As I have been reading various publications regarding denning behaviour, pup development and simply general demographic reviews of populations, it has stricken me as curious that wolves in Poland and Germany (and possibly in other countries that I have not encountered data from) appear to produce pups a bit later in spring than wolf populations which actually live in harsher conditions (larger latitude).
As far as I understand, denning period (onset of births in the wolf population) largely depends on latitude.
For a latitudinal comparison, I will present data from North America mainly because the North American wolf populations have been studied very thoroughly allowing for comparisons between populations.
‘The onset of denning ranged from 29 April to 30 May, with a mean date of 10 May, which is remarkably similar to the dates reported elsewhere in Alaska (see Walsh et al., 2016). Denning in Alaska appears to occur a couple of weeks later than in Minnesota (second week of April; Fuller, 1989), but a couple of weeks earlier than in the Canadian Arctic (late May to early June; Heard and Williams, 1992), which suggests a strong nexus with latitude.’ (Joly, K. et al., 2018)
While it may be difficult to compare data from two different continents (despite the grey wolf being a Holarctic species which exhibits overall similar ecology in North America and Europe), it appears that in the ‘far north’ of North America (on this occasion, in the Canadian Arctic and Alaska) pups are born (denning begins) on average in late May to early June and in the middle of May, respectively.
Further south, in Minnesota, denning occurs earlier (on average, middle of April) than in Canada and Alaska.
Minnesota is geographically rather parallel to the Rocky Mountain area where wolves have also been studied extensively.
However, most research in Minnesota has been acquired in the northern counties while the populations in Idaho, Montana and Wyoming (Northern Rockies) have been studied comparatively in a more southerly area relative to Minnesota research areas.
National Park Service states that in Yellowstone, female wolves ‘give birth to average of five pups in April after a gestation period of 63 days.’ (NPS website)
This could be comparable to in Minnesota.
For example, Center for Biological Diversity gives estimate, ‘In the northern Rockies, an average of five wolf pups are born any time from late March to late April or possibly early May.’ (CBD website)
Elevation and proximity to large water bodies might additionally affect climate with the Rocky Mountains being higher in elevation than Minnesota and further from influence of big lakes or seas.
It is easily seen that the birth period is not very strictly limited to one week or a couple of weeks yet there appears to be a certain tendency with most births concentrated within a specific period.
In the North America, the tendency of giving birth earlier with lower latitudes is quite apparent even when accounting for confusing factors such as elevation, habitat productivity etc.
From the publication that I have read so far, it is not clear what determines this onset but it is probably determined in the mating season (January – March) rather than in the birthing season as an evolutionary adaptation to climate, vegetation growth season, ungulate births etc.
In many species such adaptations are regulated by photoperiod or temperature that trigger the hormonal production.
Latitude (and elevation) are strongly correlated to:
- average temperature;
- snow cover;
- snow cover duration;
- snow melt initiation;
- onset of vegetation growth season;
- prey species birthing pulses etc.
Female bodies must have evolved to predict these onsets already in winter when mating begins.
It might be interesting to learn whether females give birth at different times during different years that exhibit contrasting weather patterns (earlier vs. later spring) which would inform us on the female’s ability to predict that year’s weather during pup birthing and denning.
It might also be interesting to learn whether females that have dispersed from more northerly populations or more southerly populations and that currently reside in a climate which is warmer or colder, adapt their birthing to the new location (or if their ‘biological clock’ is set genetically and not as flexible to adjust).
While I find it difficult to predict whether females might be able to adjust their oestrus with respect to the particular year’s prognosis, I believe that in grey wolves, a highly mobile species, females should be able to adapt to new climatic conditions rather readily because it is not that difficult, nor it is rare for wolves to disperse from much lower latitudes to much higher latitudes and vice versa.
Returning to the issue of wolves in Central Europe, I find it puzzling that wolves, for example, in Białowieża (one of the most intensely studied wolf populations in the world) appear to have parturition dates that are not predictive with climatic factors usually associated with latitudinal impact and that appear to be also later than birthing, e.g., in Latvia and Estonia which are more northerly countries (although less continental) and where more or less official sources cite the wolf parturition mainly with the month of April.
‘Parturition [in Białowieża] occurred between 19 April and 12 May, and the denning period lasted for 49–64days.’ (Schmidt, K. et al., 2008)
Also (quoting the same publication), ‘Compared with the years 1947–1950, in 1995–1999 the breeding season of wolves in BPF occurred two weeks earlier. A possible reason was the 1 to 1.5-degree increase in the mean annual temperature during the last 50 years.’
