A review of Sam Erlinge's Papers on Swedish River Otters
by Bob Arnebeck
From 1958 through 1966 Sam Erlinge studied otters in southern Sweden with tracking, scat analysis and observation and wrote three scientific papers: territoriality of the otter (Oikos 19, 81- 98), home range of the otter in Southern Sweden, and food studies on captive otters. He studied two areas with otter habitat, Scania and Smaland, both with terrain characteristic of all of southern Sweden: lakes connected by streams. Erlinge put several maps in his papers and glancing at them can provide some pure pleasure, for example, "Movement of two dog otters during a period of snow in 1962," which illustrates this bit of poetry:
"One dog (D1) travelled upstream along Klingavalsan. Another dog (D2) left the stream and moved to Ellestadsan. D1 stayed for two nights along Klingavalsan. D2 stayed at Ellestadsan. D1 made an excursion to Lake Sovdesjon up to the outflow of Snogeholmsan, returning again to Klingavalsan. D2 still had its haunt along Ellestadsan. D2 ran over to Klingavalsan. D1 moved downstream Klingavalsan."
I can relate to this but Erlinge being a scientist doesn't trail off at the end of his observations with an exasperated sigh along the lines of "and maybe then they went to Montreal for all I know," or in his case "...then Stockholm, perhaps."
Seriously, it is a pleasure to have this down in black and white. Unfortunately, Erlinge does have to succumb to the scientific compulsion for numbers. Fair enough that his maps explain that it is about 7 or 8 kilometers, as an otter goes, from Lake Ellestadssjon to Klingavalsan, but then from this and his other obeservations, he abstracts a generalization:
"The density of otters in the areas studied was one otter per 0.7-1.0 square km area of water or one individual per 2-3 km length of lake shore, and one otter per 5 km length of stream."
Those, I submit, are meaningless numbers on their face, and it gets worse the more you think about it especially since the theme of Erlinge's study is the fragility of the otter family. I think I've read somewhere that upon his return, years later, to the areas he studied Erlinge found all the otters gone.
He begins his "Territorality of Otters" by pointing out that there was "scant evidence" about the subject and "divergent" opinions. Most European researchers thought otters territorial, while an American, Liers, thought otters were sociable except "at breeding time... when males fight among themselves." So Erlinge first examines "reproduction, structure and density of otter population." He found, and this is a key to understanding otters, that "the rate of reproduction of the otter is low." Litter size is from 1 to 3, sexual maturity is from 2 to 3 years old and females "certainly do not breed more than once a year." American wildlife managers always lump otters with muskrats and beavers, all prime game for trappers. Those rodents are prolific with larger litters, earlier maturity and even multiple litters a year a possibility. Indeed Erlinge doubts that females breed once every year, since mothers so often remain with their young during the subsequent rutting season.
As for the structure of otter population, Erlinge found that one-third were "young of the year," a bit more than a third resident otters and the rest transients. Otter dispersion is dependent on topography with the outflow areas of lakes most popular, then the inflow areas. I've already mentioned Erlinge's numbers on otter density.
Turning to the otters marking of territory, Erlinge found that otters deposit spraints (as many call scats) at fixed sites, otters sniff before they mark, and sometimes prepare sites by scratching. Otters also frequently "visit and examine signal spots marked by other individuals," and usually they are "very common near the in- and out-lets of ditches and streams in a lake, which are the normal paths for otters entering an area." Personally, I think this simplifies the matter. However, I watch otters in beaver ponds in valleys separated by ridges. Short-cuts over the ridge are often more likely to be marked than in-let and out-let streams. And in general at least half of the otter latrines I see are not on any seeming route to, from or through the ponds.
Erlinge examines what sexual and territorial significance this marking has. Erlinge thinks that males and females are in constant communication with each other through marking, and tested to see if there was more marking during the rutting season. He found that "the signal activity was high but not exceptionally so compared with other seasons." In my own observations, by no means rigorous, I get a sense that in my area breeding males and females repair to certain areas which might mean that marking around ponds, generally used for raising otters, leave no sexual message beyond "I want to be alone" with my pups. But, let me hasten to add, Erlinge studied an area much larger than mine.
