My latest post for AAAS talks about a recent study by Elizabeth Alter and colleagues who used mitochondrial DNA (including DNA from very old samples) to explore how arctic ice and whaling have impacted the breeding and evolution of bowhead whales. You can read my post here.
In other DNA/whale news: Last week, Current Biology published a short article about the discovery of “the world’s rarest whale.” The discovery was actually of two whales–a female and her male calf. The two spade-toothed beaked whales (Mesoplodon traversii) became stranded on a New Zealand beach and later died. Prior to these specimens, the only evidence that this species existed came from a few bones. Sightings of beaked whales are rare because these whales spend most of their time deep in the ocean. Identification of spade-toothed beaked whales is especially tricky since they are difficult to distinguish from Gray’s beaked whales (a more common species). Kirsten Thompson and colleaguesused DNA analysis to determine that the mother and calf were not Gray’s beaked whales (and that they were related). Morphological examination pointed to the spade-toothed identity. This study highlights both how important DNA analysis can be for identifying rare species and just how little we know about the species living in the deep ocean–including those as large as whales.
Identify new galaxies, explore the deep oceans, and categorize bat calls with Zooniverse:Zooniverse lets citizen scientists analyze large datasets
These projects are so awesome that sometimes I wish I were retired so I could be a citizen scientist (instead of a real one). In the time since I wrote this post, four new suns were discovered by ‘armchair astronomers’ working on one of the Zooniverse projects and Zooniverse launched a new project where people can classify cancer cells. You don’t need any scientific background to work on these projects, and you might discover something new and really fascinating. Hat tip to Kyle Willett, a college friend of mine who is a postdoc working on the GalaxyZoo project, for introducing me to Zooniverse at his wedding.
A recent study published in Science looks at a question that has long vexed biologists: why do many female animals live years–often decades–after they are able to reproduce? It could be that a long life is just a side effect of being a healthy animal. Or older animals may increase the chances of survival and/or reproductive success of their offspring (thus increasing the likelihood that their genes will be passed on to grandchildren) by living past reproductive age. Killer whales are excellent for examining these ideas since they stop reproducing in their 30 and 40s and yet often live into their 90s. According to the article this is the “longest post-reproductive life span of all non-human animals.”
This study used photographic census records of killer whales living off the coast of Washington state and British Columbia to examine the impact a mother whale’s death had on her offspring. The results showed that a mother’s death had the largest effect on male offspring. In fact, for sons over the age of 30, the death of a post-reproductive mother increased the likelihood of dying by 13.9-fold (compared to 5.4-fold in females over age 30) in the first year after the mother died.
How does having a post-menopause mother whale around help older sons? This question wasn’t directly addressed by the paper although the authors mention that the mothers may help with foraging and defense. A more interesting question is why the mothers help their sons more than their daughters. There are a few considerations that might be at play here. One is that the offspring of sons are raised by other pods thus requiring less investment of time and energy into helping to raise grandchildren. More important, perhaps, is the fact that sons can reproduce their entire lifespans and thus have the potential to sire many more offspring than daughters.
Side note: There is interesting research into the “grandmother” hypothesis in humans. This study found that maternal grandmothers helped the survival of grandchildren in rural Gambia. And this study presents a model for how the benefits of grandmothering combined with the conflict of having multiple simultaneously reproducing generations could explain why women often live twice as long as their reproductive lifespan.
Tasmanian devil (nothing like the cartoon, am I right?) Image credit: KeresH
You may have heard about how DFTD or devil facial tumor disease (how’s that for a straightforward disease name?) has decimated the Tasmanian devil population in recent years and is putting the species at risk of extinction. A paper published yesterday in the Journal of Animal Ecology provides surprising insights into how this disease is spread.
Scientists had already discovered that DFTD is spread through direct contact–often through fighting (devils can be jealous bastards when it comes to their mates). But in this study, the researchers focused on the relationship between the number of bites a devil had received and the likelihood that that individual would get DFTD. While one might predict that individuals who were bitten multiple times would be more likely to develop the disease since presumably they have been the victims of attacks from multiple other devils who may be DFTD carriers, the researchers actually discovered evidence that supports an alternative hypothesis: the more aggressive animals (the ones giving the bites) were more likely to get struck down by DFTD than were the peaceful little buggers getting the bites. This could be because the more aggressive devils were actually biting the submissive devils’ tumors (eew–bad idea guys).
What’s most interesting about this finding is that it predicts evolutionary pressure on the Tasmanian devil population that could start to favor submissive animals. This is because submissive animals are less likely to contract the disease and die (and thus likely to raise more genetic offspring). And because these animals are submissive they are less likely to pass on the disease. Could this eventually bring an end to DFTD and save the devils? Or, as the authors of the study offer as a possible future direction, could humans step in and remove aggressive ‘super spreaders’ from the population to try to rein in this terrible disease? On the other hand, could this natural or unnatural selection further decrease the genetic diversity that made the devils so susceptible to the disease to begin with?
Interesting aside: A pregnant Tasmanian devil gives birth to 20-30 offspring, but because she only has four nipples in her pouch (devils are marsupials) very few survive. It’s a rough life from the start for these guys.
Microscope image of mitochondria in mammalian lung tissue. (Photo: Louisa Howard)
A person’s genome is a unique combination of genes inherited from both his or her mother and father (with some random mutations thrown in). But each person also inherits another type of DNA — mitochondrial DNA (mtDNA). Mitochondria, the bacteria-like organelles that provide power for our cells, contain their own genomes. An individual’s mtDNA is almost identical to that of his or her mother’s mtDNA because those mitochondria are direct descendants of the mitochondria in the fertilized egg cell (the sperm’s mitochondria are usually chewed up by the egg). In fact, we can take advantage of this to track how groups of people are genetically related and to trace our own ancestry. But the fact that mtDNA is only inherited from our mothers produces an interesting evolutionary phenomenon: some mutations in mtDNA are harmful only to male offspring and not to female offspring. A recent paper by M. Florencia Camus, David Clancy, and Damian Dowling published in Current Biology examines how this “Mother’s Curse” may lead to shorter lifespans in male flies.