Pick from the Past
Natural History, March 1960

Wetland Saga

Flood, drought, freezing, and predation
are the risks for a marsh muskrat population

Excerpted from Of Men and Marshes, by Paul L. Errington, with permission of the publisher, Iowa State University Press. Originally published by Macmillan in 1957. (See purchasing information.)

O the muskrat, that almost typical member of a North American marsh community, each day is something to live for in itself, whatever may be the season of the year. But since spring is the season we usually associate with biological beginnings, we shall begin here with spring to portray the seasonal fortunes of this wetland animal.

By the calendar, spring sometimes comes when the snow is as deep, the ice as thick and as hard, and the temperature as low as in the severest part of winter. But the tracks of the wide-ranging minks during their late-winter breeding season are laid down in powdery snow or in slush, on wet sand or on frozen mud, or on dry soil. Restless skunks emerge from hibernation long before the weather could tell them to, and great horned owls incubate and brood their young right on through late February and March and April blizzards. As sexual awakening progresses in the muskrat populations, living out of sight of human eyes in their lodges and burrow systems, more and more animals come out and sit on the ice on mild days, until, with the ice gone, the main dispersal away from crowded wintering quarters to easier living sites begins.

Late spring floods may cause severe losses to the marsh muskrat population. As the water rises in their burrows, they stay as long as they can and then open up the tops and enlarge the chambers just below the surface of the ground. As the water nears the chamber top or covers the ground, they must come out.

Dispersing muskrats travel along marsh and lake edges, along streams and up gullies. They may act like cautious and adaptive explorers, who know what they are doing. Or they may get started in footloose, hazardous wandering and show up on city streets or in farmyards or in any number of out-of-the-way places—if they live long enough. They may travel far or they may not. Living uncertainly or living securely, they hide or fight, doing their best to stay alive—somehow. They face flood, drought, predation, and disease, but between these various crises they may live well. They are not creatures to worry.

Late spring floods may cause severe losses to the marsh muskrat population. As the water rises in their burrows, they stay as long as they can and then open up the tops and enlarge the chambers just below the surface of the ground. As the water nears the chamber top or covers the ground, they must come out, to build nests anchored in whatever vegetation there is along the banks or out in the marsh, or to sit in the scrub willows, or to lie quietly, trying somehow to remain in the neighborhood. If there are helpless young in the burrows, the mother may deposit them, one by one, all or only part of the litter, in a drier place outside. Young of swimming size may lie in a pile in the water, in a hole or in a nest, the lower ones withdrawing to climb on the others, until some become too weak to withdraw any more, and their bodies lie, a platform of dead young flesh supporting still-living young flesh. At high flood, all the muskrats of the low bank islands may be gone, the old and the young and their nests and sitting places. Dead and living bodies may float away toward a lake, along flowing channels or off somewhere across the marsh, over the channels and banks and washed-down vegetation.

As spring grades off into summer, nothing much may happen to the muskrats, if they do not become over-crowded and the water levels are favorable to them. The midges hum, new shoots of reed and cattail appear above the water, water lilies float their new pads and flowers, masses of arrowheads take form, and, among the lodges, newly weaned muskrats swim, sit, feed, and dress themselves as if they amounted to something.

Late summer on a north central marsh is ordinarily a season of dry, hot weather. Some degree of drought is normal at this time of year over vast continental regions. It is with extreme drought that the big changes occur. The last inch or two of water on top of the marsh mud may seethe with movements—gills, mouths, backs and tails. A little more exposure, and motion ceases over the surface mud. Some creatures go beneath, some become part of a gelatinous film, some persist as shells or bony heads and tails. Here is the down of a duckling, or the woolly fur spread out from the bones of a young muskrat, or perhaps a freshly dead young muskrat with belly slashed by an older one. Or something with the shrinking skin of death still moves as carrion beetles work beneath.

Muskrat movements can be caused by too much, as well as too little, water, as when a series of wet years may be accompanied by loss of the emergent vegetation of marshes. When a marsh, in one season, turns into an open-water lake, the evicted marsh-dwellers that cannot fly away may crowd the shores and adjacent land. Muskrats, being among the sufferers from such natural cataclysms, are preyed upon by the usual hunters of shore zones. The vulnerable muskrats, while they last, live in cavities under tree roots, in badger and woodchuck holes, in tile openings and culverts, under piles of junk and in nests fashioned out of drift. They eat what they find close by and raid the corn and beans and small grain of cultivated lands. They appear in grain shocks and farmyards, and they leave their remains wherever they go until their surviving numbers are more in balance with their possibilities for living.