There is no latitudinal gradient – wolf pups in Poland appear to be born during the same period as wolf pups in Finland (‘The mating season of the wolf is in February and March. The gestation period is 60 to 63 days, and the cubs are born in mid-May.’ (Suurpedot aka Large Carnivore website)
Similarly ‘late parturition’ has been indicated for Germany, ‘After a gestation period of about 63 days, usually 4 to 6 pups are born in late April or early May.’ (DBBW aka the Federal Documentation and Consultation Centre on Wolves website)
Germany is less continental than Poland is.
For example, in a beaver-related study (Busher, P.E. et al., 2020) it has been indicated that winter temperatures may be lower, e.g., in Lithuania than in some areas of Sweden and Norway despite Lithuania’s lower latitude (54° N vs. 59° N) due to continental climate factors.
However, Germany is more exposed to oceanic and coastal influences yet this country also exhibits later pup birth dates compared to perhaps what could be expected from its latitude.
It might be important to note that the German wolf population is descendant from the Polish wolf population and they are even considered one subpopulation unit (although there is some debate regarding the subdivisions of the Polish wolf populations in different regions of Poland but not in the western region).
On an intercontinental scale, Poland and Germany might be compared to northern states of the US.
While these countries lie slightly above Minnesota and befall the southern provinces of Canada, when accounting for the influence of the Gulf Stream, I suppose Poland and Germany are not that different from Minnesota where wolf pups are mainly born in the middle of April.
If we compare climatic variables that are representative of latitude, the picture does not get any clearer.
It is difficult to obtain trustworthy climatic data for the spring months (and perhaps those are not spring months at all I should be examining but winter months when mating occurs) from the ‘general internet’.
However, I will attempt to summarize April temperatures (April being the month when wolf pups appear to be born at least to some extent outside of the arctic regions and Alaska).
April’s lowest temperature:
- 1°C – Minnesota;
- -5°C – Northern Rockies;
- 4°C – Alaska (in May!)
- 3°C – Poland;
- 3°C – Germany;
- 0°C – Finland.
Average temperature during putative pup birth peak period:
- 7°C – Minnesota;
- 2°C – Northern Rockies;
- 7°C – Alaska;
- 10°C – Poland;
- 10°C – Germany;
- 10°C – Finland;
- 7°C – Estonia;
- 7°C – Latvia.
There appears to be a slight trend (if not accounting for the Northern Rockies) where the average temperatures during putative pup birth peak are not that different although they do not follow the latitudinal trend, either.
As mentioned before, other factors could be of importance.
For example, the study by Joly, K. et al., 2018 discusses wolf denning behaviour in Alaska where females appear to choose dens respective to snow melt (den sites are selected in places where snow melts earlier in the season).
Snow melt might be important because it ensures dry den sites with proper thermal regulation.
Snow melt in Minnesota as well as the Northern Rockies begins in April.
In Poland, snow can melt throughout April but sometimes snow cover remains until late April.
In Latvia and Estonia, snow melts in the end of March or in April.
In Finland, snow beings melting in April and it can remain until the end of April (although Finland cannot really be viewed as a homogeneous region and timing of annual climatic events can be varied).
So far the pup births rather coincide with snow melt (in Poland and Finland where snow cover can remain for longer, pups tend to be born slightly later).
However, Germany breaks the trend because snow melt beings in March/early April but pups are born in late April – mid-May.
It is unlikely that wolves would adjust their birth timing to snow melt per se because wolves also give birth in areas where the snow cover persists during the denning period and wolves are capable of securing properly thermoregulated den sites under such circumstance.
Temperature and snow melt, nevertheless can be important variables that suggest the changes in vegetation growth that, in turn, affect ungulate life cycles.
Snow-melt disregarded, the similarities between Germany and Poland could be explained with the genetic origins of the Central European populations.
The wolves recolonized Germany from northeast Poland and, genetically, they form the Central European lowland population (Hindrikson, M. et al., 2017).
But the same population has also recolonized the Czech Republic and the Netherlands.
In Czech Republic, most pups are born in April (Carnivores.cz website data).
While there is no official claim regarding average pup birth dates in the Netherlands, media articles discussing wolf population development in the country mostly refers to births in April (e.g., Wilderness Society website article dedicated to the first breeding wolf pack detected in the Netherlands).
These wolves should, theoretically, represent the same genetic source as wolves in Germany or Poland and the latitudinal differences are not great enough (nor the time passed since recolonization long enough) to cause major discrepancies (although the Netherlands might have stronger influences from the Atlantic while the Czech Republic is slightly to the south from Germany and Poland).
