Monday, December 29, 2008
Tuesday, December 16, 2008
Saturday, December 13, 2008
Thursday, December 11, 2008
Descent of the testicle
This is an odd thing about mammalian testicles: they're often in a peculiar and very exposed place, bouncing about uncomfortably and dangerously, hanging out of the body in a scrotal sac. Other animals don't do anything so foolish; why mammals?
A couple of hypotheses have been proposed that might explain what advantage we derive from this peculiar arrangement.
- The display hypothesis. Males are flaunting their strength and health and virility and ability to protect a delicate package. Some species do make a show of it; some baboons have bright blue scrota, for instance. Most do not, however, and the idea seems unlikely to explain why this trait is preserved in mammals with widely diverse mating strategies.
- The cold storage hypothesis. It may not be so much the testis that is maintained at a cooler temperature outside the body core, but the epididymis. The epididymis is a storage organ for sperm. It may boost the longevity of the stored sperm if they are kept cool, maximizing the quantity of sperm that can be delivered. There are some good reasons that support this idea—some mammals do have internal testicles, but keep them organized so the epididymis is superficial—but personally, this hypothesis seems unlikely, and if it is sufficient to keep just the epididymis cool, it doesn't explain how the testicles ended up exposed in a bag.
- The training hypothesis. This one seems a bit of a stretch to me: the idea is that the testicle and epididymis are like boot camp, a tough, difficult environment that will weed out the unfit and allow only the strong to go on. Cool temperatures, more variable temperature, restricted blood flow...all prepare the sperm for the hostile environment they will confront upon ejaculation. There are many untested assumptions in this hypothesis, and it seems particularly improbable in species that practice sperm competition that they would sacrifice quantity for hypothetical quality, or that they would need to do something as risky as descensus and scrotal exposure to create an unpleasant cellular environment.
- The temperature hypothesis. The most likely explanation is that there is something in the function of the testis that is optimized for a lower temperature, and that the clumsy kludge that evolved to reduce that temperature was to let the organ hang out in the breeze. This seems reasonable, since fertility can be directly affected by temperature—a difference of a few degrees can be the difference between fecundity and sterility. The specific constraint in the process of sperm development that imposes this limitation hasn't been identified, however; one contributing factor (but definitely not the sole factor) may be that mutation rates increase with temperature, and the male germ line, which undergoes many more cell division than the female line, is much more sensitive to small changes in the mutation rate.
Werdelin and Nilsonne (1999) have taken a phylogenetic approach to examine this question. It may surprise some of you to learn that not all mammals have a scrotum. Many, like us people, have fully descended testicles contained in an external sac, the scrotum. Some have descended testicles, but no scrotum; the testicles are imbedded in the body wall. And some don't bother with that descensus nonsense at all, and keep their testicles high up in the dorsal body wall, near the kidneys, a condition called testicondy. If you'd asked me before I read this paper which animals have scrota and which do not, I wouldn't have had a clue (it really does seem like a rather personal thing, don't you think?), nor would I have had much of an idea of the distribution of scrota across the Mammalia. Werdelin and Nilsonne have peeked under the kilts, though, and summarized the distribution. Here is the abstract of the paper:
The adaptive significance of the scrotum and the evolution of the descent of the testicles and epididymis have been a focus of interest among biologists for a long time. In this paper we use three anatomical character states of the scrotum and descensus: (1) testicles descended and scrotal; (2) testicles descended but ascrotal; (3) testicles not descended (testicondy). These states are then mapped on an up to date phylogeny of the Mammalia. Three main points arise out of this mapping procedure: (1) the presence of a scrotum is either primitive in extant Mammalia or primitive within eutherian mammals except Insectivora; (2) evolution has generally proceeded from a scrotal condition to progressively more ascrotal; (3) loss of testicular descensus is less common in mammalian evolution than is loss of the scrotum. In the light of these findings we discuss some current hypotheses regarding the origin and evolution of the scrotum. We find that these are all incomplete in so far as it is not the presence of the scrotum in various mammal groups that requires explaining. Instead, it is the reverse process, why the scrotum has been lost in so many groups, that should be explained. We suggest that the scrotum may have evolved before the origin of mammals, in concert with the evolution of endothermy in the mammalian lineage, and that the scrotum has been lost in many groups because descensus in many respects is a costly process that will be lost in mammal lineages as soon as an alternative solution to the problem of the temperature sensitivity of spermatogenesis is available.
