As I have been reading my way through the Isle Royale Wolves & Moose Annual Reports, I began wondering why wolves suffered such high inbreeding rates while moose did not.
I encountered some research explaining the genetic mechanisms behind these tendencies in the two species (Kyriazis, C.C. et al., 2023) which I cannot claim I fully understood but it appears that the moose population has a higher likelihood to naturally eliminate the unwanted genetic traits because they have a tendency to become manifested in the individual in a manner that does not benefit survival and therefore the individuals carrying the inbred genes perish easier than those who do not.
This could be explained by the larger population size in moose and a relative lack of social constraint on breeding position and a prolonged breeding status tenancy (moose have a greater variety of mating choices while in wolves the same individuals can produce offspring for many years) and also by the manner in which deleterious mutations manifest in the moose (more efficient purging).
As I was reading the reports, I also began wondering about a certain aspect in wolf populations which is that of dispersal.
On the island, initially, only one large pack existed with several smaller groups around (lone wolves, duos, trios) but later on several (up to four but mostly two or three) packs took dominance over the island.
As immigration essentially stopped, it was up to the smaller groups of wolves to attempt to reproduce because only through gaining in numbers (pups that become subordinates), a new pack could be created to stand against the existing packs.
However, I was wondering – who were these individuals forming groups? Were they related or unrelated?
In mainland wolf populations, a dispersing wolf typically encounters another dispersing wolf and a pair is formed.
Other patterns are observed, as well, such as splitting of packs (but in these cases the split-off would then acquire a new breeder of the opposite sex to the dominant individual in the new group).
Sometimes dispersing wolves lure residential offspring right out of packs or even pair up with the pack’s offspring and receive inherited portions of the parents’ territory (i.e., parents of one of the mates in the new pair).
Of course, breeders in existing packs can also be replaced by immigrants (usually, one of the breeders dies or becomes chased off and becomes replaced by an immigrant of the same sex who then forms a new breeding pair with the existing breeder of the opposite sex).
Under these scenarios, it is rather clear that immigration/emigration is a must.
Certainly, on Isle Royale, emigration from the existing packs or non-reproductive units also occurred but there are two questions I would like to pose:
- Was splitting off existing packs (in duos, trios etc.) more common than dispersal by separate individuals?
- Did these individuals consider their former neighbours equally viable as prospective mates compared to the choices mate under regular, non-island conditions?
Here I should mention that ‘split-offs’ might also be defined in a non-traditional manner, i.e., as dispersed individuals who left the pack separately but later reunited with siblings forming a new group.
It is not truly a split-off but it is not a new pack formation, either, because, under such circumstance, the siblings would have failed to form a new pack with unrelated individuals and temporarily resumed associations with their own kin (while not returning to the original family group).
In these cases, the individual would not perceive the newly established unit as a breeding pack but perhaps as a ‘gang’ in order to cope with hunting, survival etc.
Namely, I wonder if dispersal was regarded in the same way on Isle Royale as it is regarded on mainland by wolves.
I believe that there is a need to pair up with someone who is not an immediate neighbour (known as a neighbour) or at least a member of a neighbouring pack must be met at a great enough distance from both individuals’ territories in order for a pair to become formed.
From the known pairing up cases that I have read, I cannot currently really think of one where two wolves from neighbouring packs actually paired up right after dispersal from their natal pack.
There might be examples but, from the top of my head, I cannot think of one.
Perhaps wolves perceive their neighbours differently than they would perceive potential mates (allies).
Neighbouring packs get to known one another (through investigating scent-marks, through howling) and their relationship is characterized by avoidance and aggression.
If a known neighbouring wolf is encountered by a dispersing individual or by an individual who is considering dispersal and formation of their own family unit, they might recognize this individual as someone to be avoided or fought – not as someone to become prospected for mating.
On an island situation where everyone is everybody’s neighbour, an obstacle might be created whereby most individuals are not regarded as potential mates because they are neighbours and even if they are, these individuals might not be able to find a place where to consort without enticing the aggression by either of the packs (a Romeo and Juliet type of situation).
During the periods when the island is dominated only by two packs or all three packs have common territorial boundaries with one another (one of the packs is smaller and crammed between the other packs the territories of which still meet above and/or below the smaller pack), it might be difficult for dispersing wolves to actually leave the pack and to attempt to settle anywhere because they are the ‘known enemy’.
It might be easier to disperse commonly with siblings (strength in numbers) but if the hostility is not overcome and if the neighbouring dispersing individuals do not attempt to join this new group (which still represents ‘the enemy’), there might be a high risk of inbreeding between related individuals (within the split-off group).
Thus, it might take time for any two unrelated individuals to actually meet and to become ‘forgotten’ as enemy (dissociated from their natal pack through temporal and/or spatial distancing over a longer period).
It is possible that such two individuals only meet when their original ‘buddies’ have died and even then they have a hard time reproducing because the island is dominated by these influential families that claim most of the resources.
Meanwhile, an individual who attempts to approach an existing larger pack, will not be considered a viable candidate for breeding because their social status is very low and they might also have become weakened through prolonged inability to take down moose on their own.
I am not completely certain if, statistically speaking, there is a real tendency to avoid neighbours and thereby to limit breeding prospects to emigration/immigration across a greater distance (at least beyond one pack’s territory).
Dispersal might be extremely dangerous in a population where travel occurs not merely on strangers’ territories but on neighbours’ territories.
Meeting up with a prospective mate might also become perilous if this mate comes from ‘the known enemy pack’.
Thus, the likelihood increases that split-offs have a greater chance to persist but these split-offs are groups of already interrelated individuals who might find it difficult to recruit new breeders.
The situation could be aggravated by the formidable prey and the subsequent weakening of lone individuals who probably find it more difficult to take down moose on their own.
It could be nearly impossible for lone wolves to survive long enough in order to become forgotten as the enemy and to find a spot where to meet and to attempt reproduction.
The island set-up might naturally facilitate inbreeding over ‘regular dispersal’ unless new individuals arrive across ice from Canada (which is less and less probable due to the global warming).
These ‘true immigrants’ might have much better chances at mating because they have not been anybody’s neighbours.
Notes by Ieva 