Last week I was both reconstructing the history of Snake River Pack (NE Oregon) and reading a few publications about wolf pup development (van Ballenberghe, V. & Mech, L.D., 1975; Roffler, G.H. et al., 2023).
Snake River Pack had a rather constant pup production rate of 3 surviving pups per year (perhaps 4 because there seemed to be a consistent discrepancy between the subordinates in the group in the following year and the number of suppositioned yearlings although it was, of course, possible that two-year-olds stayed for another year; namely, it appeared that the pack had a tendency to retain yearlings but not 2-year-olds, however, there was that one extra wolf who could have been a pup of the previous year unaccounted for in the end of year count).
As Snake River Pack lived in a rather remote and wild landscape (Hells Canyon bordered by rugged terrain as well as the wide and mighty Snake Rive), I began wondering if 3 or 4 surviving pups suggest a good productivity in a wolf pack or not and whether there is a difference between pup production (survival) when wolf packs are compared that live in entirely wild vs. anthropogenically altered areas.
For example, in Voyageurs National Park and the Greater Voyageurs Ecosystem, quite frequently, zero pups survive and pup survival rate of 7 is considered extraordinary (see this video dedicated to Thuja Pack (7 surviving pups in fall) by Voyageurs Wolf Project).
Voyageurs National Park is a wild ecosystem but so is Yellowstone National Park where after wolf reintroduction 5 pups often survived until December and several packs had two litters.
The difference between these relatively wild areas (NE Oregon original recolonizing packs also having settled in comparatively wild habitats) is perhaps the prey availability.
Yellowstone wolves became reintroduced into an area of elk overabundance while Voyageurs National Park wolves are often struggling to get by, especially, during late summer/early autumn which appears to be a crucial period for pup survival.
Also, in Voyageurs, the historical deer densities are declining.
Similar picture was observed in Isle Royale where during higher moose abundance periods wolves produced litters but after the moose population crashed, for example, in 1996 (lack of food, severe winter weather, heavy tick loads; see Annual Report of 1995-96, 1996-97 and 1997-98), pup survival and pack productivity also declined (other examples are available, e.g., during the years of low senior moose availability which is the most vulnerable and exploited prey group on Isle Royale).
Accordingly, it seems (and it is only logical) that pup survival is affected greatly by prey availability and this is no news.
However, I would like to focus this discussion toward the differences between areas of low vs. high prey abundance and the reasons behind this abundance.
For example, there are not that many wild ecosystems left but it appears that wild ecosystems with historically intact predator-prey communities and with a high mature forest cover would perhaps have lower prey availability because prey species have been regulated by predators (unlike in Yellowstone) and because prey species are not exposed to overabundant food resources.
Meanwhile, most of our habitats have been altered greatly by humans and these modifications are frequently beneficial for ungulates resulting in ungulate densities higher than would be observed in purely natural habitats.
Interestingly, these alterations (while perhaps more pronounced now than ever) are not recent and throughout the history of Holocene (after the last ice age), humans have impacted landscapes in manners that probably support ungulates but that push large predators into the more marginal forested and highland habitats.
I wonder if this has affected reproductive evolution in grey wolves.
My reason for wondering is that wolves are largely a K-selected species – their investment in the survival of their pups is very high and they rather invest in quality vs. quantity of offspring.
Wolf pups are not subject to significant predation, either (many species that produce offspring which is preyed upon by many predators might choose r-strategy).
Due to the varied distribution of wolves along the climate gradient and important adaptation such as denning belowground or in other thermoregulated cavities, it does not seem that they would have had to adjust to high pup mortality probabilities due to weather.
Meanwhile, their litter size at parturition is often greater than the pups surviving by the end of the winter.
As wolves are so cooperative and demonstrate great affection and concern for their pack members, I find it strange that this species would not have been selected for smaller litter size at birth.
A smaller litter size would be beneficial for the pups and evolutionary advantageous because, as mentioned before and as discussed in the study by van Ballenberghe, V. & Mech, L.D., 1975, attaining a great enough body weight until September (in Minnesotan wolf populations) determines whether the pup would survive until winter and beyond the winter, or if it will die.
In fact, fall mortality is so evident that it can give false results if one studies, for example, pup weights in wild wolf packs.
If the studies are conducted after the major pup ‘die-off’ in early fall, the results suggest that all pups are near or above proper weight because the underweight pups have all succumbed by that time.
Wolf parents (and older siblings) are highly dedicated at providing for their offspring and having a smaller litter at birth would give a better chance to all the pups that are being taken care of thoroughly.
Wolf parents would, of course, keep providing for as many pups as there are born (and even for adopted pups as observed in the cases of captive-born endangered wolf species/subspecies pup introduction into wild wolf dens) and this is even beneficial for them because large litter size can ensure survival and protection during winter.
Nevertheless, I am not certain that it would make as much sense for wolves to invest in attempting to raise as many pups as possible (for the purpose of having a larger pack) over raising as many pups as they can conceivably provide for.
