Friday, October 25, 2013

Mighty mouse uses scorpion venom as painkiller

Here we go: two weeks in a row. I think that technically counts as a habit, right?
om nom nom nom nom
Photo by Ashlee Rowe via /
Anyway, this is a pretty cool study out of the Department of Zoology at Michigan State University (actually it looks the lead author did most of the research at the University of Texas but I am choosing to ignore that; cf. sucks, Texas). The adorably vicious grasshopper mouse in this video is obviously unphased by the stings it receives from its lunch: a common bark scorpion. Apparently, the venom of this scorpion is strong enough to kill other mammals of the same size and is extremely painful to larger animals including humans. The venom normally works by specifically binding to and opening one of the sodium channels (Nav1.7) that is expressed in pain receptor neurons. Opening of one of these channels causes depolarization of the membrane and thus an action potential which travels to the central nervous system and creates the sensation of pain. What this group found out is that, in grasshopper mice, the venom also binds to a different sodium channel (Nav1.8). Both channels are found on the same neurons and serve the same function. When the venom binds to the 1.8 channel, however, it has an inhibitory effect which means that it closes the channel and keeps it from opening which prevents action potentials from propagating. In other words, not only is the mouse immune to the venom, but it actually has an analgesic effect. But why doesn't the same thing happen when the scorpion stings other animals? Apparently the venom does not bind to the Nav1.8 channel of other mammals. The reason is a substitution mutation of a single amino acid in the protein structure of the Nav1.8 channel that has only been found in this particular variety of mouse. The authors note that they do not have the full story and there is likely more to the mechanism of the venom that may contribute to the mouse's immunity, but it is a nice illustration of the neverending evolutionary arms race that goes on between predators and prey. The bark scorpion has few predators precisely because of its potent venom, but a single mutation carried by the grasshopper mouse has rendered the scorpion completely defenseless. For the mouse it means that it is not only safe from the venom, but it has access to a food source that no other competing predator can use.

Friday, October 18, 2013

A new hominid fossil may simplify the story of human evolution

My goal this year was to get into a habit of posting something interesting every week. Considering that it is now October and I have posted nothing since school started, I would say that I have not exactly succeeded. But failure is no excuse for giving up and this story sounds like a pretty major development in the area of human evolution. The original study is in Science, but most of the good stuff is behind a pay-wall. The BBC has a pretty good write-up though with some good pictures and an interview with the leader of the research team. What they found was a skull from a hominid that lived about 1.8 million years ago in what is now the Dmanisi region of Georgia. There are at least two things that are interesting about the find. First, it is a nearly complete skull including most of the face and brain-case and even most of the teeth. No skull that is this old has ever been found in such good condition. Second, the scientists are not sure how to classify it because it possesses features that are characteristic of several previously identified hominid species. This means that the skull is either a hybrid of several different species or that those groups are not really different species at all and the differences among them simply reflects individual variation. In support of the latter hypothesis, other skulls have been found in the same area over the years that are from a similar time period and show considerable variability when compared to each other.  
The most likely explanation is that all these individuals are members of a single population of a single species. If that is true, it would suggest that many of the hominid fossils that have been found in Africa and Europe are members of the same species and not of separate species as they have been classified in the past. This is a common problem in paleontology: whenever multiple individual fossils are discovered that are not exactly like each other, it is hard to tell if they are members of the same species. Sometimes, two individuals that are considerably different from each other are placed in separate species only to be reassigned to the same species once more specimens are found that share characteristics of both groups.

Tuesday, July 30, 2013

The puzzle of monogamy

This article from Discovery News compares two recent studies that try to understand the evolution of The first one from Opie, et al. at University College in London looked at the primate family tree and used statistical models to show that appearance of monogamy coincided frequently with the appearance of infanticide. In this case, infanticide refers to the practice of males who kill the offspring of females that they have not mated with. This strategy evolved because females of many species will immediately become fertile if the young they are caring for are killed. Thus, the males create a mating opportunity for themselves and eliminate the offspring of a rival male. Of course a male that engages in infanticide runs the risk of losing his own offspring to another male, so monogamy may have evolved as a way for fathers to protect their children from other males.