The mink trails are sometimes littered with clam shells where the marsh is lakelike, or with dryland prey when the minks have access to an inviting source of mice, ground squirrels, grasshoppers, or land birds. On duck marshes, there may be duck bodies of every age on the mink trails or pulled into the den holes; on muskrat marshes, muskrats are among the conspicuous victims.

Desert marshes of the West afford great contrasts in life and lifelessness and, sometimes, in life and death. Along the east side of Great Salt Lake, muskrats that live in the low river and dike banks, and out in the bulrush patches, disperse from the choicer environments in response to population pressures. Occasionally, one of them may find a freshwater spring in which it can stay alive in such an unpromising place as an island in the Lake, surrounded by miles of brine and desert.

The usual hunters of the shore zones in lowland marshes of course include the minks, and virtually any discussion of North American muskrats is likely to get round to minks—water weasels with which muskrats have had long experience. The mink trails reflect the day-by-day drama of the marsh edge. Either in the trails or at the openings of dens—such as holes in the upper parts of old muskrat burrows—mink “signs” tell of a staple diet of crayfishes and frogs, or of water beetles or dragonflies or blackbirds or young coots. The mink trails are sometimes littered with clam shells where the marsh is lakelike, or with dryland prey when the minks have access to an inviting source of mice, ground squirrels, grasshoppers, or land birds. On duck marshes, there may be duck bodies of every age on the mink trails or pulled into the den holes; on muskrat marshes, muskrats are among the conspicuous victims.

Mink predation upon muskrats has been studied with particular thoroughness on Iowa marshes and proves to be mainly a matter of the minks utilizing part of the biological wastage of muskrat populations. Up to midsummer, nearly all of the muskrats preyed upon by minks in the Iowa areas are the strife-battered excess males that are forced to wander off and live in risky places because of the intolerance of their better-situated fellow muskrats. In mid- and later summer, most of the mink victims are either surplus young, similarly forced into the dangerous shore zones to keep out of the way of their intolerant elders, or animals placed at overwhelming disadvantage through storm or drought emergencies. Mink victims, therefore, are not just any individual muskrats that minks care to prey upon, but rather those that have fallen into special classes of vulnerability.

Frosts of the Indian summer night leave thicker and thicker ice on the ponds in the mornings. In shaded, quiet places, the ice may not melt all day. Where a muskrat swims, there may be a swirl of bubbles and pieces of broken ice film, or the animal travels submerged, breaking the ice film only where it surfaces. One evening, icy needles form a crystalline lacework on the lakeside rocks, the water’s edge is still and films over, and the lake, also except where the ducks sit. During the night, the new ice film thickens, cracking now and then, with water oozing from the cracks to freeze in turn. Muskrats sit beside the open holes. New lodges and feed houses appear over the marshes. Faint, muddy prints of both muskrats and minks mark the new ice. Lakeshore ice, in particular, may have a little blood, some bits of vegetation, fish scales, or skin about the rocks and openings where the minks and muskrats feed.

In the course of a killing freezeout, the last places on a marsh where fishes keep alive include the plunge holes and channels of muskrat burrows and lodges. The water there may be packed with bullheads; almost all other marsh fishes die before this stage.

Early winter may bring no great problems for marsh animals adapted to live beneath the ice. The outside temperatures may vary from forty above zero to forty below without making much difference to the life in the water—as long as the essentials for living remain at hand.

With the continued sinking of the frost line, however, the seeming isolation from winter’s problems may be transformed into a series of patent emergencies. In the course of a killing freezeout, the last places on a marsh where fishes keep alive include the plunge holes and channels of muskrat burrows and lodges. The water there may be packed with bullheads; almost all other marsh fishes die before this stage. The bullheads gulp air, trying to live, whether the oxygen is all gone from the water or not, whether the water reeks with hydrogen sulfide or not. A few die, or many may, and the living wriggle among masses of floating or partly submerged dead.