What they see in the phylogenetic distribution of scrota is that our common, ancestral proto-mammal had probably evolved a scrotum as a solution to its fertility requirements, and really, probably the best answer to why we have this odd scrotal arrangement is that that is the way great-great-greatn-grandpa did it. It is the primitive condition, inherited from mammal-like reptiles of the Permian, and mammalian lineages have been independently ditching the idea as fast as they can...which isn't very fast. Here's one of their summary diagrams:
Results of mapping the three character states onto a phylogeny of the Mammalia. Testicle position unordered: red, testicond; gray, descended and ascrotal; black, descended, scrotal; blue, marsupial; white, equivocal.
The black bars are us, the species that let it all hang out. The most parsimonious explanation for their distribution, the one that requires the smallest number of novel adapations, is that they represent the ancestral condition. Some lineages, such as rhinos and tapirs and bats and whales (whales and seals, by the way, still keep their gonads cool by shunting blood vessels that pass through the skin back to the testicles—they essentially have water-cooled balls), still have descended testicles but don't have scrota; these are in gray. And the rare red lineages, elephants and manatees most prominently, have dispensed with the whole silly arrangement and keep their testicles safe and deep in the body cavity. (This is very useful knowledge to have if you ever get angry at an elephant and are tempted to kick him in the nads...don't bother. He'll just laugh at you.)
Here's the rough history of the mammalian testicle, in summary:
Studies of the origin of endothermy in mammals indicate that incipient endothermy may have evolved in mammalian ancestors as early as the middle Late Permian, at least 260 million years ago, which is some 40 million years before the origin of the Mammalia and 130 million years before the origin of crown group mammals, i.e. those for which we have here demonstrated that the scrotum may be primitive. A plausible, though at present untestable, scenario is that in the course of the evolution of mammalian endothermy, core body temperatures eventually reached levels at which spermatogenesis was disrupted. At this point, the testicles had to be cooled, and descensus into a scrotum was one way of achieving this result. However, because of the various costs of descensus the evolution of the male reproductive system of mammals since then has revolved around finding alternative solutions to the original cooling problem. This means that testicondy probably evolved only once among Eutherians, strengthening the conclusions herein.
Source: The evolution of the scrotum and testicular descent in mammals: a phylogenetic view.
Opinions from Pharyngula
Wednesday, December 10, 2008
Basic Male Tool Kit is Under Threat
The male gender is in danger, with incalculable consequences for both humans and wildlife, startling scientific research from around the world reveals.
The research shows that a host of common chemicals is feminising males of every class of vertebrate animals, from fish to mammals, including people.
Backed by some of the world's leading scientists, who say that it "waves a red flag" for humanity and shows that evolution itself is being disrupted, the report comes out at a particularly sensitive time for ministers. On Wednesday, Britain will lead opposition to proposed new European controls on pesticides, many of which have been found to have "gender-bending" effects.
It also follows hard on the heels of new American research which shows that baby boys born to women exposed to widespread chemicals in pregnancy are born with smaller penises and feminised genitals.
"This research shows that the basic male tool kit is under threat," says Gwynne Lyons, a former government adviser on the health effects of chemicals, who wrote the report.
Wildlife and people have been exposed to more than 100,000 new chemicals in recent years, and the European Commission has admitted that 99 per cent of them are not adequately regulated. There is not even proper safety information on 85 per cent of them.
Many have been identified as "endocrine disrupters" – or gender-benders – because they interfere with hormones. These include phthalates, used in food wrapping, cosmetics and baby powders among other applications; flame retardants in furniture and electrical goods; PCBs, a now banned group of substances still widespread in food and the environment; and many pesticides.
The report – published by the charity CHEMTrust and drawing on more than 250 scientific studies from around the world – concentrates mainly on wildlife, identifying effects in species ranging from the polar bears of the Arctic to the eland of the South African plains, and from whales in the depths of the oceans to high-flying falcons and eagles.
It concludes: "Males of species from each of the main classes of vertebrate animals (including bony fish, amphibians, reptiles, birds and mammals) have been affected by chemicals in the environment.
