Mortality in wolf pups

The main monograph on the grey wolf by Mech and Boitani (2003), which still provides the basic and unequalled knowledge on the species biology, informed the following. “Almost 30 years ago, Keith (1974) concluded that “the factors which provide wolf pup mortality during the first 5 months are almost wholly unknown. This is probably the single greatest enigma in wolf biology today.” Though some strides have been made toward identifying these factors, this is still a much needed area of research.”
While reading this in 2004, we already were much aimed by the question. Moreover, the opportunity to find out something really new in the wolf biology accelerated these our research efforts. Nowadays, we may say that, at least, for the region of Belarus mortality in wolf pups is known more or less. Also, we assume that the factors, which impact the survival of wolf pups in Belarus, act in other regions of European forest zone. At the same time, we suppose that the only main causes of wolf pup mortality were found out, whereas many smaller questions remain unresolved.

This begs the question of how a such complicated research (i.e. what factors are likely to influence pup mortality) was posed. In fact, during searching for wolf dens with early day pups and older pups, being in wolf family habitats year-round and for many years, we faced many times with situations of dying or dead pups. The data collected finally clarified the hidden question of wolf pup mortality in Belarus. Also, in 2014-2017 applying of many camera-traps that were well-set helped a lot.

Mortality causes and estimates of mortality rate in wolf pups

While analyzing mortality and survival in wolf pups, it is worthwhile to begin with the analysis of the influence of environmental factors in connection with complexity of wolf denning behavior. The gained initial data suggests that burrowing by parents leads the higher survival in pups until the coming winter. Long distance replacing of pups leads their lower  survival. During rainy weather, more complicated denning behavior of breeder wolves (more denning sites, more dens each denning site and longer distance of replacing between the denning sites)  leads the lower pup survival rate. Conversely, during more or less dry weather more complicated wolf denning behavior leads the higher survival rate in pups. Number of mosquitos, rainy weather and human presence drive the complexity of denning behavior and may increase the mortality in wolf pups indirectly.

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It is important to get know how and from which kinds of impact wolf pups die. Therefore, by describing and estimating mortality in wolf pups, first of all, we would like to list the revealed cases of pup deaths and respectively which factors were responsible for that.

First, it is a deliberate leaving by wolf parents of some pups to die , while replacing the litter from one den to another one. During our long-term experience of wolf pup searching we discovered on the recent former den one abandoned pup for 5 times, two pups – twice, and once there were four definitely abandoned pups. Mostly the abandoned pups were dead. Normally such a deliberate leaving of a few pups to die happened with a large litter consisted of more than 7 pups (in our materials it was in 5 out of 6 cases, in 2 cases the number of pups was unknown).

The above-mentioned cases of pup abandonment relate to early days pups. However, even older pups of several months old may be abandoned by parents. In the mid and late summer of 2013 in Naliboki Forest we faced with that, at least, for several times.

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There is another way, an indirect one, how breeding wolves condition to small pup death. In case of multiple breeding in wolf packs mothers steal pups from each other quite often. They steal not all pups; usually only 1-3 pups are taken from another litter. Actually, in our research experience and the practice of three other wolf pup searchers (altogether about 160 wolf litters that were looked through) such a denning behavior of stealing pups was registered for 6 times. In the respective litters there were pups of different ages. These mixed litters consisted of 3 and 4 pups of distinctive ages; 4 and 1; 5 and 1; 3 and 2; 7 and 1; 5 and 4. In such a case wolf litter consists of bigger and smaller pups. Sometimes, this size difference may be too much, and bigger pups suppress smaller ones . Such a suppressed small pup die somehow, finally being trampled in the den bedding materials. Twice we faced with such a case of a dead pup or several dead pups in the mixed litters: in one case there were found 2 small pups alive, one small dead pup and 4 markedly bigger pups; in another such a case – 3 small pups alive, 2 small dead pups and 4 markedly bigger pups.