The muskrats may or may not feed upon fish, depending principally on whether they have access to an abun-dance of choice plant foods, to root-stocks or tubers of cattails, bulrushes, reeds, or duck potatoes. When the better plant foods are in short supply, the muskrats may feed upon the fishes almost to the extent that the minks do, either upon their own victims or upon those dragged out and left by the minks. The stomachs of nearly half of one sizeable lot of midwinter muskrats from a northern Iowa marsh contained bullhead flesh.

The muskrats, not being as dependent upon the water as the fishes, have more leeway in meeting freezeout crises, but they, too, can winter-kill. As a rule, those living in a food-rich environment with little water get along better than those in food-poor environments with more water. In the shallows that are grown to cattails and bulrushes, the muskrats can still dig out nutritious underparts, even though considerable freezing of the bottom muck occurs.

If the muskrats are forced to come out on the surface to feed in cold weather, some of them may still find something edible in the exposed plant crowns and rootstocks. They gnaw the favored parts right down into the ice and may almost stand on their heads, hindquarters braced above, as they tug and twist. They eat the river bulrush rootstocks sticking here and there from the outside of a lodge. When the entire food supply of the muskrats becomes encased in ice—as is often true where nothing but coontail and other submerged water plants occur in a foot or two of water, and all of this freezes to the bottom—their situation becomes one of deadly crisis almost in a matter of hours. They cut out through the sides of lodges to travel over the ice, going from one frozen lodge to another. They fight with and eat upon the bodies of their fellow muskrats or leave the marsh to wander over the countryside—tails, eyes, and feet freezing, always vulnerable to whatever predators prey upon muskrats that are trying to live at a hopeless disadvantage.

The muskrat tail tip is still frozen to the ice beside the lodge, but the rest of the animal is gone. Fox tracks center about this spot, and they lead off in a straight line toward the shore.

Here is a beaten group trying to weather a cold snap. They huddle, a half-dozen of them, in the eaten-out and reworked shell of a small lodge. Some openings to the outside are plugged with mud, fragments of water lily rootstocks, and miscellaneous debris—even with the frozen bodies of bullheads. Other openings are partly plugged; others are not plugged at all, and inside the muskrats sit—with upper parts frosting and lower parts wet. The inside ice-glaze has bullhead bodies in it but the muskrats are no longer eating bullheads. They are no longer doing anything except sitting or rearranging themselves. A wet tail tip sticks out of an opening and freezes to the ice outside. I have stroked the backs of such animals with a hatchet handle, and they just turned to look at me, without otherwise moving.

Next morning, the whole top of the lodge shell is open, empty of muskrats, and powdered by a trace of snow. A mink-killed muskrat lies smeared with blood on the ice, and a drag trail represents another victim. A third muskrat lies on the ice without a wound on it but with lungs congested from pneumonia. The trail of a live muskrat can barely be distinguished; after tracking around the wreckage of the lodge, the animal headed for shore, where it worked the rushy and weedy fringes before crawling under a boat. The muskrat tail tip is still frozen to the ice beside the lodge, but the rest of the animal is gone. Fox tracks center about this spot, and they lead off in a straight line toward the shore. A crow alights by the mink-killed muskrat; after a little pecking, it walks over to feed on a big bullhead that somehow got on the ice away from the lodge. The mink returns to its remaining victim of the night, but the blood-saturated under-fur is now frozen too solidly to the ice for the mink to wrench it free. The mink finally drags away the muskrat with the pneumonic lungs, following the same drag trail it had made earlier.

There are different degrees of security among wintering muskrat populations of glacial marshes, even where practically all individuals are able to survive. Some have to do little except sit, sleep, swim, feed, and show a minimum of wariness toward the more ill-disposed of their fellow muskrats and their racial enemies, the intruding minks. Others have to work hard for their lives, as the ice thickens and the water recedes under the ice. Some live in dry passageways and run along dry trails between nests and feeding sites. Thaws cause the flimsier structures to collapse, or the ice or snow melts about the pushed-up slugs of vegetation, mud, stones, and sticks over the burrows; and then the occupants have more problems of repairs. Plugging of openings may go to extremes, as when a muskrat continues to push wet vegetation through a small hole to the cold outside, and, with each new push, a horn of vegetation protrudes farther on the outside until it topples over from its own weight.

