The publication by Mayer, M. et al., 2017 which is also included in the PhD Thesis by Mayer, M., 2017 (that offers several highly interesting papers), ‘The 7-year itch: non-adaptive mate change in the Eurasian beaver’ discusses reproductive partner change in a long-lived and largely monogamous species.
Out of 62 observed partnerships, 25 resulted in a mate change.
The lost or displaced mate was usually replaced by a younger partner (a disperser).
A frequent cause of mate change was mortality (in the particular study area it was human-caused mortality accounting for 20% of mate changes).
Monogamous species (Eurasian beavers being both socially and biologically monogamous) would ordinarily replace a partner lost to mortality.
Another potential cause for mate change in many species is reproductive failure.
While there is reproductive senescence observed in beavers, the mate change did not appear to improve the reproductive success and therefore, it was unlikely to have been caused by a determination to improve mating chances.
In fact, the reproductive rates were even slightly lower after the mate change.
Also, mate change appeared to occur more frequently at lower population densities when dispersal rates tend to be higher.
This indicates at the displacement of the same-sex breeder by the dispersing individual (i.e., the resident beaver does not appear to be actively seeking out a new partner).
The authors attribute it to the ‘forced-divorce’ hypothesis where the divorce is not a result of deliberate decision of one or both of the former mates but is rather ‘forced upon them’ through the arrival of a new individual who seeks to establish themselves as a territory holder and a breeder.
On one hand, the observed results appear to have been explained through the ‘forced-diverse’ hypothesis.
On the other hand, there are many questions that remain unanswered.
For example, in most cases the fate of the displaced individual was unknown and only in 2 cases death was confirmed (although it was not stated conclusively whether death occurred as a result of breeding rights disagreement or as a result of displacement from the former territory and the consequent threats due to loss of foraging grounds, shelter etc.).
Dispersing beavers are normally younger than dominant residence holders (especially, at lower population densities where younger beavers disperse compared to higher population densities where subordinates tend to ‘wait out’ until even 3 – 4 years of age).
In many species, younger individuals might be considered fitter than older individuals.
However, beavers do not acquire their full adult size by the age of 4.
This means that dispersers should be, most frequently, smaller than older residents.
Also, subordinates do not engage in as much of territory patrolling as dominant individuals (resident holders, older beavers) (Mayer, M., 2017).
Dispersers (except the rare cases where former resident holders have dispersed from their previous territories) have been subordinates in their natal ranges and they might be at disadvantage respective to experience during conflicts.
Two adult resident beavers (even if older and somewhat senescent) should be able to overpower a single, young, experienced, smaller beaver if not in all cases then in many cases.
Personally, I believe that this suggests – mate change is at least partly voluntary in at least one of the former mates and if the mate does not actively encourage it, they might refrain from preventing it.
This leads to the following question – why then would the resident(s) refrain from exerting full assertiveness toward the younger, smaller, inexperienced claimant?
I would like to propose a theory which is related to territory quality.
Another reason why a pair would break its bond, e.g., in monogamous birds is related to poor nesting success and sometimes – where it is more difficult to find a new entirely vacant nest site but it is less difficult to find a partner with an available nest site, the pair might split up with one of the mates dispersing to seek out a new partner (and nest).
While the reproductive success was decreasing with age, the new partnership in beavers did not result in an apparent reproductive improvement.
However, I was wondering if the reason for mate change was not one of attempting to improve the immediate reproductive success but one of attempting to remediate a deteriorating habitat.
Senescence is not the only consequence of long-held partnerships in monogamous mammals.
If these mammals are territorial, long-time mating bonds would also lead to long-time tenures of the same territory.
Beavers are known to be highly selective, for example, in their foraging efforts.
They have their preferred tree species, foraging distances from the shore (on average up to 15 m), tree diameter that is selected for at different distances from the shore (e.g., average 2.5 cm in North American beavers at Isle Royale National Park, US; with smaller maximum diameter further from the shore and larger minimum diameter felled further to the shore, Belovsky, G.E., 1984).
These preferences are caused by foraging considerations but alongside the dam construction efforts as well as food caching efforts (it has not been estimated to what extent beavers can prognosticate the winter needs for the specific yearly season but they probably have adapted at least to the regional climate, i.e., the overall length of the cold period), this points at a thorough understanding in the beavers of their habitat and its resources.
Beavers also can move upstream or downstream when food resources become depleted or due to other reasons and this adaptability is likely dependent on the territory size.
However, long-term bonding and long-term tenancy by the same dominant individuals as well as senescence might lead to established routines that become difficult to alter resulting in a deterioration of the territory.
When beavers have lived together for a long time, they might have established routines including ’emergency routines’ (i.e., adaptations to very localized resource depletion) and if their cognitive faculties are not ‘as sharp as they used to be’, these routines in the established pair might be proven hard to change.
Over time, this could lead to decrease in habitat quality.
The reduced reproductive success due to senescence could be also partly explained by poorer resources due to depletion and lack of regeneration.