"Feminisation of the males of numerous vertebrate species is now a widespread occurrence. All vertebrates have similar sex hormone receptors, which have been conserved in evolution. Therefore, observations in one species may serve to highlight pollution issues of concern for other vertebrates, including humans."
Fish, it says, are particularly affected by pollutants as they are immersed in them when they swim in contaminated water, taking them in not just in their food but through their gills and skin. They were among the first to show widespread gender-bending effects. Half the male fish in British lowland rivers have been found to be developing eggs in their testes; in some stretches all male roaches have been found to be changing sex in this way. Female hormones – largely from the contraceptive pills which pass unaltered through sewage treatment – are partly responsible, while more than three-quarters of sewage works have been found also to be discharging demasculinising man-made chemicals. Feminising effects have now been discovered in a host of freshwater fish species as far away as Japan and Benin, in Africa, and in sea fish in the North Sea, the Mediterranean, Osaka Bay in Japan and Puget Sound on the US west coast.
Research at the University of Florida earlier this year found that 40 per cent of the male cane toads – a species so indestructible that it has become a plague in Australia – had become hermaphrodites in a heavily farmed part of the state, with another 20 per cent undergoing lesser feminisation. A similar link between farming and sex changes in northern leopard frogs has been revealed by Canadian research, adding to suspicions that pesticides may be to blame.
Male alligators exposed to pesticides in Florida have suffered from lower testosterone and higher oestrogen levels, abnormal testes, smaller penises and reproductive failures. Male snapping turtles have been found with female characteristics in the same state and around the Great Lakes, where wildlife has been found to be contaminated with more than 400 different chemicals. Male herring gulls and peregrine falcons have produced the female protein used to make egg yolks, while bald eagles have had difficulty reproducing in areas highly contaminated with chemicals.
Scientists at Cardiff University have found that the brains of male starlings who ate worms contaminated by female hormones at a sewage works in south-west England were subtly changed so that they sang at greater length and with increased virtuosity.
Even more ominously for humanity, mammals have also been found to be widely affected.
Two-thirds of male Sitka black-tailed deer in Alaska have been found to have undescended testes and deformed antler growth, and roughly the same proportion of white-tailed deer in Montana were discovered to have genital abnormalities.
In South Africa, eland have been revealed to have damaged testicles while being contaminated by high levels of gender-bender chemicals, and striped mice from one polluted nature reserved were discovered to be producing no sperm at all.
At the other end of the world, hermaphrodite polar bears – with penises and vaginas – have been discovered and gender-benders have been found to reduce sperm counts and penis lengths in those that remained male. Many of the small, endangered populations of Florida panthers have been found to have abnormal sperm.
Other research has revealed otters from polluted areas with smaller testicles and mink exposed to PCBs with shorter penises. Beluga whales in Canada's St Lawrence estuary and killer whales off its north-west coast – two of the wildlife populations most contaminated by PCBs – are reproducing poorly, as are exposed porpoises, seals and dolphins.
Scientists warned yesterday that the mass of evidence added up to a grave warning for both wildlife and humans. Professor Charles Tyler, an expert on endocrine disrupters at the University of Exeter, says that the evidence in the report "set off alarm bells". Whole wildlife populations could be at risk, he said, because their gene pool would be reduced, making them less able to withstand disease and putting them at risk from hazards such as global warming.
Dr Pete Myers, chief scientist at Environmental Health Sciences, one of the world's foremost authorities on gender-bender chemicals, added: "We have thrown 100, 000 chemicals against a finely balanced hormone system, so it's not surprising that we are seeing some serious results. It is leading to the most rapid pace of evolution in the history of the world.
Professor Lou Gillette of Florida University, one of the most respected academics in the field, warned that the report waved "a large red flag" at humanity. He said: "If we are seeing problems in wildlife, we can be concerned that something similar is happening to a proportion of human males"
Indeed, new research at the University of Rochester in New York state shows that boys born to mothers with raised levels of phthalates were more likely to have smaller penises and undescended testicles. They also had a shorter distance between their anus and genitalia, a classic sign of feminisation. And a study at Rotterdam's Erasmus University showed that boys whose mothers had been exposed to PCBs grew up wanting to play with dolls and tea sets rather than with traditionally male toys.