Dead wolf pup that was trampled in the couch-den bedding

We registered that the wolf pup mortality was significantly higher in a larger litter. Usually, it is like that. If there are 9-12 pups in the mid-May, it means there will be 2 to 4 of them in July; whereas in litters that initially consisted of 1 to 4 pups almost all of them may survive till the first winter. Generally, we suppose that such a dying of wolf pups through a fault of their parents comprise 10-20% of overall pup mortality till the first winter.

Second factor of wolf pup mortality is predation lynxes, particularly by adult male lynxes. Indeed, we found that adult lynxes kill wolf pups quite regularly. As to this impact, we actually registered the following. In Naliboki Forest, 3 wolf litters up to 2 months old were definitely killed by lynxes and killing of 3 other wolf litters up to 2 months old by lynxes was found very plausible. Also, we discovered 8 wolf pups of 2-11 months old, which were killed by lynxes. Mostly adult male lynxes deliberately hunt on wolf pups. While roughly estimating, we suggest that in Belarus, at least, a half wolf pups are killed by lynxes in the habitats, where the lynx population density is 2 inds per 100 km2 and higher (may be up to 6).

Wolf pup that was killed by lynx

One well traced and documented example was quite impressive, at least, for us. In Naliboki Forest in the late April of 2017 in wolf pack consisting of 3 pregnant females and 2 adult males (a case of multiple breeding in a wolf pack) there were born three litters. During two weeks all three litters were extirpated by one big male lynx. Applying camera traps, we documented the appearance of the male lynx at the wolf burrows with pups. In one case there was the mother wolf at the den, but, when it detected the lynx, it escaped, permitting for the lynx to kill the pups.


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Potentially red foxes, badgers, raccoon dogs, golden eagles and white-tailed eagles may kill wolf pups up to two months old, when parents are absent at the den for foraging and other necessary activity. Nevertheless, we have never faced with that. However, one of the two wolf pup searchers, whom we know life-long and trust, evidenced that he registered wolf pups that were presumably killed by a red fox. Perhaps, therefore breeding wolves try to extirpate all red foxes in their denning sites. Concerning the other four species of medium-sized predators, we have never heard that in Belarus they attacked wolf pups although it is possible. Nevertheless, in the neighboring Latvia it was found that golden eagles kill and consume wolf pups sometimes (Bergmanis et al., 2012).

Third factor of wolf pup mortality is killing of them by wild ungulates. According to our own findings and information that we received from three other wolf pup searchers, this happens not rarely. Twice there were discovered by us the remains of wolf pups, which were killed and even eaten by wild boars and once we found two carcasses of pups, which killed by roe deer. For three times a lot of fresh elk tracks were found nearby wolf dens; it looked like the elks tried to kill small pups with their hooves, while the pups tried to escape within fairly dense wood cover (dry last year and new grass, dead tree materials with uprooted trees, bushes). None of the killed wolf pups was discovered on the places, but, if they were crashed by the elks, perhaps, their carcasses might be taken by the parents afterwards. Twice the same situation was registered in relation to red deer. In one of these cases carcasses of three wolf pups were found. Once we saw place (grassy opening not far from forest edge), where wolf pups about 2 months old were trampled by bison herd. We suppose that in Belarus killing of wolf pups by wild ungulates usually constitutes up to 10% of overall pup mortality till the first winter. However, if wild ungulates are spread widely in the whole habitat variety and their population density is high (15-20 and more inds per one km2, except of roe deer), up to a quarter of present wolf pups may be killed by wild ungulates till the first winter. We think that stags (adult male red deer) and both sexes of adult elks may be mainly responsible for such a death of wolf pups.

