I had long thought, and even taught, that sheep and goats couldn’t form viable hybrids, as they are very distantly related. Timetree puts them at a divergence of about 7 million years, which is the equivalent of a human and a chimp producing a viable offspring. (It’s been tried, believe me, and it doesn’t work. See WEIT for details.) I would sometimes tell students that cross-fertilization between the species produced hybrid embryos that would die early in development, an example of species distinctness enforced by “hybrid inviability.”
It turns out that’s largely true, but not entirely. Reader Merilee sent me a picture of a newborn geep, an apparently healthy hybrid between the two species. That led me to a news website in Arizona that published a piece on a newborn geep on August 1 and showed and a video of the cute little thing. Here are the facts:
Butterfly may be Arizona’s only geep, a tiny tot with goat hooves and a goat head, but covered in wool. Her mom, Momma, is a sheep. Her dad, Michael, is a pygmy goat. Hence, geep. She was named Butterfly for her spots.
As expected, Wikipedia has a good piece on the hybrids. It says, among other things, this:
A sheep–goat hybrid (sometimes called a geep or toast in popular media) is the hybrid offspring of a sheep and a goat. Although sheep and goats seem similar and can be mated, they belong to different genera in the subfamily Caprinae of the family Bovidae. Sheep belong to the genus Ovis and have 54 chromosomes, while goats belong to the genus Capra and have 60 chromosomes. The offspring of a sheep-goat pairing is generally stillborn. Despite widespread shared pasturing of goats and sheep, hybrids are very rare, indicating the genetic distance between the two species. Though sometimes called “geep”, they are not to be confused with goat-sheep chimerae, which are artificially created.
Knowing nothing about geeps, I would have guessed that any viable ones would be female (and probably sterile), in accordance with Haldane’s rule: the generalization that if only one sex of hybrids between species is either viable or fertile, that sex is almost invariably the “homogametic” one: the one with similar sex chromosomes. Since male mammals are the “heterogametic sex” (XY) and females are the homogametic (XX) sex, I’d expect geeps, if viable, to be female, and to be sterile because of the evolutionary distance between the parental species.
An aside: I worked on Haldane’s rule for much of my career, as it’s one of the few “laws” of biology–i.e., generalizations that are rarely violated–and for many years my students and I sought a general explanation for a phenomenon seen not only in mammals and insects–males heterogametic– but also in birds and butterflies, groups which females are heterogametic and in which, in accordance with the rule, females are the sterile or missing hybrids. We think we have a good explanation for Haldane’s rule now, but if you want to read about it, see chapter 7 of Speciation by Coyne and Orr. It’s technical and a bit complicated.
Well, Butterfly is female, but we don’t yet know about her fertility. But Wikipedia suggests that Haldane’s rule has been violated in these species–the geep was both female and fertile.
On May 12, 2011, a healthy and fertile geep was born in Bant, Flevoland, the Netherlands. The geep mated with a ewe and on December 25, 2012 two healthy lambs were born.
Below is a video of a geep born on a farm in Ireland, and you can find pictures of other hybrids here. I expect reader Linda Grilli to weigh in soon, as she keeps goats and also rises very early. She will, I hope, tell us more about geeps, or about this hybrid.