In cutting through ice, the muskrats are adept if they work from below, and they may work well at this in water. They may gnaw upward through the ice in the middle of a marsh, and then heap up the vegetation for a new lodge around the new hole. Once I happened to be walking across the bare ice of a marsh center when a muskrat cut through ahead of me. It pulled its wet body out, and there it sat amid ice splinters beside a hole leading down through a foot or so of ice. A muskrat, working from below, may similarly gnaw the ice covering the plunge hole of a solidly frozen, abandoned lodge and rehabilitate the lodge in a few hours.

A heavy thaw imposes its own problems upon a muskrat population that got along well in the labyrinths of layered ice and subsurface channels left by receding water during cold weather. The pouring of water from above does not necessarily endanger muskrats through drowning, for only under exceptional conditions are north central muskrats unable to reach air; but they may need to do some quick adjusting when forced out on the surface of land or ice or into the upper parts of previously unmaintained lodges and burrow systems. At such times, many of the animals are briefly vulnerable to minks and other predators—or to other muskrats if the emergency forces more intimacy than muskrats are disposed to tolerate, particularly as the spring breeding season approaches. If a cold snap follows the thaw while the muskrats are still in process of adjusting to the flood waters, their troubles are compounded. The luckier of the unlucky ones get by with only frozen tails.

The minks are the specialists in seeking out and exploiting dead creatures. They are adept at detecting the scent of the dead as it penetrates to the outside from snowdrift or muskrat lodge or burrow. They are also sufficiently good diggers to break through many frozen surfaces and eat the accessible flesh of coots, ducks, and muskrats partly imbedded in ice.

Within a week or so after the onset of a deadly freezeout, the muskrat population of a marsh may be classed as either the dead or the safe—safe, that is, until the frost line reaches another stage or until something else goes wrong for them.

That the north central marshes really belong to the minks during the winter months is manifested in many ways. Minks may prey upon parts of muskrat populations that become vulnerable with the descent of the frost line. The “sign” shows where a muskrat ventured from an opening in the side of a frozen lodge, to make futile explorations about other frozen lodges. After tracking up new snow with its walking, digging, and gnawing, it began its return trip. Then the walking trail changed to a bounding trail that was joined by the trail of a big mink. The two animals rolled as the mink seized the muskrat by neck or shoulder, grappled with forelegs, and kicked and clawed with hind feet. On the snow is blood and a little fur. A dragging trail leads away across the open spaces, through the cattails and bulrushes and reeds, through marginal weed patches, between and over snow-drifts. Partly blown away in the wind, partly drifted over, the trail leads past one open lodge after another, then into a hole, several hundred yards from the site of the killing. A slightly older drag trail, likewise ending at the hole, backtracks to a place where mink and muskrat wrestled over the snow outside of another lodge. A couple of still older drag trails (but still datable to the same day or to the night before) also lead to the same mink’s cache of earlier muskrat victims.

Within a week or so after the onset of a deadly freezeout, the muskrat population of a marsh may be classed as either the dead or the safe—safe, that is, until the frost line reaches another stage or until something else goes wrong for them.

One type of mink predation that can be conspicuous even at times of favorable wintering conditions for the muskrats is centered upon certain individual muskrats that show restlessness with the approach of the spring breeding season. The relatively few individuals of the well-situated population that start coming out on the ice during early winter or midwinter thaws, and persist in doing so for any reason, have limited life expectancies, whether their predatory enemies be minks, foxes, dogs, coyotes, or birds of prey. These unhappy or restless individuals vary from full-fleshed animals of either sex at the peak of physical prowess to the thin and chewed-up social misfits of either sex that almost every muskrat population has. They include the senile or the mere “kits,” the last-born young of the last breeding season. They include trap cripples, the sick, or members of the usual surplus of males. They may not fall in any handy classification except that they are muskrats living dangerously while most of the local muskrats are living securely. When these individuals are sooner or later killed off by the opportunistic predators that can do it, or die from fight wounds inflicted by other muskrats, or are otherwise eliminated, the muskrats of a marsh may again be classed as the dead and the safe—and the period of safety for the safe part of a muskrat population may extend far past the spring breakup and the dis-persal from wintering to breeding quarters, the presence of muskrat-hungry, enemy minks notwithstanding.

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