Many species adjust their timing of the birth of the young.
This can be achieved, for example, through delayed implantation or reduced breeding period (monoestrous species).
The goal appears to be to produce the young during periods when food is more abundant to satisfy the needs of the pregnant/lactating female as well as the developing young during and after the weaning period.
However, I am somewhat puzzled over the timing of the birth in wolves that are a monoestrous species and exhibits a strategy in synchronizing mating under social mechanisms (e.g., in Ethiopian wolf populations).
The mating in grey wolves occurs mainly in January – March and the pups are born in April – June.
Still, in most regions, to my knowledge, mating occurs in February and pups are born in April.
But is April the best time to bring pups into the world?
The late winter die-offs have likely subsided (although they might have contributed to mother’s fitness which is important and it would be important to confirm whether wolves are capital breeders and to what extent mother’s fitness prior to birth impacts litter size, litter weight and other such parameters).
It appears that female body size significantly determines litter size and litter survival (Stahler, D.R. et al., 2016) although it has not been specified whether body mass in the particular breeding season is of as great importance as the female’s overall body size (a given trait that is constant irrespective of mass gains or losses).
It is of importance in many species and might also be of importance in wolves.
It has been mentioned in some publications (e.g., an early publication by Mech, L.D., 1995) that females might not be able to conceive if the food resources were too scarce and their could not accumulate the necessary body mass and/or reach the minimum optimal fitness level to ovulate.
Therefore, it can be assumed that the timing of breeding depends on the prospective mother’s body condition and it is planned not in accordance with parturition dates but in accordance with the temporal resource distribution allowing the female to ovulate.
The timing of birth might, accordingly, result from processes occurring during winter rather than later in spring and summer.
If body mass also impacts litter success, it would make more sense for wolves to breed in February because during pregnancy, the wolf mother would be able to feed on late winter carrion and kills of the winter-weakened ungulates.
It is even
However, as indicated in some publications, e.g., by the Voyageurs Wolf Project (Freund, D.R. et al., 2023) April is a month that can create a gap between late winter kills/scavenging and the birth of the ungulates (fawns becoming available in May, June).
Wolves get by – fishing, catching beavers etc.
Still, during lactation the female’s energy needs must be high.
In some mammalian species (e.g., bats) lactation is considered more energy-demanding than pregnancy itself although this likely depends on the reproductive strategy (capital vs. income breeder) and on the offspring ability to supplement nursing with solid or partly solid food (which in wolf pups is attained at about three weeks of age when partly solid food is brought to them by adults and regurgitated).
Then the fawning season ensues providing a greater abundance of resources.
However, the end of the summer (July, August) when the wolf pups are still developing and before they are able to travel with the pack thus restricting the pack movements and not participating in the cooperative hunting yet, is, once again, a period of famine when, e.g., in Voyageurs Ecosystem wolves even gorge on blueberries to have their fill (Gable, T.D. et al., 2017 and Homkes, A.T. et al., 2020).
The end of the summer can become a time of significant mortality in wolf pup demographic group.
On the other hand, it is also possible that the timing is good enough because otherwise the pups would experience their first winter underdeveloped and incapable of facing the cold as well as holding back the entire pack whose metabolic needs have to be met during the harsh season.
The wolves might have chosen a trade-off where the early pup development (first month) is highly energy demanding but perhaps less so than the time when pups begin consuming solid food, as well, in addition to mother’s milk and where the pups develop rapidly acquiring biomass at great rates.
Thus, the first month may have been ‘risked’ at lower prey abundance in order to compensate for the intense needs of the following development as well as to ensure that the pups reach biomass and mobility that is necessary to begin moving with the pack during late autumn and winter.
Additionally, if the fawning/calving time coincided precisely with the pup birthing period, wolves might ‘run out of’ vulnerable prey.
Currently, July and August already can be a treacherous time in many wolf populations when the fawns and calves have surpassed high vulnerability moving alongside their mothers (and sometimes herds), capable of escaping predators.
If the ungulate births occurred even earlier, the gap between vulnerable prey and winter-weakened prey might be too long and coincide with a time of rapid and crucial pup development.
However, this trade-off has a high cost of frequently losing at least some of the pups early on during the denning period.
I believe that the mismatch between the timing of pup birth and ungulate birth has occurred later in wolf species evolution than the adaptive development of gestation period because otherwise wolves might have evolved to gestate longer (giving birth to perhaps more developed, a bit less altricial offspring) and to reduce litter size.