Larger packs are not necessarily better at getting resources (food availability per wolf can decrease significantly in larger packs) although they are better at protecting the resources.
However, large packs that consist of overworked adults that have spent the entire summer caring for pups many of whom die before they can return the favour (this sounds very pragmatic and it is my formulation and not how wolf parents perceive it), does not make much sense, from the perspective of evolution.
Then why is the litter size at parturition (2 – 10, most often 4 – 7) sort of bigger than actual pup survival until the end of the year in wilder ecosystems (ecosystems resembling the ‘natural state’)?
I wonder if this discrepancy and the lack of evolutionary adaptation to produce slightly smaller litters (or, more precisely, litters that befit the survival patterns in truly wild ecosystems) is the result of the landscape that wolves have had to live in since after the last ice age.
It has been a landscape predominated by semiwild and agricultural habitats that favour ungulate abundance.
Accordingly, many wolves have had to live habitats where, on one hand, there is high prey abundance along the marginal areas between anthropogenic and wild areas.
Meanwhile, in order to protect themselves, wolves have had to mainly stay reclusive from the most prey-abundant habitats.
I believe that during winter wolves probably roamed more widely (as they tend to do) and fed on the prey closer to human settlements where prey abundance is often too high.
Meanwhile, during summer, they had to restrict themselves to deeper forests or rugged terrains where humans could not access their dens and pups.
This would have caused a discrepancy between prey availability during winter and summer.
As a result, the female wolf might not have been informed by her body that she should limit the litter size because during winter she would have had access to more abundant prey resources and by February (the typical mating period), she would have more or less recovered after the strain of the summer provisioning.
Meanwhile, during summer, prey availability would have decreased as the wolves retreated to den further from anthropogenic impacts and the respective prey abundance areas.
Historically, this period might have been proven even scarcer because massive logging probably did not occur and the forest interior was more resemblant of Voyageurs National Park than of modern-day intensely managed forestry landscapes.
As a result, the litter size and the pup mortality rate would not have been fully compatible.
The strain of having to provide for the pups under low resource circumstances combined with the need of the breeding to sustain themselves and to share with subordinates, could have exhausted the breeding pair renewing the tendency for them to frequent overabundant prey areas when the pups became more mobile.
Also, this could have encouraged dispersal by subordinates leaving the pack vulnerably small during winter and leading to ‘keeping the litter as large as possible’ scenario.
This could have been one of the reasons why large litter size wolf packs were not selected against naturally – those that managed to raise as many pups as possible would have succeeded at defending their breeding territory.
Additionally, as the wolf pack resumed roaming more widely, they would have been subjected to wintertime hunting for wolf pelts.
As a result, the pack count would have dropped, and, once again, it would have been vital to have as many surviving pups as possible.
These conditions resemble some of the life aspects of r-selected species.
In our day, we often hail wolf reintroduction as a method of reducing the overabundant prey species.
More rarely, we consider how this overabundance (and lack of management adaptations to reduce ungulate populations to more natural levels through transforming our forestry and agriculture methods) would affect the wolves we reintroduce to save what we have damaged.
Firstly, there is a great possibility that the initial overabundance in wilder ecosystems with lost predator-prey associations would first lead to large packs with big litters and then to extremely high intraspecific strife and increased wolf mortality as the prey populations return to their sustainable levels.
Should we not consider the history we are setting for these wolf packs and the dramatic fates they will be subjected to as they heal the ecosystems from abnormal to normal conditions?
Secondly, most wolves outside of nature preserves and on the edges of nature preserves are subjected to conditions where during winter they can hunt also in relatively anthropogenic landscapes but during summer they must observe a reclusive lifestyle.
If we create conditions where there is a huge contrast between these two habitats, we are subjecting wolves to high pup mortality rates or to inadequate ability to retain the necessary number of subordinates during the summer/fall period and to teach them important life skills that they will need after they disperse and establish a family of their own.
Thirdly, if most wolf packs live in areas where large litter size is not selected against due to prey overabundance (in winter and/or summer), as a species, this highly vagile animal would not adapt to the ‘true wilderness’ conditions and in our nature preserves (such as national parks) as well as under sustainable forest management conditions wolves might not be able to provide for their large litters.
We are creating evolutionary conditions that imply high stress for the breeding adults, high pup mortality in natural areas and species adaptations to abnormal habitats.
We are forcing a K-selected species with its respective psychology, physiology and intense investment in each offspring, to lead a partly r-selected lifestyle.
I do not believe that the anthropogenic influence in ungulate abundance and distribution has caused a larger litter size in wolves.
Many canids have similar-sized litters and perhaps so did the ancestors of wolves.
However, large predators (K-strategists) tend to have smaller litter sizes or even give birth every other/third year (bears).
I believe that other traits in wolves (such as physical/psychological devotion to family and investment in all pups) are suggestive of a species that would rather profit from a slightly smaller litter.
Perhaps we have created environmental conditions that have interfered with certain aspects of the grey wolf evolution that would have otherwise taken the direction of a slightly reduced litter size (at parturition already).
Notes by Ieva 