A new mate might offer a chance to establish entirely new routines (a novel approach to the use of the habitat) improving the resource regeneration and use in the territory.
If the territory deterioration is observed also in the quality of its residents, this could explain the inability of the dominant residents to fend off a young disperser.
However, personally, I believe that it is not merely due to physical incapacity the mate change occurs and I think that it is also deliberate.
I think that the mate change is accepted in order to recover the deteriorating range.
While the immediate reproductive success might not be thereby improved, it is also difficult to say whether even poorer reproductive success has not been avoided due to collapse of the territory resources.
It can be easily seen why it benefits the non-displaced mate but I will later in the post attempt to consider why it might benefit the displaced mate, as well, regarding the continuation of genes.
However, I recognize that my theory will not be accepted readily because I believe it benefits the potential continuation of genes in a specific territory (and not the continuation of genes per se) and while I think that beavers value their home and form an attachment to it that might surpass even the pair bond, such ideas are not accepted in wildlife biology where the importance of territory inheritance or home range loyalty have been recognized but not as forced overriding other aspects of fitness (e.g., lifetime reproductive success).
Returning to the ‘territory quality salvaging hypothesis’, the 7 year period that has been mentioned in the study by Mayer, M. et al., 2017 might be indicative of the reasons behind the mate change.
On one hand, 7 years could be the average time for some beaver cognitive processes such as routine establishment or senescence of some mental faculties (flexibility).
On the other hand, 7 years might be indicative of vegetation regeneration processes.
For example, if beavers prefer trees of certain diameter (because the costs of energy felling them are compensated for by the resources and energy acquired consuming the biomass obtained from the trees of such diameter), there could be thresholds of regeneration that need to be reached in order to satisfy the beavers’ dietary requirements.
If trees are not regenerating to reach that size, the beavers might also notice it (even if unawarely) and this apprehension of greater effort and smaller gain could result in stress that affects reproductive success and general fitness.
7 year period (on average because this probably depends on the region, tree species, habitat conditions etc.) could be suggestive of some changes in vegetation dynamics that lead to lack of suitability of the habitat for the beavers, especially, on some ranges.
The habitat might even still be optimal but the decrease in quality might result from the established routines by the residents.
During my private observations, I have noticed that beavers have their preferred foraging spots and times (that are, however, seasonally variable) attesting to at least some routine in space and time.
They also probably have developed stricter routines during the energy-demanding kit-rearing period when ‘every move counts’ and high efficiency is mandatory.
Routines can be crucial in survival and reproductive success and one of the reasons behind long-term pair bonding involves established routines (cooperation and traditions).
Such routines, meanwhile, in time could result in depletion of certain patches and perhaps an inability to switch over to new routines involving other patches.
A new mate might alter these routines while retaining the former resident holder’s (the non-displaced mate’s) traditions and knowledge of the local resources.
There is another factor that partly contributes to this theory.
A greater number of older subordinates (i.e., less dependent subordinates that are experienced enough and might take initiative) could offer alternative means of introducing change into the established routines.
Beavers disperse at a young age when the population densities are lower which coincides with the mate change frequency (a lower number of subordinates in the colony).
Also, at higher densities a greater number of subordinates reach an older age until they attempt dispersal (retaining older, more capable individuals in the natal colony for longer).
Older subordinates might also act aggressively toward a potential usurper of one parent’s breeding position.
I believe I have read somewhere that mate change is less likely with more subordinates in the colony but I cannot find that reference at the moment.
I will attempt to include it if I recall its source.
The number and age of subordinates does not substantiate my claim directly because, as I have mentioned before, it might be more difficult for a new breeder to establish in a larger family group in which subordinates likely regard the newcomer as competition.
Also, as stated before, mate change occurs more often at decreasing densities but decreasing densities also promote dispersal at a young age which might lead to fewer older subordinates in the family.
It is even possible that the senescent dominants are no longer capable of achieving specific tasks without the assistance of younger beavers (but older than kits).
The acceptance of a new mate might also be related to the necessity in physical youth in order to perform, e.g., building and maintenance activities.
This does not contradict my theory because recover of a deteriorating habitat might necessitate major ‘make-overs’ (construction of new dams rather than maintenance of the old dams).
Without older subordinates, the parents and the kits might not be capable of such refurbishments of their home.
If my theory is at all true, it is important to understand what drives the acceptance of a new mate.
It could be the gradual decrease in fitness (both regarding the health and reproduction of the former dominants that they might predict to further decrease with deteriorating habitat that they are lo longer capable of sustaining together).
In this case, it would be a simple outcompeting scenario.
However, I have developed a subjective notion that home is very important to beavers.
Generally, we do not speak about values too often in wildlife biology.
While we refer to altruism or some social help systems in animal societies, we also attempt to explain it all with the fundamental objective – to survive and to reproduce or to ensure one’s gene transference into the future populations (even if through siblings or more distant kin, not one’s own offspring).
There is not much discussion over other values.