Communities heavily polluted with gender-benders in Canada, Russia and Italy have given birth to twice as many girls than boys, which may offer a clue to the reason for a mysterious shift in sex ratios worldwide. Normally 106 boys are born for every 100 girls, but the ratio is slipping. It is calculated that 250,000 babies who would have been boys have been born as girls instead in the US and Japan alone.
And sperm counts are dropping precipitously. Studies in more than 20 countries have shown that they have dropped from 150 million per millilitre of sperm fluid to 60 million over 50 years. (Hamsters produce nearly three times as much, at 160 million.) Professor Nil Basu of Michigan University says that this adds up to "pretty compelling evidence for effects in humans".
But Britain has long sought to water down EU attempts to control gender-bender chemicals and has been leading opposition to a new regulation that would ban pesticides shown to have endocrine-disrupting effects. Almost all the other European countries back it, but ministers – backed by their counterparts from Ireland and Romania – are intent on continuing their resistance at a crucial meeting on Wednesday. They say the regulation would cause a collapse of agriculture in the UK, but environmentalists retort that this is nonsense because the regulation has get-out clauses that could be used by British farmers.
Sniped from The Independent
The research shows that a host of common chemicals is feminising males of every class of vertebrate animals, from fish to mammals, including people.
Backed by some of the world's leading scientists, who say that it "waves a red flag" for humanity and shows that evolution itself is being disrupted, the report comes out at a particularly sensitive time for ministers. On Wednesday, Britain will lead opposition to proposed new European controls on pesticides, many of which have been found to have "gender-bending" effects.
It also follows hard on the heels of new American research which shows that baby boys born to women exposed to widespread chemicals in pregnancy are born with smaller penises and feminised genitals.
"This research shows that the basic male tool kit is under threat," says Gwynne Lyons, a former government adviser on the health effects of chemicals, who wrote the report.
Wildlife and people have been exposed to more than 100,000 new chemicals in recent years, and the European Commission has admitted that 99 per cent of them are not adequately regulated. There is not even proper safety information on 85 per cent of them.
Many have been identified as "endocrine disrupters" – or gender-benders – because they interfere with hormones. These include phthalates, used in food wrapping, cosmetics and baby powders among other applications; flame retardants in furniture and electrical goods; PCBs, a now banned group of substances still widespread in food and the environment; and many pesticides.
The report – published by the charity CHEMTrust and drawing on more than 250 scientific studies from around the world – concentrates mainly on wildlife, identifying effects in species ranging from the polar bears of the Arctic to the eland of the South African plains, and from whales in the depths of the oceans to high-flying falcons and eagles.
It concludes: "Males of species from each of the main classes of vertebrate animals (including bony fish, amphibians, reptiles, birds and mammals) have been affected by chemicals in the environment.
"Feminisation of the males of numerous vertebrate species is now a widespread occurrence. All vertebrates have similar sex hormone receptors, which have been conserved in evolution. Therefore, observations in one species may serve to highlight pollution issues of concern for other vertebrates, including humans."
Fish, it says, are particularly affected by pollutants as they are immersed in them when they swim in contaminated water, taking them in not just in their food but through their gills and skin. They were among the first to show widespread gender-bending effects. Half the male fish in British lowland rivers have been found to be developing eggs in their testes; in some stretches all male roaches have been found to be changing sex in this way. Female hormones – largely from the contraceptive pills which pass unaltered through sewage treatment – are partly responsible, while more than three-quarters of sewage works have been found also to be discharging demasculinising man-made chemicals. Feminising effects have now been discovered in a host of freshwater fish species as far away as Japan and Benin, in Africa, and in sea fish in the North Sea, the Mediterranean, Osaka Bay in Japan and Puget Sound on the US west coast.
Research at the University of Florida earlier this year found that 40 per cent of the male cane toads – a species so indestructible that it has become a plague in Australia – had become hermaphrodites in a heavily farmed part of the state, with another 20 per cent undergoing lesser feminisation. A similar link between farming and sex changes in northern leopard frogs has been revealed by Canadian research, adding to suspicions that pesticides may be to blame.