Indeed at the outset of his discussion of territorial marking he suggests "intense signal activity occurred in meeting zones of otters coming from different areas." Erlinge found the most intense scatting where two dog otters, as he calls the male, from different territories met. He also found signal activity strictly dependent on otter density and when density dropped old scatting sites 'became overgrown by grass." Erlinge also noted intense marking by family groups and by juveniles a year or two from being sexually active.
And so he begins tracing the travels of the dog otters who seemed to respect territory while still traveling on average 10 kilometers a night. Here again, I am out of my element, since I usually see otters during the day and my own range on the end of a large island is perhaps 4 or 5 kilometers. Erlinge found that the dog otters extended territory when another dog otter was shot or disappeared, and then another dog otter moved in to take territory and the territory of others contracted.
"Females with cubs occupied areas within the dog otters territories." And the ranges of family groups changed during the year, widely separate when the cubs were young, then coming closer to each other as the family dispersed, and in one case overlapping slightly. Finally, Erlinge found that this pattern seemed almost traditional. Here I am on familiar territory and am pretty confident that in seven years of otter watching only one family group at a time spent the year in the area I watch.
There are other otters in the mix: females without cubs and 2-3 year old otters. Erlinge observed them as always being transient. A pair that stayed close with each other still moved on. I've seen single otters join the otter family in foraging, and recently what appeared to be an adult otter spelling the mother and caring for the pups. Erlinge makes the point that these extra otters, so to speak, didn't den with the family. However, I've seen them so close to the family that, I find it hard to believe that they didn't den with each other. In his observations of several generations of river otters in California, Scott Shannon has showed how other adult otters, previous offspring of the mother, often join the family group.
The point of sharing my own experience is not to denegrate Erlinge's studied. It is one of my ambitions to follow the trail of a dog otter and perhaps, if the large river freezes totally (which is possible) and the snows show the tracks and slides, I might get my wish. What I'm trying to do is highlight Erlinge's strength which is his understanding of the dog otter, which in turn raises a caveat: his generalizations must be checked to be sure that they don't project on all otters that which pertains only to dog otters in finest form.
Anyway here's what I envy Erlinge for. He can write:
"The dogs are mainly solitary, and although they often occurred in the proximity of families they seemed to pay little attention to them. Sometimes, especially in autumn and spring they travelled together for part of the night.... In winter, the dogs are more mobile, paid regular visits to the haunts of the females. They remained together for 1-2 days, then the dog was away for some days travelling in others parts of its territory."
Erlinge never saw dog otters fight in the wild, but he relates the experiences of two captive males fighting, the loser fleeing as far away as possible and then later refusing to even approach the cage of the victor. This information is an unfortunate addition to the paper. I have seen otters fight and the one that was driven off made a great point of acting like he was still quite at home in the pond. Indeed, I theorized that it was a female fighting off a male. Anyway, my point is that otters can suffer loss of face and still maintain the general otter characteristic, in males and famales and juveniles, of agressive readiness. Indeed, I've frequently seen them faced down by beavers and still swim off as cocky as ever. For a captive animal, losing a fight might just be the last straw. I've worked with men in prison. The metabolism is different, the terrain is different, the consequences of every act are different.
Erlinge is on better ground when he describes the behavior of what he describes as the dominate and subdominate dog otters in an area. The former rules the terrain while the latter is more prone to be shuffling off to the side. Erlinge also found that during the winter the dog otter influences the movement of the family, and that if the dog otter finds another female, the family will move off. I think I've seen this also. While a mother and three pups seemed to have the freedom of the ponds I watch during the Fall, during the winter they deferred to a pair of otters, very likely a male and female about to mate.
While carefully establishing the territory of the otters he watched, Erlinge recognized that the boundaries changed as the year progressed and from year to year. However, otters do maintain their territory actively. Erlinge never saw wild otters fight so he thinks otters rely almost entirely on marking to facilitate avoidance. Intrusions, which may be frequent, were exceptions which more or less proved the rule. Because of marking, the intruder regulated his behavior so as not to confront the dominant dog otter.
Erlinge suggests there are two types of otter territory: the best feeding areas are reserved for mothers and their pups and the large territory maintained by the dog otters encompass several females. Such behavior, Erlinge argues, "serves to disperse the individuals of the population, and may be important in controlling population densities." Finally, Erlinge notes that otter territories and populations seem more stable than that of the smaller mustelids and suggest that is because fish populations are more stable than the population of the rodents hunted by martens, weasels and stoats.
by Bob Arnebeck
return to: tracking.html