Another possible factor of wolf pup mortality is piroplasmosis disease. When we look through wolf pups, many of them had ticks: pups that were up to one month old – 43%, n=634; pups of 1-5 months old – 100% (n=32). We do not know how often wolf pups may be ill on piroplasmosis disease and die from that. We merely have an indirect argument for that. Our dog of 13 years old spent almost all its life within our study areas in the places with the same abundance of ticks as the local wolves. This our dog was ill on piroplasmosis disease for three times during these 13 years and could die from that each time. So, we assume that this disease is a significant factor of wolf pup mortality, but it is not so essential as in the cases of killing of wolf pups by lynxes and wild ungulates. During our long-term being in the habitats of wolf families we discovered only two wolf pup carcasses (about 4 months old) that died from an unknown cause, may be piroplasmosis disease or may be other problems. Another wolf amateur and game worker reported that once he faced with two dead wolf pups, and the cause of their deaths was hard to determine, just by inspecting them externally. Anyway, it looks like diseases (e.g. piroplasmosis, rabies etc.) may be responsible for a relatively low portion of wolf pup mortality, at least, in the Paazierre Forest and Naliboki Forest in Belarus during the period of studies since the mid 1990s. Otherwise we or somebody else could find markedly more wolf pup carcasses in the habitats.

Among other causes of mortality in wolf pups it should be mentioned killing of them by wolf pup searchers as one of the ways of the species population control. In Belarus it constituted of an essential part of wolf pup mortality until the mid-1980s. That time in more or less well-set game areas (a third part of Belarus), at least, a half of wolf litters were discovered and killed by wolf pup searchers every spring. Then wolf pup searching sharply declined, mostly because people, who smart in the wild and easily doing with wildlife began disappearing in the countryside. That was evidently connected with village life declining, replacing of countryside locals in towns and cities, while urban residents are evidently not capable for such a job. Afterwards (the 1990s-2010s) approximately one per 30 to 50 wolf litters was killed by people mostly game-related ones. So, this factor of mortality in wolf pups became not really important nowadays in Belarus.

Road kills of wolf pups and any wolves are rare in Belarus still. During the long-term period of study on wolves in the study areas we revealed only two road kills of wolves. One of the killed wolves was a pup of 5 months old. Normally, information about wolf road kills spreads widely, and we get know easily about such a case.

By evaluating the mortality in wolf pups, we revealed that it is really high. While tracing 33 wolf litters in the period from their birth (mostly first half of May) to the beginning of winter in November-December in Paazierre Forest, the survival in wolf pups was rather low – 0.48 on average (i.e. 52% of pups died during the first 6-7 months).

As to mortality of wolf pups in Naliboki Forest, we separated our material for the three periods characterizing by distinctive state and density in the local population of the lynx: in 1999-2011 there was low number of lynxes, up to 1.11, on average 0.45 inds per 100 km2of forest massif; 2012-2016, 1.16-1.84 lynxes per 100 km2 of forest massif; 2016-2017, 2.21-3.16 lynxes per 100 km2of forest massif. These three periods were checked separately,  because of  the marked difference in the lynx population density was found and taking into account that lynxes kill many wolf pups. Respectively, we evaluated that in  1999-2011 with the  low number of lynxes the mortality in wolf pups was about 30% (n=14 litters traced); in 2012-2016 with markedly more lynxes – 63%  of wolf pups died before winter (n=10); in 2016-2017, when lynxes got common, the extremely high mortality in wolf pups was registered –  96% (n=9). For instance, in the warm season of 2017 in Naliboki Forest there were located 7 wolf litters, and in September-October only two pups were registered altogether (one pup in two wolf packs).

So, the above-presented data again suggest the crucial role of  lynx predation for survival of wolf pups. It is becoming evident that, when in a forest massif the lynx gets a high population density, the local wolf population appears in a “trap” due to reproduction “pitfall”. The reproduction problems in wolves are based not only on  just the lynx interference towards wolves, and also on the manner of wolves to raise pups with leaving them for quite long in a sheltered site, when the parent wolves need foraging. Thus,  wolf pups are  like exposing for an easy killing by lynxes. Since the lynx-related problem in the wolf reproduction in a forest massif appears, the local wolf population becomes persisting not due to reproduction, but because of immigration from adjacent rural-forest mosaic terrains, where pups survive easier with a few or no lynxes in the habitats.