For example, while many species demonstrate high loyalty to their natal range or to their traditional breeding range, we do not consider it to have been caused by their system of values (loyalty to home) but we consider it to have been caused by developing a specific strategy where it is beneficial to stay at home or to return to the same home year after year.
Both ideas should not necessarily be exclusive but we tend to detach moral values from practical gains.
Our societies have developed to regard morality as ‘impractical’ because what is moral, should not be based on material benefits and certain types of success.
Our morality is frequently a morality of self-denial and abstinence, and humility.
It is difficult for us to accept that moral decisions might lead to practical benefits, or that the two are somehow connected at all.
I think that beavers have developed a certain value system in which home is one of their top priorities.
I even happen to believe that a beaver who has been a dominant territory holder, would be capable of giving up its security and its status to ensure the long-term prosperity of a home it has loved and managed for many years (on average, 6.25, which is the duration of first-partner mate bond in the publication by Mayer, M. et al., 2017 but which differ in other study areas or study periods).
However, I do not have proof yet to substantiate such claims.
It is easier to see how the stay-behind partner would benefit from a younger mate.
They would continue reproducing (not at increased rates but at approximately equal rates) while potentially improving the house-keeping and perhaps delaying their own physical deterioration with the regenerating resources.
As humans, we are probably not prepared to a sacrifice of both our mate and our home in order to secure their (and not our own) future, especially, because ‘unknown fate’ (the most common result of displacement) can entitle mortality.
The Telemark beaver population is studied thoroughly and only 1 replaced female re-paired in another territory.
If other replaced individuals dispersed outside of the study area, they might have re-paired, as well, unbeknownst to the scientists.
However, mortality is very likely because beavers are highly territorial, the riparian corridor is probably settled without gaps and surviving without a territory is almost impossible.
From the perspective of current paradigms in wildlife biology, an individual would be foolish to make such a sacrifice because it lowers the chances at future reproduction as well as at survival itself.
It does not appear to benefit gene transmission ‘into future’, either.
I am, nevertheless, not as assured regarding this last factor.
I have a suspicion that this action is aimed at maintaining the territory at ‘tip-top’ shape until it can be inherited by the original pair’s progeny (grandchildren).
For example, if the senescent pair kept living on their range, they might both become old enough to be displaced by non-related dispersers.
Or they might both die, or one of them might die leaving the survivor too old to be an acceptable mating choice or a viable defender of the range.
If they had older offspring still around, these children could inherit the territory at once (as sometimes happens, indeed).
But if they did not have older offspring, the young kits (their own kin) would be forced out by dispersing subadults/adults.
This would, once again, result in loss of their home to ‘complete strangers’.
Through accepting a younger mate, the territory would be defended more efficiently through the physical strength of the new mate and the experience of the older individual.
The likelihood of losing the home to strangers would be reduced although the replaced mate’s genes would no longer be in the picture.
Furthermore, I wonder what the probability is of the home range to eventually become ‘reowned’ by dispersers that are related to both the original male and original female.
Currently, I do not know enough about beaver life histories to make assumptions but at some point the old territory holder (who was not replaced and displaced) would die, as well.
And what if the genetic turnover would have been substantial enough to allow for immigration into this territory by a disperser who carries both the genes of the original male and female but who is not close kin and can thereby mate with the offspring of the widowed mate and the non-replaced mate (or with the widow if a new re-pairing occurs)?
It is not as important with whom the ‘carrier of genes’ mates but inbreeding should be avoided (and is probably actively avoided by dispersers who can recognize kin scent; Sun, L. & Müller-Schwarze, D., 1997; and who would thereby opt out of breeding chances in territories with close kin)
For example, the average duration of the first pair bond was 6.25 years.
The average duration of the second pair bond was 3.08 years.
If the original pair had produced offspring in the first year of their pair-bond, then at the time of the ending of the second pair bond, their oldest offspring would be around 9 years old.
If the first mating occurs at 2.5 – 3 years of age, the oldest offspring of their oldest offspring (conceived with an unrelated mate) would be ca. 6.5 years old.
Such offspring would be a cousin to the new kits of the original pair.
Their oldest offspring (conceived, once more, with an entirely unrelated individual) would be ca. 4 years old and this offspring would be cousin once removed if I am not mistaken (because these kinship networks confuse me).
Their oldest offspring would be at the age of dispersal when the second pair bond ends and they would be cousin twice removed to kits if those kits would have been born to the original pair but at some point during these generational processes, the original pair has also broken its ties and a new, unrelated individual has replaced one of the breeders.
Thus, the kits living on the original home range and the kits born at the time of the potential dissipation of the second pair bond are not even cousins twice removed.
I do not know if this suffices to avoid inbreeding but it would be curious to know what the probabilities (and quantifiable rates through genetic confirmation) are of the original pair’s genes remaining in their territory after their death (through direct inheritance or through potential of distantly related gene-carriers immigrating into this territory and taking over).
What if the beavers opt for an even more complicated objective – not merely to increase the chances of their genes surviving but also of their genes inheriting their old home?
Notes by Ieva 