Inbreeding, or the mating of more genetically similar individuals than the average relatedness of individuals in a population, has been shown to reduce fitness through the increase in frequency of deleterious recessive alleles being expressed in offspring (Cohen & Dearborn 2004). Throughout the animal and plant world, strategies have been developed that decrease the likelihood of inbreeding and the negative effects it can produce (Darwin 1876, Charlesworth & Charlesworth 1987, Pusey & Wolf 1996). However, in certain populations of organisms including birds, it does occur due to limited dispersal ability, random mating processes with respect to relatedness and natal breeding site fidelity (Bohonak 1999, Cohen & Dearborn 2004). But is this the ONLY reason we see inbreeding occurring in bird populations?
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| The male great frigatebird (Fregata minor) shows off his inflated red gular pouch to attract a female, but is this really what the female is inspecting? Could this be used by the female as an indicator of genetic similarity? Image sourced from http://www.pelagicodyssey.ca/styled-3/page37/. Viewed on 3rd April, 2017. |
The great frigatebird (Fregata minor) population
of Tern Island in Hawaii has been shown to display high levels of
inbreeding, but not due to any of the reasons mentioned above (Cohen & Dearborn 2004). Great frigatebird breeding ecology would make it improbable
that inbreeding would occur, as the species does not suffer from any of
the constraints that would "force" genetically similar individuals to
breed with each other - females aerially assess the displays of up to
several hundred males, males can travel hundreds of kilometers to
display to females on other islands and interisland genetic diversity is
reasonably high (Dearborn & Ryan 2002, Dearborn et al. 2003). While
great frigatebirds do show a degree of natal site fidelity, females
have a broad range of males to choose from as the operational sex ratio
is male biased at the time the sexes are pairing-up to breed (Dearborn et al. 2001).
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| Great frigatebirds (Fregata minor) (foreground), and red-footed boobies (Sula sula) (background) both breed on Tern Island in Hawaii. Photo by P.D. Tillmam https://commons.wikimedia.org/wiki/File:Great_frigatebirds_and_red-footed_boobies_at_Tern_Island.jpg. Viewed on 3rd April, 2017. |
Despite these breeding behaviours and opportunities for
females to pick genetically dissimilar males, inbreeding is widespread
throughout the population (Cohen & Dearborn 2004). This would suggest that females are actively picking genetically similar males to breed with (Cohen & Dearborn 2004). Sexual imprinting, descibed by Irwin & Price (1999) as "the means by which a young bird
learns species-specific characteristics that enables it to find a
conspecific mate when adult", has been suggested as a possible mechanism for inbreeding in the great frigatebird, but there is still much uncertainty about the resulting fitness consequences and whether they are beneficial or detrimental (Cohen & Dearborn 2004). There would have to be an advantage associated with inbreeding in this case if the postulates of sexual selection apply wouldn't there??? A very interesting but still unanswered question!
References
Bohonak, A.J. 1999, "Dispersal, gene flow, and population structure", The Quarterly Review of Biology, vol. 74, no. 1, pp. 21-45.
Charlesworth, D. & Charlesworth, B. 1987, "Inbreeding depression and its evolutionary consequences", Annual Review of Ecology and Systematics, vol. 18, no. 1, pp. 237-268.
Cohen, L.B. & Dearborn, D.C. 2004, "Great frigatebirds, Fregata minor, choose mates that are genetically similar", Animal Behaviour, vol. 68, no. 5, pp. 1229-1236.
Darwin, C. 1876, The effects of cross and self fertilisation in the vegetable kingdom. J. Murray.
Dearborn, D.C., Anders, A. D. & Parker, P. G. 2001, "Sexual dimorphism, extrapair fertilizations, and operational sex ratio in great frigatebirds (Fregata minor)", Behavioral Ecology, vol. 12, no. 6, pp. 746-752.
Dearborn, D.C., Anders, A.D., Schreiber, E.A., Adams, R.M.M. & Mueller, U.G. 2003, "Inter‐island movements and population differentiation in a pelagic seabird", Molecular Ecology, vol. 12, no. 10, pp. 2835-2843.
Dearborn, D. C. & Ryan, M. J. 2002, "A test of the Darwin–Fisher theory for the evolution of male secondary sexual traits in monogamous birds", Journal of Evolutionary Biology, vol. 15, pp. 307–313.
Irwin, D.E. & Price, T. 1999, "Sexual imprinting, learning and speciation", Heredity, vol. 82, no. 4, pp. 347-354.
Pusey, A. & Wolf, M. 1996, "Inbreeding avoidance in animals", Trends in Ecology & Evolution, vol. 11, no. 5, pp. 201-206.
Totally cool! My question is whether or not inbreeding always has to incur a cost? If you look at species like naked mole rats that are highly inbred, could it be that frigatebirds just don’t suffer costs of inbreeding, and the benefits of choosing genetically similar mates are high, encouraging selection for this breeding strategy?
ReplyDeleteYes, this would make sense or why would they do it? I think there is still a lot to learn regarding this. I found plenty of information saying how detrimental inbreeding can be but also examples such as this case where it seems that animals actively choose this breeding strategy. Surely this breeding strategy would not persevere if it resulted in reduced fitness???
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