Holidays in Shetland, continuation: today is dedicated to the common guillemot (or thin-billed murre), Uria aalge, family Alcidae. As before, photos taken by my daughter are labelled with (Jo).
This one shows an very elegant monocle, complete with its lace. In the Shetland, about 10% of the guillemots bear this ornamentation; they are called “bridled”. Polymorphism is always interesting for biologists; below the photo are some results of their studies (some references at the bottom of the post):
- The frequency of bridled guillemots follows a cline, from 0 % in the most southern part of its distribution (Portugal) to about 50 % in the northern part (Bjørnøya, an island halfway between continental Norway and Spitzbergen).
- This polymorphism is apparently due to a single gene, the bridled allele being recessive (that is, b/b guillemots are bridled, NB/b and NB/NB ones are not).
- The probability of mating with a bridled partner (both for a bridled and a non-bridled bird) is proportional to the number of bridled birds in the colony: neither sexual selection nor assortative mating seem to occur.
- Mixed pairs (bridled x not bridled partner) produce stronger and larger chicks than either bridled x bridled or not bridled x not bridled pairs. It’s a case of selection for heterozygotes, which would insure the maintenance of a stable polymorphism in a given colony. [JAC: this “heterosis” is also seen in sickle-cell anemia, where heterozygotes don’t have the sickle-cell disease but are more resistant to malaria than “normal” homozygotes and of course far healthier than individuals homozygous for the allele causing the disease.]
- In northern Norway, the winter survival of bridled individuals is better during cold winters (cold sea surface temperature), the survival of non bridled individuals better in mild winters. The differences are very slight, but significant. That would be expected if the “bridled” mutation first appeared in the northern part of the species’ distribution and is partly linked to genes dealing with climatic conditions.
- In northern Norway and in Bjørnøya, the northern tip of their distribution, common guillemots live in huge mixed colonies together with the arctic Brünich’s guillemot (Uria lomvia), which has a thin white “moustache” (Clark Gable style) but is never bridled. These species occasionally hybridize; it is suggested that Brünnich would-be partners more efficiently avoid pairing with bridled common guillemots, which would save the latter from losing genes in sterile (or at least poorly fit) hybrids and therefore would favour the bridled form. From these northern mixed colonies, the “bridled” mutation would have slowly diffused to the south during cold periods like the little ice age.
(Jo) In this partial photo of a colony , showing about 120 individuals, I counted 12 bridled ones. Can you spot the (probably abandoned) eggs ? Guillemots lay their eggs directly on the rock and incubate them on their feet. The eggs are pear-shaped, which prevents them from rolling too far away and falling down the cliff. They are very variable in coloration and pattern, improving individual recognition.
Three non-bridled individuals:
For comparison: the arctic species, the thick-billed murre or Brünnich’s Guillemot, Uria lomvia (Spitzbergen 2014):
Looking underwater in search of fish.
(Jo) Diving: the guillemots use their wings to “fly” underwater and actually open them before going under with a little jump. Their very efficient underwater flight can be seen here [JAC: I’ve embedded the video below]. Aerial flight is straight and fast (up to 80 kmh, between 300 and 400 wingbeats per minute); the delicate moment is the landing on the colony, however crash landings are strongly over-shown on the web.
(Jo) Thin-billed murre indeed!
Jeffries,D.J. and J.L.F. Parslow (1976): The genetics of bridling in guillemots from a study of hand-reared birds. J. Zool., Lond. 179,411-420.
Kristensen D.L., Erikstad K.E. et al. (2014): Differential breeding investment in bridled and non-bridled common guillemots (Uria aalge): morph of the partner matters. Behav Ecol Sociobiol 68: 1851.
Taylor, S.A., Patirana, A. et al. (2012): Cryptic introgression