It looks like such a declining situation in wolves (i.e. the reproduction “pitfall” in forest massifs) was not forever. Lynxes as well as wild ungulates were not so common in the primeval forest on the territory of Belarus in the Middle Ages, i.e. in the Great Lithuania. Indeed, there are not so many mentioning about lynxes in the game documents of 17th-19th centuries for Naliboki Forest and the central Belarus on the whole, whereas wolves were mentioned at the same time a lot  (Belarusian archive departments 694, chapter 2, and articles 201, 1587, 3984, 4353 in it and chapter 3, and articles 2012, 2015, 1217;  Zavisha, 1862(2011); Niamtsevich, 1868; Shyshyhina-Patotskaya, 2007). As far as we can realize from the historical documents there were markedly fewer  lynxes in the more primeval forest – approximately 1-5 inds per 1000  km2.

Then the intensification of logging with early reforestation on clearcuts conditioned markedly higher habitat carrying capacity for wild ungulates and lynxes. Lynxes got manifold (about ten times) commoner, and frequency of wolf-lynx interference increased a lot. Thus, we hypothesize that just logging is responsible for such situation of the reproduction “pitfall’ in wolves that inhabit forest massifs.

The material that we obtained through the age determination of wolf skulls can bring some more knowledge on the survival in young wolves.  The ratio of the yearling number over the juvenile number suggests that the survival in young wolves aged from 6 to 18 months constituted about 0.62 in the period of fairly low hunting pressure. When wolf persecution by human became heavy, the survival in young wolves aged from 6 months up to 18 months was markedly lower (0.28) than that after the period of fairly low hunting pressure. So, winter mortality in wolf pups in Belarus is determined a lot by the pressure of wolf persecution by human.

In the question of mortality in wolf pups, there is another important aspect that is tightly connected with multiple breeding in a wolf pack. In most cases sometime in July-September one of the mother wolves begins to live alone with its pups after its sending away by another stronger mother wolf. Quite often the expelled mother is young (2+ only or even 1+ sometimes) , and such a young mother is not able to hunt good enough to feed itself and the pups. Also, it is hard for such a vulnerable mother to establish own territory alone. Therefore, such weakened wolf pack mostly scavenges around villages, kills dogs, takes carrion and try to hunt domestic ungulates. We traced four stories of such wolf packs of young mothers and their pups. Concerning these four mothers, it looked like two of them overwintered, other two were killed. Concerning their pups, only 2 out of 20 of them that were registered in November-December could overwinter, and their winter survival was about 10% only. Also, it is worthwhile to notice that all these four packs lived in the conditions of presence of a few lynxes in the habitats. Perhaps, if lynxes were common, the stories of these packs might be even worse.


Decline in the wolf reproduction with implication for wolf pack composition

n Naliboki Forest, when lynxes got common since the autumn of 2015 (more than 40 inds), the reproduction in wolves almost stopped due to disappearing most of the pups. In turn, and family-origin pattern of wolf packing was substituted for subordination-origin one. As you learnt from the previous item, the disappearance of wolf pups is strongly connected with the deliberate predation of adult lynxes (in particular males) on wolf pups.

With respect to this, it is worthwhile to compare the wolf number, portion of pups in the local wolf population in November-December (when pups are still recognisable), patterns of wolf pack forming in October-January. As to a pattern of wolf pack forming, it is either family-origin, i.e. parents with pups or subordinating, i.e. a strong wolf has subordinated other wolves that ranged around.

In early winter of 2008-2012 in Naliboki Forest and its forest-agriculture surroundings in the area about 2700 km2, there were 51-70 wolves. The portion of pups of the year (0+) was 25-44% and 86% packs were of family-origin (perhaps, some of such packs included a few non-relative subordinated individuals).

In early winter of 2015-2017 on the same area in Naliboki Forest and its forest-agriculture surroundings, there were 40-44 wolves. The portion of pups of the year (0+) was 14-18% and the majority of the packs (at least, 63%) were entirely or mainly of non-family origin, i.e. mainly consisted of non-relative subordinated individuals.

Thus, in 2008-2012 the local wolf population in Naliboki Forest existed being supported by its reproduction, while nowadays in 2015-2017 it still exists due to the immigration of adult wolves during mating season.