Male alligators exposed to pesticides in Florida have suffered from lower testosterone and higher oestrogen levels, abnormal testes, smaller penises and reproductive failures. Male snapping turtles have been found with female characteristics in the same state and around the Great Lakes, where wildlife has been found to be contaminated with more than 400 different chemicals. Male herring gulls and peregrine falcons have produced the female protein used to make egg yolks, while bald eagles have had difficulty reproducing in areas highly contaminated with chemicals.
Scientists at Cardiff University have found that the brains of male starlings who ate worms contaminated by female hormones at a sewage works in south-west England were subtly changed so that they sang at greater length and with increased virtuosity.
Even more ominously for humanity, mammals have also been found to be widely affected.
Two-thirds of male Sitka black-tailed deer in Alaska have been found to have undescended testes and deformed antler growth, and roughly the same proportion of white-tailed deer in Montana were discovered to have genital abnormalities.
In South Africa, eland have been revealed to have damaged testicles while being contaminated by high levels of gender-bender chemicals, and striped mice from one polluted nature reserved were discovered to be producing no sperm at all.
At the other end of the world, hermaphrodite polar bears – with penises and vaginas – have been discovered and gender-benders have been found to reduce sperm counts and penis lengths in those that remained male. Many of the small, endangered populations of Florida panthers have been found to have abnormal sperm.
Other research has revealed otters from polluted areas with smaller testicles and mink exposed to PCBs with shorter penises. Beluga whales in Canada's St Lawrence estuary and killer whales off its north-west coast – two of the wildlife populations most contaminated by PCBs – are reproducing poorly, as are exposed porpoises, seals and dolphins.
Scientists warned yesterday that the mass of evidence added up to a grave warning for both wildlife and humans. Professor Charles Tyler, an expert on endocrine disrupters at the University of Exeter, says that the evidence in the report "set off alarm bells". Whole wildlife populations could be at risk, he said, because their gene pool would be reduced, making them less able to withstand disease and putting them at risk from hazards such as global warming.
Dr Pete Myers, chief scientist at Environmental Health Sciences, one of the world's foremost authorities on gender-bender chemicals, added: "We have thrown 100, 000 chemicals against a finely balanced hormone system, so it's not surprising that we are seeing some serious results. It is leading to the most rapid pace of evolution in the history of the world.
Professor Lou Gillette of Florida University, one of the most respected academics in the field, warned that the report waved "a large red flag" at humanity. He said: "If we are seeing problems in wildlife, we can be concerned that something similar is happening to a proportion of human males"
Indeed, new research at the University of Rochester in New York state shows that boys born to mothers with raised levels of phthalates were more likely to have smaller penises and undescended testicles. They also had a shorter distance between their anus and genitalia, a classic sign of feminisation. And a study at Rotterdam's Erasmus University showed that boys whose mothers had been exposed to PCBs grew up wanting to play with dolls and tea sets rather than with traditionally male toys.
Communities heavily polluted with gender-benders in Canada, Russia and Italy have given birth to twice as many girls than boys, which may offer a clue to the reason for a mysterious shift in sex ratios worldwide. Normally 106 boys are born for every 100 girls, but the ratio is slipping. It is calculated that 250,000 babies who would have been boys have been born as girls instead in the US and Japan alone.
And sperm counts are dropping precipitously. Studies in more than 20 countries have shown that they have dropped from 150 million per millilitre of sperm fluid to 60 million over 50 years. (Hamsters produce nearly three times as much, at 160 million.) Professor Nil Basu of Michigan University says that this adds up to "pretty compelling evidence for effects in humans".
But Britain has long sought to water down EU attempts to control gender-bender chemicals and has been leading opposition to a new regulation that would ban pesticides shown to have endocrine-disrupting effects. Almost all the other European countries back it, but ministers – backed by their counterparts from Ireland and Romania – are intent on continuing their resistance at a crucial meeting on Wednesday. They say the regulation would cause a collapse of agriculture in the UK, but environmentalists retort that this is nonsense because the regulation has get-out clauses that could be used by British farmers.
Sniped from The Independent
Monday, December 1, 2008
"An atheist before Darwin could have said...
'I have no explanation for complex biological design. All I know is that God isn't a good explanation, so we must wait and hope that somebody comes up with a better one.' I can't help feeling that such a position, though logically sound, would have left one feeling pretty unsatisfied, and that although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist."
Richard Dawkins.
Richard Dawkins.
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