This will be a short post referencing the study by Zedrosser, A. et al., 2007 where dispersal was analyzed in a Scandinavian brown bear population (in two study areas).
While the objective of the study was not related to inbreeding, the phenomenon was briefly discussed due to the possibility that males disperse in order to avoid inbreeding (due to the matriarchal clustering and natal philopatry of females).
The study mentioned that inbreeding rates were altogether rather low in the population (2% of all litters resulted from incest between a father and a daughter which might occur when the relatedness of the females in the area becomes very high and the male has perhaps lived there for more than a decade).
The study also refers to the only male which did not disperse and the resulting incestuous inbreeding,
‘The only nondispersing male reproduced with his mother, and both of these individuals had the same father (i.e. two-generational incest). This is the only known case of mother-son mating in our study among 107 known breeding pairs based on reproductive analyses.’
The apparent ‘heritability’ and repeatability of incest in this case as well as the extraordinary non-dispersing manner of the male made me wonder about biological, intrinsic reasons for incest outside of such situations where the founder population effect or reduced dispersal/immigration are determinants of availability of potential mates.
Still, it seems that inbreeding avoidance is a strong genetic trait which results even in behaviour that is costly and involves high mortality risk to the individual that attempts to avoid mating with close kin (e.g., dispersal is a high risk life event).
The study areas in the Scandinavian research did not have significant shortage of males.
There was reduced male immigration and fewer adult males in the north compared to the south but the overall inbreeding rate was low and therefore it appears that everyone could find a mate.
Also, it is worth noting that the presence of adult males (number of adult males in the area) did not affect dispersal in younger males (regardles of their size which is an important dominance factor) and all young males but one dispersed in both study areas.
I find it curious that this young male which did not disperse also bred with his mother.
To me, it suggests the male suffered a condition which perhaps at the same time prevented dispersal and prevented close kin recognition (in order to avoid inbreeding).
Unlike in some mammal species (including brown bear females), brown bear males are likelier to disperse if they are larger and more fit.
‘Several studies evaluating the effect of size and condition on dispersal in mammals have found that larger individuals and those in better condition were more likely to disperse.’ (Zedrosser, A. et al., 2007)
The lower state of fitness could have been the result of inbreeding itself (the male had been sired by his grandfather).
However, I find it unlikely that such substantial impairments would occur in the first generation of inbred offspring.
His mother apparently demonstrated a lack of inbreeding avoidance, as well, because she bred with her own father.
Perhaps there are unfortunate gene mutations that disable the inherent mechanisms in individuals to avoid inbreeding and perhaps these mutations can be passed on (from mother to son or even from father/mother to daughter to grandson).
It is not really known how inbreeding avoidance is inherited (whether it expresses itself as certain behaviours, e.g., propensity to dispersal, or emotional states, or physical states).
These issues would be impossible to tease apart presently but I began wondering if there could be other factors influencing inbreeding avoidance (or lack thereof) in some individuals.
These factors could be related to olfactory impairment.
Studies in rodent demonstrate the significance of olfactory cues in kin recognition (Mateo, J.M. et al., 2003).
One of the mechanisms of inbreeding avoidance could be related to differential sensory reactions to olfactory signals by kin or non-kin (kin signals not stimulating reproductive motivation).
Bears are not rodents but there are similarities between these taxons.
For example, adult female rodents, as well, retain their daughters close to their own home sites (just like female brown bears do) in order to ‘insulate’ themselves against agonistic non-related females or perhaps to ensure sharing resources with individuals who would pass on their own genes (resident fitness hypothesis; Zedrosser, A. et al., 2007).
Bears rely primarily on olfactory cues in their foraging.
Olfactory cues are also used to advertise one another’s presence in the area (e.g., Clapham, M. et al., 2014 and Clapham, M. et al., 2023).
Therefore it is not improbable that olfactory signals would also assist in kin recognition and the related inbreeding avoidance.
If the sensory organs or the respective neural pathways have been impaired, this could result in both lower fitness (preventing the male from dispersing) and reduced ability to avoid inbreeding.
The male’s mother who bred with her father might not have been a small bear (size being determinant of the physical state of later offspring) because daughters who remain on their natal range (close to mother and accordingly father), tend to be larger (although this tendency decreases with increasing mother’s age), Zedrosser, A. et al., 2007.
The male’s father (grandfather) also could have been not of the smallest bears if he ensured breeding position (while he inbred, he still bred which is somewhat of a success).
Thereby, it is possible that the young male’s size was not the limiting factor to dispersal (and also, in this particular study population males of different sizes dispersed).
He could have been otherwise impaired and, as explained above, this impairment could have led to both lack of dispersal and lack of inbreeding avoidance.
This trait could have been inherited and I would suppose it was passed down from grandfather because the grandmother was the only individual in this sequence who did not inbreed.
It could have been difficult for the male’s grandfather to avoid inbreeding with his own daughter due to proximity (if the daughter remained philopatric, she would have lived ca. 40 km away from her own father).
Fathers do not participate in caring for the cubs which further complicates recognition and avoidance because there is no memory in the female cub of her father and she must avoid breeding with him through other mechanisms.
Later the mother’s cub would have been born with the same impairment (perhaps augmented by the incestuous origin), unable to disperse.
After a period of estrangement (in the study population, mothers usually cared for cubs until one year of age but brown bears do not reach reproductive maturity and full size until several years later which would have resulted in inability by the mother to recognize her son from such morphological attributes as size etc.).
Thus, it was likelier for them meet one another after ca. 4 years and to fail recognizing their relatedness.
I find this assumption curious and worth studying because such failure to avoid inbreeding could be present in other species and in populations where inbreeding risk is very high, the impairments resulting from incest might be augmented resulting in even higher inbreeding rates.
Notes by Ieva 