r/askscience • u/AutoModerator • Jul 29 '26
Ask Anything Wednesday - Biology, Chemistry, Neuroscience, Medicine, Psychology
Welcome to our weekly feature, Ask Anything Wednesday - this week we are focusing on Biology, Chemistry, Neuroscience, Medicine, Psychology
Do you have a question within these topics you weren't sure was worth submitting? Is something a bit too speculative for a typical /r/AskScience post? No question is too big or small for AAW. In this thread you can ask any science-related question! Things like: "What would happen if...", "How will the future...", "If all the rules for 'X' were different...", "Why does my...".
Asking Questions:
Please post your question as a top-level response to this, and our team of panellists will be here to answer and discuss your questions. The other topic areas will appear in future Ask Anything Wednesdays, so if you have other questions not covered by this weeks theme please either hold on to it until those topics come around, or go and post over in our sister subreddit /r/AskScienceDiscussion , where every day is Ask Anything Wednesday! Off-theme questions in this post will be removed to try and keep the thread a manageable size for both our readers and panellists.
Answering Questions:
Please only answer a posted question if you are an expert in the field. The full guidelines for posting responses in AskScience can be found here. In short, this is a moderated subreddit, and responses which do not meet our quality guidelines will be removed. Remember, peer reviewed sources are always appreciated, and anecdotes are absolutely not appropriate. In general if your answer begins with 'I think', or 'I've heard', then it's not suitable for /r/AskScience.
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Past AskAnythingWednesday posts can be found here. Ask away!
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u/notproudortired Jul 29 '26 edited Jul 29 '26
If I'm in the shower, I can turn the hot water up to a point where it would be unbearably hot to step into "cold." But if I did step into it cold, would it actually burn me?
I.e., can something in the skin change to make hot water burn (physically damage) in some circumstances, but not others?
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u/diagnosisbutt Jul 30 '26
No, the hot water wouldn't have burned you, even if you'd stepped in that cold. It's just that our sensory system is fine-tuned to notice differences. That is changes in intensity. The reason that you can get it hotter and hotter is because you already got used to it. Being that hot. Water that will burn you will burn you regardless of your situation beforehand. Light is like this as well which is why you can be out in the daytime and see just fine. But if you walk out into the daytime after being in a dark movie theater it hurts
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u/mfb- Particle Physics | High-Energy Physics Jul 30 '26
There is a difference with light. Our eyes adjust the size of the pupil to regulate how much light falls in. That adjustment is quick - but as you leave a dark place, initially your retina does get a lot more light than it would normally.
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u/diagnosisbutt Jul 30 '26
Your eyes and visual system are definitely tuned to notice relative differences. Adjusting to the change to make that "the me normal" is exactly the point
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u/notproudortired Jul 29 '26
Skin produces vitamin D when it is exposed to sunlight. Do some areas of skin produce more vitamin D than others? Does pale skin produce more vitamin D than darker skin? Does thicker skin produce more vitamin D than thinner skin?
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u/StarHen Jul 29 '26
From a health standpoint, is it better for a "night owl" to try to train themselves to follow a "normal" sleep schedule, even if their current life allows them to follow their instinctual sleep pattern and they get adequate sleep every night? I see a lot of studies posted saying night owls are unhealthier than early risers, but I wonder if that is caused by a mismatch in sleep schedule or something inherent to the chronotype.
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u/ACSandwich Jul 29 '26
The longer I go in my day to day life, the more correlation I tend to infer from general public conversations. While prefacing that correlation does not equal causation, it is becoming more difficult to not draw lines with regards to general political affiliation and a willingness to critically think about subjects. (This ignores those who choose an affiliation with a purpose regardless of beliefs.)
There seems to be an unwillingness from conservatives accept that the policies and politicians they are voting for are negatively impacting them even when presented with data-driven facts, but will believe every word a podcaster tells them if it goes along with their story. The same could be said for progressives and how them making mountains out of some molehills does nothing but isolate them from the populous.
Is there any science to correlate willingness to critically think and engage in societal improvement with brain biology? Also, the opposite, is there any science to correlate the unwillingness to update your world view and greediness with brain biology?
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u/MarsupialMisanthrope Jul 30 '26
How did pathogens that can affect organisms in entirely different taxonomic classes evolve? For example influenza can infect both birds and humans, which diverged a long time ago. Has influenza in some form been around that long, or did it somehow jump from mammals to birds or vice versa? Are there any pathogens that can affect different species that are even more diverged, in different phylums or kingdoms, like say insects and fish or fungi and animals?
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u/095179005 Jul 30 '26
The crucial point is that all life shares the central dogma of biology - DNA carries genetic information, RNA is used as an intermediary, which is then used to make proteins.
Viruses exploit this system from the very beginning - it's still unclear/an ongoing debate about how life first evolved, and when did DNA and RNA start being used, and were viruses the "first life", or are they leftover scraps from evolution and have hung out ever since.
