Twinning in avian species
Keywords: avian, twins, triplets, freemartinism
Image size: 1352 x 734px
Note: this is a serious entry, only the image is somewhat tongue/egg-in-cheek
I was amazed to crack open two large eggs and later, to have two sets of twin yolks sitting there on my toast. Being compulsively academic, but essentially mammalian in orientation, I began to wonder about the incidence of twinning, identical twins, mixed sex twins conjoined fetuses and even Freemartinism in birds.
Here is a summary of what I learned:
Twinning appears to be common in birds, with twins arising from eggs with double yolks, eggs with one yolk and two blastoderms, or eggs with one yolk and a single blastoderm. In the last two cases, one presumes that the chicks are identical twins. Indeed, both fraternal and identical twins do occur from single eggs in birds. In rare cases, triplets and even quadruplets may occur. In a remarkable photograph from a paper published in 1940, H.H. Newman shows triplets and conjoined chicken embryos in their early stages of development.
Twins have been reported in many species of birds and may be induced by stress. In support of this statement, the incidence of twins rises in laying hens exposed to hypothermia.
As expected, individual twins weigh less than singlets and twins may be of the same or mixed sex. Vascular anastmoses develop during gestation in twin chickens and in mixed sex chickens a freemartin-like condition has been reported in female chicks.
References:
Bassett, S.M. et al. 1999 Genetically identical avian twins. Journal of Zoology. 247: 475–478
Burke, W.H. and Sharp, P.J. 1989. Sex Differences in Body Weight of Chicken Embryos. Poultry Science. 68: 805-810.
Griffith, S.C. and Stewart, R. 1998. Genetic confirmation of non-identical embryonic twins in the House Sparrow Passer domesticus. Journal of avian biology 29:207-208
Lutz, H. Lutz-Ostertag, Y. 1975. Intersexuality of the Genital System and “Free-Martinism” in Birds. pp. 382-391 in Intersexuality in the Animal Kingdom. Springer Berlin Heidelberg. Print ISBN 978-3-642-66071-9
Newman, H.H. 1940. Twin and triplet chick embryos. Journal of Heredity. 31: 370-378
Pattee, O.H. et al. 1984 Twin embryos in a peregrine falcon egg. The Condor 86:352-353