The second study by Lukas and Clutton-Brock from the University of Cambridge is similar but it looked at monogamy across all mammalian species. But they did not find a correlation between monogamy and infanticide. Instead, their analysis suggested that monogamy evolved as a consequence of females living in solitary territories. Males in this situation have to defend a large area if they want to mate with multiple females and prevent other males from mating with their partners. Presumably then monogamy evolved because it was more efficient to only defend one female against rival suitors.

Monogamy is an interesting puzzle in evolutionary biology, because on the surface it seems like a bad idea. If the name of the game is to get your genes copied into the next generation then you would want to mate with as many partners as possible. This only applies to males though because females are usually limited by the number of eggs they can produce or the number of offspring they can gestate at one time in the case of mammals. Many hypotheses have been proposed over the years to try explain the phenomenon. For example, it was once assumed that monogamy evolved for the benefit of the offspring. Having two adults looking after you has obvious benefits for survival over just one. However, even though these two studies do not agree on what was the original function of monogamy, they both failed to support the paternal care hypothesis. They showed that fathers started actively caring for their offspring after monogamy evolved which suggests that paternal care may have appeared as a side benefit of a strategy that originally only benefited the fathers.

Thursday, July 25, 2013

False memories implanted in mouse brains

'Total Recall' for Mice | Science/AAAS | News

A very cool study out of MIT showing that it is possible to create false memories in a mouse brain. They first put the mice in a chamber and let them form a memory of that location. Then they put the mice in a different chamber and gave them painful electric shocks while simultaneously stimulating the same neurons in the brain that had been activated in the first chamber. Then, when the mice were put back into the first chamber, they froze as if they were recalling the memory of the electrical shock. They way they identified the neurons associated with the memory and reactivated them later is an impressive technological feat in and of itself. The mice were genetically modified to express a protein called channelrhodopsin-2 which was only expressed in active neurons. Essentially , this protein acted as a tag for activated neurons which means it labels the neurons that are involved in storing the initial memory. This protein is also light-sensitive and it causes the neuron it is expressed in to become active when exposed to certain colors of light. So, before being placed in the shock chamber, they put a fiber optic cable into the brains of the mice which would shine light onto the parts of the brain where the memory was stored and activated them. This is an example of a new field called optogenetics which is allowing lots of exciting new questions to be asked about brain activity. The Science article discusses the possibility of understanding how false memories are formed in the human brain, but the ability to create and manipulate memories in the brain may someday be a way to treat lots of psychological problems such as PTSD.

Wednesday, July 24, 2013

Yes, dogs can see in color.

Dogs See the World in Living Color - D-brief |
This study from the Russian Academy of Sciences should put to rest the old myth that dogs can only see in black and white. Scientists already knew this because dogs have two different kinds of cone receptors in their retinas which means they can differentiate between at least two wavelengths of light. Humans, of course, have three kinds of cones, so dogs are not as good as us at color vision, but we are well behind some other animals like the mantis shrimp for example which has up to 16 kinds of cones. So dogs essentially have about the same degree of color vision as a colorblind human. The term 'colorblind' is probably the reason why people misunderstand this condition. It does not mean the inability to see any color; it is just means an insensitivity to certain colors. In the case of dogs, they cannot see red or orange parts of the spectrum. They have no problem with green, blue, and yellow, however. The study described in the article above shows that dogs not only are capable of discerning color but actually use color cues when making decisions; even more so than other cues like brightness. The study design ingeniously simple: the dogs were trained to associate a reward with a cue that differed from non-reward cue in both color and brightness (dark yellow vs. light blue, or light yellow vs. dark blue). Then the cues were switched: if the dog had learned that dark yellow was the rewarded cue, for example, it was given the choice between light yellow and dark blue. If brightness were a more salient feature, the dog would look for a reward next to the dark blue cue even though it had associated a reward with dark yellow previously. However, 70% of the time, the dogs chose the cue that matched the color that they had seen before (i.e. if they had learned that dark yellow was associated with reward in the training trials, they chose light yellow in the test trials). Of course it is no surprise that dogs are not as good color vision as much as humans. Dogs rely on other senses such as scent much more than humans.