like  black and white back patterns, a red patch on the head, etc. But why? 

Perhaps this “similarity phenomenon” among woodpeckers is the result of mimicry.  The larger woodpecker species such as the Pileated, Redheaded, and Red Bellied woodpeckers are well protected with long, sharp pointed bills, and powerful neck muscles able to powerfully drive their sharp beaks. Their bill or beak is covered with durable keratin, has a sharp chisel point and is driven by powerful neck muscles which in total effect, can be a effective protective weapon that may dissuade predators from attack. 

Could it be that by advertising “I am a woodpecker with a dangerous weapon on my nose” warns predators away.  Woodpeckers as a group may be an example of converging evolution resulting in similar feather patterns — i.e. they mimic each other—because these patterns  provide a survival advantage.   Looking much like a long billed Hairy, Red-bellied or Pileated woodpecker might just save a smaller woodpecker from attack—just enough times to make its similar plumage an inheritable advantage.  Then it takes only long periods of time—geologic time—thousands of years and generations to result in similar feather patterns and colors in the woodpecker gang. 

Thus it now seems that the Hairy and Downy woodpeckers have slowly evolved from very different unrelated species to look like each other for several reasons. Woodpecker colors and feather patterns must provide a level of protection from predation, and as well, these particular patterns are especially effective in the North American  woodlands that both inhabit.  The differences in their size may be simply their more specific adaptation to different sizes of tree limbs and branches which they exploit for food and nest sites in their native forests. 

These musings are those of a curious mind, and seem to lead to more questions rather than certainty. This is a good and natural process. One is always fascinated by the complexity and questions wonderful Mother Nature awards us with. May we all wise up and protect her better than we do now.   

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