However, I have been wondering whether the current reproductive ‘planning’ in grey wolves could not have become slightly skewed due to ancient inheritance from climatic conditions where winters might have been longer and some other factors (e.g., more diverse prey species with different birthing times) used to be at play.
The adaptations by species are usually highly effective but they also do not evolve over a day, not even over a decade, nor a hundred of years.
It would be interesting to consider cases where species adaptations (such as reproductive timing in wolves) might have turned into a slight mismatch due to changes in climate, prey species composition, habitat attributes etc. and where the ‘perfect timing’ is not truly observed anymore and it has deviated from the ‘ideal’ by a month or two due to the passage of millennia and millions of years since the time the adaptation was first developed in the specific environment with the particular abiotic and biotic interactions.
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Lately I have begun wondering if the prey size effect might be a significant factor in the adaptations to denning period.
While pups are nursed by the mother for the first three weeks and after the three weeks they receive regurgitated meat, adults also begin bringing pieces of solid food to the den.
I have seen videos of as early as the end of April with wolves carrying parts of prey (e.g., legs of larger ungulates, whole fawns, hares, beavers or parts of the smaller prey) back to the den (later – to the rendevouz site).
Pups cannot travel to kill sites until they reach a certain stage of mobility but it is likely that this stage is only reached after pups begin consuming non-regurgitated food and developing skills of handling prey.
Also, prey is killed in different locations and some might be closer to the den/rendevouz site and some might be further (within the reaching distance of pups or outside their current mobility scope).
As a result, it might be adaptive for wolves to prey on smaller-sized ungulates (as well as other smaller prey) during the denning/rendevouz site period because parts of these ungulates (prey) can be carried more easily in teeth back to the pups.
It might be crucial to get the timing right during the period when pups are relatively immobile but they also need to start eating solid, non-regurgitated food.
During this period smaller prey is more readily transported to the site where pups can consume it thus learning the handling techniques of prey but also developing the necessary gut microflora as well as teeth.
If during this period only very large prey was available (difficult or impossible to be transported even when separated into pieces), pups could only become exposed to actual meat and bones on occasion the prey is killed near the pup location or the pups have reached a more mobile stage in development where they can travel to the kill site.
It might not be sufficient for pup development to be exposed only to minimum amount of actual meat and bones.
The adverse effects would probably be more pronounced on teething.
Additionally, I wonder if there could be some aspects to the nutritional contents (or otherwise chemical contents) of prey that are considered by wolves as they choose food for their pups.
Perhaps it is easier for pups to digest the tender meat of calves/fawns (or alternatively, meat of beavers) than it is to digest the meat of adult ungulates.
Maybe these differences among nutritional availability by prey items are significant even under condition that the meat is first regurgitated (pre-digested).
As the pups grow up, their digestive capacity might become better adapted to consuming a range of prey.
Therefore, it might be important that pups receive fawn/calf meat during the stage in their development that follows nursing (when milk is the food item) and precedes feeding on kills by adults (including kills of mature ungulates).
If this were true, the timing of wolf birth relative to fawn/calf birth might be better adjusted because fawns/calves would be mostly available during the post-weaning period when meat is consumed more or less directly (not in the form of milk).
It might be interesting to study nutritional availability of ‘young/old’ ungulate meat for pups in a digestive context (including in its pre-digested, regurgitated form).
There might also be differences in pup development between home ranges where there is a larger availability of young ungulates vs. home ranges where pups have to consume mature ungulates in a greater proportion since a younger age (due to relative scarcity of fawns/calves).
Similarly, the nutritional availability of beavers might be studied.
I also wonder if the choice of rendevouz sites depends at all on the distribution of typical kill sites and whether the pup-rearing patterns are different between wolves that consume smaller-sized prey (e.g., white-tailed deer) vs. larger-sized prey (e.g., moose).
For wolves who prey on very large ungulates, it might be important to choose rendevouz sites that are likelier to be in proximity to potential kill sites.
On the other hand, it might be more viable for such wolves to simply choose rendevouz sites after having made a kill because large ungulates also last longer.
Thus, homesite choice pattern might be different between wolf families that can transport their prey more easily (even moose calves are larger than, e.g., deer fawns) vs. wolf families that prey on very large prey which cannot be transported by carrying.
Similarly, there might be differences between wolf families that prey on ungulates producing longer-lasting carcasses (which could serve as home-base; but they might also attract potential enemies for pups and pup protection might become an issue necessitating the allocating of babysitters while larger prey also demands a greater number of wolves to kill it) vs. smaller ungulates that produce more fleeting carcass resources (consumed in one or two sittings).
Notes by Ieva 