However, it still does not explain why these wolves would give birth later than wolves in more northern regions approximating timing in, e.g., Finland.
One explanation that comes to mind early on is related to the extinction of wolves in most of Europe.
The Polish wolf population never went wholly extinct and it persisted in small subpopulations in the eastern regions of the country.
In many places, wolves found refuge in more mountainous areas that are less densely population.
However, wolves also continued living in the eastern lowlands of Poland.
Elevation could explain later birth but the Poland’s northeastern population (whence the western Poland and Germany were largely colonized) is not characterized by great elevation (elevation in Białowieża – 140 – 200 m above sea level).
It would be possible that the latency in parturition is a general remnant of a more or less ancient time in wolf evolution when the ancestors of the current Poland’s populations lived in colder areas (e.g., higher in the mountains or descendant from more northern latitudes).
This assumption would contradict my hypothesis of wolves being highly adaptive to the local environment.
It is, of course, possible that timing in birth is not adjusted if the adjustments are not altogether necessary (if the change between the habitats is not great enough to warrant an adaptation and the subsequent alteration of parturition date).
In order to determine how adaptive wolves are, I thought I could compare timing of birth in early years of Yellowstone Wolf Project because the wolves introduced to Yellowstone were mostly transported from Canada where the timing should be, theoretically, different from that in the Northern Rockies (May vs. April).
Data from the YWP Report 1995 – 1996 indicates that:
1.
The wolves released in 1995 came from Jasper National Park, Alberta (53°N vs. 44°N in Yellowstone; 1060 m above sea level vs. 2357 m; parturition in Alberta is indicated at late April – early May).
These wolves were captured in January and acclimated in pens until March.
This means that conception occurred in the pen (after arrival).
The wolves released in 1996 came from Williston Lake Area, British Columbia (56°N; 651 m; pups born in April – May).
It can be seen that pups should be born both in April and May to continue the ‘tradition’ but some climatic trade-off could be expected exchanging YNP elevation for JNP and WLA latitude.
YNP latitude: 44° N.
YNP elevation: ca. 2360 m above sea level.
Precise dates are presented in Table 8 (for some individuals) in Report 1995-96 and Table 2 in Report 1997.
The Report 1995-96 Table 8 gives the following radiocollaring dates that should be slightly later than birth dates:
- 5/4/96;
- 13/4/96;
- 25/4/95;
- 25/4/95.
- 26/4/95;
- 26/4/95;
These are not all pups that were born during 1995 – 1996 (6 out of 23 pups born). These are radio-collared pups.
This suggests that pups were actually born in the first half – middle of April.
The YWP Report 1997 states, ‘Pups were born from April 6 to May 3.’
The data of the Table 2 gives birth dates of around:
- 6/4/97;
- 6 – 8/4/97;
- 6 – 10/4/97;
- 9/4/97;
- 12/4/97;
- 12 – 18/4/97;
- 13/4/97;
- 16/7/97;
- 20/4/97;
- 22/4/97;
- 26/4/97;
- 3/5/97;
- 3/5/97.
In 1997, the dates appear more varied but also closer to April (with many births occurring rather early in April).
Overall, it appears that the translocated wolves adapted almost immediately containing most births in April and perhaps even giving birth slightly earlier (beginning of April).
It suggests that wolves are capable of adapting and that perhaps female oestrous is influenced by local climatic conditions and not by genetics.
However, it makes the case in Poland and Germany more curious.
If the parturition dates in Central Europe were somehow related to inheritance, this would mean the wolves in North America and in Europe differ in their plasticity (this is possible if adaptations are determined by genetic diversity which has suffered in European populations due to bottlenecks caused by extirpation and even historic habitat losses since perhaps earlier than in NA).
If they are not related to genetics, what are they caused by?
The central issue is likely that of what wolves have evolved to plan their parturition in accordance to.
For example, the differences within the Central European population cannot really be explained by ungulate species richness because Poland has a great number of wild ungulates, especially in Białowieża while in Germany the wild ungulate availability might be lower.
Thus, it is not as permissible to compare Poland + Germany to, e.g., Czech Republic of the Netherlands because Poland and Germany already contrast one another.
I do not really have the answer and these are merely musings.
Average or earliest/latest pup birth dates have not been estimated in these regions officially (not to my knowledge) and it is also possible that I am extrapolating on assumptions (in the case of Germany, Czech Republic and the Netherlands but no in the case of Poland where data is available and published) rather than facts.
Still, this would be an interesting area of research because it could provide information about one of the most important stages in wolf life history and what wolves have evolved to account for when planning their demographic future.