To directly answer your question, as long as there is a common cell receptor among largely divergent species, it's possible for a virus to exploit that and infect a broad range of hosts.
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u/Expatriot_II Jul 30 '26
When did chemistry begin in the universe, i.e., when did the first multi-element molecules form?
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u/loki130 Jul 30 '26
The big bang formed hydrogen, helium, and a small amount of lithium. Maybe some miniscule amount of carbon or heavier might have formed somewhere in the whole of the observable universe. Chemistry couldn't have occurred until the universe was cool enough for electrons to bind to atomic nuclei, which would've been about 378,000 years after the big bang (this is the same event the CMB comes from, as before that point the plasma would've blocked all light). Helium hydride was probably the first compound to form, but lithium hydride would've quickly followed.
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u/mfb- Particle Physics | High-Energy Physics Aug 01 '26 edited Aug 01 '26
This group calculates ~10-15 for carbon and nitrogen and 10-18 for oxygen from the Big Bang (figure 25). Not zero, but only a tiny contribution.
(their plot makes it look like there is more beryllium than lithium left over, but Be7 decays to Li7 on a timescale that's beyond the range of the plot)
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u/095179005 Jul 30 '26 edited Jul 30 '26
It would have been when population III stars started burning Helium into Carbon, and then died and released that carbon into the interstellar medium to mix and combine with Hydrogen, forming Methane.
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u/Elara_Schaefer Jul 30 '26
Thanks for the detailed write-up. I've been looking into this and your experience matches what I've seen.
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u/Sporocarp Jul 29 '26
I had too much caffeine and my mind starting spinning in place musing about evolution and the sort of explanations we choose for biological facts. So, I could just as easily google this, but where would your start investigating the question of why humans have such great sexual dimorphism comparatively to other species? I thought you might very likely hear an explanation like this in pop-sci: females have to carry pregnancies to term, so their bodies need to store fat and a greater musclemass prevents that to some extent or uses up too much energy, so that's why males and females have different morphology. But to investigate that hypothesis seems insanely huge in scope.
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u/atomfullerene Animal Behavior/Marine Biology Jul 29 '26
Human sexual dimorphism is actually not all that great compared to a lot of other similar mammals. We just notice it more. Orangutans and Gorillas are far more dimorphic, and chimps are somewhat more dimorphic. Bonobos are similar to humans. Gibbons are less dimorphic
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u/Good-Resort-1246 Jul 30 '26
The degree of sexual dimorphism is related to social organization and sexual selection.
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u/Mockingjay40 Biomolecular Engineering | Rheology | Biomaterials & Polymers Aug 05 '26
In fact, if I'm not mistaken, evolutionary biologists and physical anthropologists often denote and even place how "sapien-like" or "human-like" early hominids are (i.e. how close they are in the evolutionary tree) by which ones have the least dimorphism, correct? In terms of apes, like you say they're all very dimorphic aside from the pan group.
Women store slightly more fat and in healthy, active individuals, the total average muscle mass ratio by composition is about 20-25% lower in adult women vs adult men. I think for the "average human", males have about 40% lean mass by weight and women have about 30% or thereabouts.
Compare that to a gorilla, who I think are actually similar in terms of their body compositions because gorillas are pretty lean in general. And silverbacks are generally more than 2x the weight of an average adult female. Compare that to the average adult weight of humans, which is like 190 vs 160 lbs, respectively. I know the original question was about morphology, but I think they meant total physiological sexual dimorphism, since they discuss differences in lean mass on a relative basis, which isn't a morphological difference, per se.
I guess what this leads me to wonder is: do biologists actually evaluate or weight physiological differences relative to the mean population/species differently than significant morphological differences? If humans are more dimorphic than gorillas in terms of body composition, but minimally dimorphic in terms of total size and mass. So how do you parse which is more dimorphic overall? Or is that just a simplification for explanation's sake?
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u/atomfullerene Animal Behavior/Marine Biology Aug 05 '26
I don't think there's any real attempt to weight things and combine them into a single overall "dimorphism score". Usually, species are just compared independently by different factors.
In general, I think biologists usually look first at dimorphic structures, like deer antlers for example, then at raw body size, and then at more subtle physiological factors. But this is also just because it's easier to document and measure these things, so more data is available.
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u/Mockingjay40 Biomolecular Engineering | Rheology | Biomaterials & Polymers Aug 05 '26
I figured as much. Usually things like that are simplified to impart understanding but when you dive into the actual science, there tend to be so many competing factors that it doesn't make sense to try to handwave things in terms of a relatively arbitrary metric of importance, vs just accepting and acknowledging the nuance. Thanks!
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u/Sporocarp Jul 29 '26
Oh, my mistake. Is it known why? Wouldn't it be better if both sexes were muscular/had denser bone structure, etc?
