Readers, here's a partial list of things you should not try with elephants: a memory contest, jump rope and castration.
See that elephants, in addition to their eerie memory and their strong relationship to gravity, have their testicles embedded deep in their bodies, right up to their kidneys. This is unusual: in most other mammals, testicles form near the kidneys during embryonic development and then sink to either the lower abdomen or an outer scrotum at the time of birth of a male.
Biologists have been thinking about this discrepancy for decades. Have the earliest mammals kept their testicles, like elephants, or did they drop their family jewels? A new study, published Thursday in PLOS Biology, says it was the latter.
A German team that examined the DNA of 71 mammals concluded that the testicular origin is an ancestral form that was later lost in so-called Afrotherians. This group consists of elephants, manatees, and some insectivores that live or originate from Africa
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In four African subgroups – manatees and dugongs, elephant shrews, goldmolen and tenrecs (small insect-like hedgehogs) – the authors
Scientists often rely on geological fossils to study evolutionary history but this study shows that there is also a "fossil entry in the genome," said Mark Springer, a biology professor at the University of California, Riverside, who was not involved in the research.
These "molecular fossils" are abundant in the tree of life. "For just about any species, you'll find on the order of a hundred or more broken genes that existed in the past and have been lost," said Michael Hiller, a senior research group leader at the Max Planck Institute for Molecular Cell Biology and Genetics in Dresden , Germany, and senior author of the new work.
He and the lead author of the study, Virag Sharma, did not begin to target testicles.
For years, their group had developed a computer-aided method to study different genomes on broken genes with high accuracy. They observed remnants of genes rendered useless by evolution: enamel-producing genes in edentulous whales, fat digestion genes in sugar-dependent fruit bats, and DNA repair genes in Armadillos with armor to protect them from harmful UV radiation.
In addition, they noted that two genes called RXFP2 and INSL3 were inactive in several African species.
From a literature review, the researchers learned that when these genes are released in male mice, the rodents' testes are not descended. They also learned that evolutionary biologists have long debated whether this absence of testicular exit – called Testicondy – is a primitive property, or one that uniquely evolved Afrotheriane.
"It became clear that we would be able to resolve this debate," Dr. Hiller.
Based on the fact that genes begin to get mutations as they lose their function, the researchers worked backwards, estimating that tysticondyly arose at least four times, about 25 million years ago at Cape Golden Moles before about 80 Millions of years ago in Cape Elephant shrews.
This also meant that the Testicondy evolved after Afrotherians separated from other placental mammals about 100 million years ago, suggesting that the common ancestor of all mammals actually existed her testicles lowered, said Dr. Hiller.
But secrets remain. Not all Afrotherians have Testicondyardarks, for example with descending testes. And although elephants and rock hyraxes (which resemble guinea pigs) do not have descending testicles, RXFP2 and INSL3 are still intact in both.
It may be that researchers only "look at part of the picture" and these other genes and the processes involved have not yet been identified, said Ross MacPhee, a mammalian curator at the American Museum of Natural History, who is not working on the new study participated.
There is also the question of why testicles ever crash. Since they have valuable, life-giving content, why should they carry them in vulnerable bags? Scientists know that optimal sperm production requires lower temperatures than the rest of the body, but they do not understand why.
The answer may lie in further study of the Afrotherians, especially why and how they kept their testes so close.
Earlier reports on mammalian secrets