***
If wolf breeding season is planned not in accordance with the timing of birth but rather in accordance with female’s body condition and her ability to conceive and gestate, as mentioned before, it might of relevance to compare impacts of winter variables between study areas and not spring variables.
Such variables would be the timing of major die-offs of ungulates in late winter or the customary weakening of the large prey following the harsh winter period (and the corresponding climate factors).
***
It is also possible that wolves adapt to the new conditions when there is an actual need and they do not adapt if the conditions are similar or if their former breeding cycle fits well enough with the resource allocation and climate in the new area.
For example, wolves in the North America might adapt faster because climate can be much harsher in, e.g., the Northern Rockies and it is crucial to use the resources to their best potential (to fit very closely with the favourable weather and prey variables).
Adaptations might also depend on other factors such as other stressors.
For example, if the Polish wolf population, for some reason, is late-breeding and it has given rise to populations in Germany and the Czech Republic, wolves in Germany might not have adapted as closely to the new conditions because Germany is highly urbanized and industrialized perhaps forcing the wolves to, first and foremost, adapt to human influence.
Perhaps the wolf’s body is not capable of simultaneously processing this many variables and the demands to adapt to human presence might cause imbalances (e.g., hormonal imbalances) impeding other, more natural and biological adaptations.
The Czech Republic is less densely populated and possibly has more forest cover that is not fragmented resulting in lower stress levels (lower physiological pressure) and faster adaptations to the local climate.
In fact, there also could be differences between wolves in Europe and North America.
Some species in Europe (Eurasia) do not demonstrate local adaptations (e.g., lynx coat patterns in the study by Darul, R. et al., 2021) which could be a result of the frequent historical persecutions, habitat loss and habitat alterations, colonization and recolonization events and anthropogenic pressure that interferes with the localizing of the population.
However, once again, this does not explain the population in, e.g., Białowieża which demonstrates ‘belayed’ pup birth but which is one of the best preserved regions in the world.
Meanwhile, highly conserved areas with rich ungulate communities might not call for special adaptations and adaptions could be of greater importance (as well as of greater attainability) in regions of moderate human influence (moderate wilderness value).
***
I have been reading about wolves in Canada (e.g., Stronen, A.V., 2000) where some wolf packs follow migratory caribou that move between wintering grounds and calving grounds.
Thus, the ‘delay’ in wolf denning would be explained with the ending of the main migration wave as the caribou become more restricted in their movements, as well (due to calf birth).
This likely correlates with latitude through climate (vegetation growing season) and would not shift the wolf denning considerably from the latitudinal gradient but it is an example how wolves can adapt also to their prey species behaviour (birthing times).
It is possible that in multi-prey systems wolves might have adapted to their selected prey species birthing season even if the species is not migratory (on occasion the young constitute the only or the predominating resource in wolf diet during denning and alternative resources are scarce; or if there are changes in one prey species behaviour during birthing that make its young particularly vulnerable and profitable to prey on).
This could account for localized differences in the timing of the denning.
***
The study ‘Denning phenology and reproductive success of wolves in response to climate signals’ offers some cues regarding denning phenology in NW North America (Mahoney, P.J. et al., 2020).
While not discussed specifically, it mentioned the synchronicity of the timing of denning among packs within a population.
It made me wonder about recently (re)colonizing wolves and newly established populations that might avoid localizing their phenology rather keeping to the phenology patterns of their source populations.
Perhaps wolves that are only (re)colonizing an area perceive themselves as belonging to the source population and also rely on synchronicity because it is not easy to meet partners when the population density is very low and dispersing wolves move into areas where encounters with other wolves are also scarce.
First of all, while exploring the new range, there might be a lag observed between the wolf arrival and their subsequent adaptation to the new conditions.
In order to ensure optimal performance under unknown circumstances (while acclimating), (re)colonizing wolves might use cues of the source population (because, all in all, according to the publication by Mahoney, P.J. et al., 2020, denning success was not based as much on locally specific climate as on annual weather patterns).
On the other hand, in areas with scarce mating opportunities, meeting a partner who is ready to breed as the dispersing individual who encountered them, is crucial and therefore wolves might rather rely on breeding cues from their source populations in order to maintain synchronicity.
It is possible that for a highly vagile species, wolves generally cannot overlocalize their adaptations to local climate cues because this would not benefit the recruitment of immigrants from genetically differentiated populations into their own population (it is better to also maintain the synchronicity between more distant populations in order to facilitate genetic exchange).
Notes by Ieva 