It’s March – the month when longer days and melting snow tease us into thinking about spring.
At this time of year as the ground color changes from milk white to toast brown, some animals also begin to change their appearance. Snowshoe hares, along with short-tailed and long-tailed weasels, will soon begin to replace their white winter coats with soft brown fur. This molt, triggered by changing day length, enables them to be well camouflaged regardless of the season (assuming there is plenty of snow during the winter).
In addition to helping animals hide against a backdrop of snow, white fur maintains warmth. Fur is white when there is an absence of the pigment melanin, which produces various shades of brown and black. The lack of the pigment creates space in the cells of the fur. Air fills that space and provides 27 percent more insulation capacity than pigmented fur. Thus the color change has adaptive benefits that outweigh the energy required to create a new pelt twice a year.
Not all color variations are as regular or as beneficial.
Recently, there have been reports of white mammals and birds around the Concord area. Were the white porcupine in Canterbury and the nearly white robin in Epping trying to replicate the winter coat advantage of the hare and weasels?
No, these unusual individuals exhibited genetic mutations that cause a complete or partial lack of melanin. Albinism occurs when there is no melanin in the skin, fur, feathers or even eyes. Without color in the eyes, the blood vessels are visible, giving a distinctive and eerie red-eye stare to true albinos.
Yet, not all white creatures are albinos. In some cases the eyes or a portion of the fur or feathers may retain color. This phenomenon is called leucism. Leucistic creatures can exhibit varying degrees of unusual color patterns or fading, ranging from all white to a pied or calico appearance. It is understandable that wildlife specialists get called when people see these creatures; they just don’t look normal.
Another color variation is called melanism. This is the opposite of albinism and occurs when there is an excess of melanin causing an animal to appear completely black. While being black is normal for some species such as blackbirds, black racer snakes or black bears, melanistic versions of other colored mammals, birds and reptiles occasionally appear in the wild.
Squirrels are well known for this genetic variation. Black versions of the Eastern Gray Squirrel can be seen in New Hampshire. There are well known populations of them in many Boston suburbs, around Springfield, Mass., and in other cities across the country.
Each of these pigment variants is a genetic mutation that can be passed down from generation to generation. Yet because they are recessive traits they are not commonly exhibited. So why do some unusual color morphs show up more than others?
When the variations show up, the impact on an individual will determine how readily those genes are passed along. For instance, a melanistic squirrel may blend in with surroundings and avoid predation.
However, a white robin will be more visible to a predator than a red-breasted one and possibly more susceptible to being attacked. Yet various studies show that predators may avoid white oddities because they do not recognize them as food.
A bigger problem for the albinos and leucistic animals is that they are not attractive to potential mates. Therefore reproductive success is greatly reduced and they are less likely to pass on their recessive genes to offspring.
In addition, the lack of pigment in the eyes creates poor vision for albinos, rendering them less able to find food or protect themselves. For birds, non-pigmented feathers are weaker than colored ones and are more prone to damage. Flying ability and heat regulation are therefore compromised, also reducing survivorship for pure white individuals.
Albinism has been recorded in at least 300 species of North American birds, many mammals, reptiles, amphibians, fish and even invertebrates. Yet it is estimated that only one in every 10,000 mammals is an albino. Because these mutations are in recessive genes and generally don’t produce beneficial traits, it is extremely unusual to observe them. Their rarity is one thing that makes these animals really fun to see. But keep your eyes open, you never know when one might show up.