Many species of animals have evolved behavioral rules or mechanisms that help them avoid pairing, mating, and producing offspring with close kin. Plants face this danger too, and the majority of plant species have evolved their own ways of not fertilizing themselves (“selfing”) or doing so only as a last resort. The reason to avoid such inbreeding is a simple one; mating with one’s self or a close relative increases the likelihood that two unfavorable alleles (“versions” of a gene like the “A”, “B” and “O” types in the human blood system) can come together in an offspring and produce an unfavorable outcome.
I thought I understood the mechanism for inbreeding avoidance in common loons. Twenty-nine years of research supported a simple conclusion. Loons avoid mating with close kin, it seemed, because males never settle to breed on their natal lakes. Thus, they avoid the obvious danger of forming a mated pair with their mothers.
Let me back up. Male loons, like males of most birds, do not move very far from their natal lakes to breed. Indeed, males often claim breeding territories on lakes adjacent to where they were hatched and reared. Females show much longer distance natal dispersal, typically breeding a half-county or so away from where they were raised. So the main inbreeding threat that loons face is the possibility that young males will return and pair with their mothers. Father-daughter pairs are not an issue.
In fact, males do occasionally pair with their mothers. On four occasions we have recorded young males to return to the breeding grounds and settle on lakes next door to their natal lake, only to meet and pair with their mom there. How? Females are sometimes evicted from their territories. When this happens, they commonly re-settle near the territory they lost. This places females in the path of any son from four or years before that also happens to also settle there.
The fact that males and their mothers only interbreed in instances when female eviction coincides with settlement of a son appeared significant to me. I reasoned that, since young adult sons were unlikely to happen to choose the same lake where their mother had taken refuge following eviction, loons were using a pretty reliable rule to avoid mother-son pairings. The rule seemed to be that males never chose to settle on their natal lake, thus avoiding the great risk that they would pair with their mother on the lake where she reared him.
Many of you are thinking: “Wait, why would close kin ever form mated pairs? Don’t loons recognize close kin?”. We do not know the answer to this question definitively, but the fact that there are any mother-son pairs at all suggests loons do not know their relatives. If either moms recognized sons or vice-versa, then that recognition would cause them to avoid incestuous pairings.
Let’s turn our attention to Arrowhead Lake in 2021. The tragic fracture of the breeding male’s humerus in 2020 created an opening for a new male on Arrowhead. But the non-return of the female in 2021 meant that both male and female breeding slots were open. The female that took over ownership was a 16-year old from Vilas County. This event was unsurprising. But the new male — at least as of five days ago — was the first-ever male to settle on his natal territory. What is remarkable about the settlement of this four-year-old is that it coincided precisely with his mother’s disappearance from the lake.
As a scientist, I habitually view coincidences not as such but as meaningful statistical patterns that can teach us about the world. So I have an inkling that the first-ever settlement of a male on his natal lake is the beginning of a revealing pattern. Perhaps loons really do recognize close kin in some instances. Perhaps this male, only four years removed from the egg, remembered his mother, recognized that the new female on his natal lake was not her, and seized a rare opportunity to settle at home while dodging inbreeding.
The problem with my hypothesis is that it will take a long time to test. I suppose that we can satisfy ecologists’ desire for statistical significance if we detect three more settlements of young males on natal lakes just after their mother’s disappearance. At a rate of one such event each 29 years, it will take 87 more years to reach statistical significance.
So…..we have some time. That being the case, take a moment to study the photo I took today on Long Lake, which shows two eggs that became a raccoon’s snack a few nights ago. Don’t worry. The pair is safe and sound and has plenty of time to attempt a second nest.