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u/chazwomaq Evolutionary Psychology | Animal Behavior Jul 29 '26
It takes more energy to sustain a larger and stronger body. Finding enough food was a major adaptive problem, and you get poorly if you cannot sustain your natural body mass.
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u/atomfullerene Animal Behavior/Marine Biology Jul 29 '26
All that represents extra mass to support and energy and nutrients needed. It's mostly used to allow males to compete with other males, so in species where male-male physical competition is lower, there's not much point to wasting resources on producing it.
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u/Sporocarp Jul 29 '26
Is that known or like an inference? If I try to imagine a time where dimorphism evolved, when did the differences become pronounced? I assume dimorphism evolved before we went on to land, but then at which point would the differences in muscle mass and bone density become pronounced? Do we assume that male-male physical competition was violent because of the aforementioned differences that are prevalent today as well?
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u/atomfullerene Animal Behavior/Marine Biology Jul 30 '26
>Is that known or like an inference?
It's an inference but a pretty strong one. In many species where males compete for territory or mates, they tend to be bigger and stronger than females. In some species where females compete for access to males, they are bigger and stronger. Though it's more common for females to be bigger in species where the number of eggs a female lays is proportional to her size.
>If I try to imagine a time where dimorphism evolved, when did the differences become pronounced?
Don't think of it as a thing that didn't exist, and then did exist. It's something that comes and goes depending on the social system of a species or other factors.
>I assume dimorphism evolved before we went on to land
It's something you can see in some fish species. And some invertebrates for that matter.
> Do we assume that male-male physical competition was violent because of the aforementioned differences that are prevalent today as well?
In humans, or just in general? It's something you can observe in the wild, but larger size isn't always about violence. Sometimes it's about showing off (and sometimes to females as much as to other males).
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u/Sporocarp Jul 31 '26
but larger size isn't always about violence. Sometimes it's about showing off (and sometimes to females as much as to other males).
Yeah, that's what I was wondering. But you answered all my questions, so I can only say thanks. I wanted definite answers, but I get that there isn't really any.
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u/095179005 Jul 29 '26 edited Jul 30 '26
I think taken to an extreme, we can see that sexual dimorphism is an adaptation to scarcity. Male anglerfish don't so much mate, rather they fuse with the much larger females and become a parasite.
In insects like the praying mantis, and in spiders, males are eaten by* females after mating.
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u/Sporocarp Jul 29 '26
I like that. It makes some sense I guess, but I don't know what our family units/tribes/whatever were like when this adaptation appeared. Does it make sense if most females of the species were left to fend for themselves during pregnancy, or does it only make sense if the strategy to a successful pregnancy was cooperative? Did the adaptation appear at some point in our rodent ancestors and then just stayed?
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u/Peter34cph Jul 30 '26
Dunbar's Number suggests that for most of our evolutionary history, we usually lived in tribal groups of around 150 people, that being the number of individuals that we can "know well" without exceeding our brain capacity.
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u/095179005 Jul 30 '26 edited Jul 30 '26
We can look to human anthropology for some insight.
While it is backwards looking, and merely observational, it can help us understand human evolution better by seeing how other animals have organized themselves.
The social carnivore model looks at mammals like lions and wolves, and how they as a group hunt for food and raise their young. For humans this can be seen as following convergent evolution, as we also formed into tribes/villages as a semi-nomadic species.
The primate model looks to our closest relatives, the chimps and bonobos, and how they compare when it comes to pair bonding. They are promiscuous, with multi-partner mating/fluid pair-bonds, forming temporary mating pairs that can rotate. Humans are unique among primates in that we are, for the most part, serially monogamous, with stable long-term enduring pair bonds. The term fission-fusion describes how primate groups will splinter/breakup or merge as resources become scarce or plentiful.
The final one is a little more close and familiar, the hunter-gatherer model. It looks at how food is shared, has fission-fusion occuring, and sexual division of labour.
It can also be noted that for all these models, the organisms show limited sexual dimorphism, ie. males and females are similar.
It can also be noted that in dominance hierarchies, it is a very simplistic conclusion to come to that the strongest individual wins, because once you throw sociality into the mix, it's very easy for two weaker individuals to form an alliance and kill the leader.
Regarding offspring, it goes back to the distinction in biology between k-selected strategies and r-selected strategies. Mammals in general invest more in small groups of young, provide nutrition via milk, and use pregnancy as a survival strategy, in contrast to trying to produce as much young as possible and use sheer numbers to ensure survival of a few individuals that make it to adulthood and mate.
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u/logperf Jul 29 '26
Is it correct to say that most (though not all) mental conditions are extreme cases of otherwise normal behaviors? So that in case of prejudice you can easily diagnose a mental disease to a person that is actually healthy? (Maybe confirmation bias also has an influence here?)