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Sometimes men wake up with an erection in the morning. Why does this happen? Shortly speaking: REM (Rapid Eye Movement) is one phase of sleep. During this phase we dream and some of our neurotransmitter are shut off. This include norepinephrine, which is involved in controlling erection. Norepinephrine prevents blood to enter the penis (preventing the ...


12

The female stood with the tail held sharply to one side, and the quills on the back lying very flat. The male stood on his hind legs, while the front legs grasped the sides of the female. There was no repetition of the act. The male's urethra is 115-120 mm long, and his penis is 75 mm, so the he doesn't need to be as close to the female as one ...


11

Women have erections too! These erections are called clitoral erection. These erections are usually accompanied by vaginal lubrification. Just like men, the absence of norepinephrine during the REM phase of the sleep causes erections. In women, this phenomenon is called Nocturnal clitoral tumescence while it is called Nocturnal penile tumescence in men. In ...


10

As both @Rory M and @Alexander Galkin suggest, there are various non-visual mating behaviors to allow these species to select mates and also allow taxonomists and researchers to identify these species. And they hit on the two major ones, courtship rituals (mating calls, throat bulging, dancing) and pheromones. Let's have a look at some two examples: The ...


5

There is absolutely no way to tell to be honest because mulberry trees have the capability of changing sex (reference). From a paper titled "Diversification of mulberry (Morus indica var. S36), a vegetatively propagated tree species", I quote The sex expression of plants appears to be a function of hormonal control; there seems to be evidence that ...


5

Of course. There's several fertility tests available to determine the quality and quantity of egg reserve, and whether or not the there are abnormalities in the uterus. However, some of them are invasive procedures, and all of them should be performed by a professional. I doubt the layperson would know what they're looking at on an Ovarian Ultrasound. If ...


5

It's probably around 7 cm for men and 6 cm for women. The Evolution of Adult Height in Europe, which is a source for some of the statistics reported on that Wikipedia page, gives averages, standard deviations, and sample sizes for both sexes across 10 european countries (and across a number of different age cohorts). Taking their numbers for men and women ...


4

I remember overhearing a botanist some 10 years ago, that he cannot tell male poplar from female when they are not flowering or producing seeds. Poplars and willows are related, so it is a weak indication for you, you will likely not find any visible features in willow neither. But it is an old information, too. (Are you interested in DNA sequencing ...


4

In principle, Meiosis is only the process in which the haploid egg or sperm are generated. Have a look at this figure, which shows Meiosis I and II (from the Wikipedia): During Meiosis I homologous recombination between homologous chromosomes can happen, the chromosomes are then distributed normally among the daughter cells. In Meiosis II the cells split ...


3

I will be able to answer only a part of the question. Sex determination in Zea Mays Initially, all the flowers in the plant are perfect, ie they have both the male and the female flowers. This is followed by abortion of stamen or pistil resulting in unisexual flowers. Which part of the flower undergoes abortion is determined according to the location of ...


3

Sex (depending on how you define it), evolved very early (more than a billion years ago). Even species like yeast have a type of "sex" (see below). Also, this doesn't mean there is necessarily a big difference between the two sexes. The yeast Saccharomyces cerevisiae, for example, also has two "sexes", called mating types. One is called alpha and the other ...


3

r/K selection has become more of an heuristic for characterizing a species' strategy of reproduction and less a matter of predicting what sort of species will emerge from a given environment as it had originally been proposed by MacArthur and Wilson in 1967. The paper you cite tries to determine whether rotifers become more r or more K selective in different ...


3

I can only offer a partial answer on the theoretical aspects. I don't know if you are familiar with the mid-90s papers by Otto et al. (Otto & Goldstein,1992, Otto & Marks, 1996), but these are definately relevant to your question. They deal with the "masking hypothesis" of diploidy, i.e. that deleterious mutations can be masked by "healthy" alleles, ...


3

This is a little tricky. First of all lets be clear about 'bringing together favorable alleles' (or any alleles) represented by site mutations on 2 chromosomes: --------A------------------ X ------------------B-------- If the two dashed lines are two copies of the same chromosome, then a recombination event at X may produce: ...


3

Interestingly it does seem as if this is at least in part the case. Testosterone levels are tied to sexual impulses, but also risky behavior and aggression. Its responsible for sexual drive in women as well. Low testosterone can cause depression and loss of the ability to make decisions. You'll note that the work cited is done on criminals, patients ...


3

Shuster, SM & C Sassaman (1997) Genetic interaction between male mating strategy and sex ratio in a marine isopod. Nature 388: 373-377 As described in this paper, the chromosomal system of sex determination in Paracerceis sculpta is ZW=females, ZZ=males. Genetic evidence indicates that the morph of a ZZ male is determined by a second locus, Ams ...


3

Cases of homosexuality have been described in numerous species, including primates, and the wikipedia link posted by @shigeta lists and describes some of them. It could be difficult to untangle the difference between an animal not being able to identify sexes very well, or, deliberately mating to anyone and those actively making a choice to engage in ...


3

If you are talking potential for fertility. There are a few... Having regular periods Healthy weight. Extremely thin women tend to stop ovulating. Obesity also impairs fertility by altering hormone levels. Whether she has been using birth control (i.e., "the pill") in the recent past. It takes a while for the body to get back to normal. Smoking impairs ...


3

Does Oxytocin increase more so for women than for men after sex? Answer: Yes, Oxytocin level shoot up for female after sexual. It is highest at orgasm, which is the main cause women could have multiple orgasm. Someone told me that it increases 4-fold for men and 12-fold for women (unconfirmed). Answer: The level of oxytocin actually increase 5 ...


3

Could not fit in a comment… I don't quite know how to answer. I think that the only answer one could give is why not?. Various mechanisms may evolve under different life-history, different environment, different ecological strategy, different mutations occuring,.. In some species (including some amphibians) the evolution of sex determination system is ...


2

In most species, females are pregnant as often as they can be. Since the availability of sex is rarely a limiting factor, it seems likely that female bonobos are as "always" pregnant as other chimps and socially living primates. The actual rate of pregnancy will be hormonally limited by things such as reduced fertility during lactation and so on.


2

The short answer is that in dioecious plants the sex of an individual plant is determined by its chromosomes. Several familiar crop plants are dioecious: asparagus, kiwi, hop, papaya, spinach and yam. The evidence indicates that dioecy has evolved independently in different plant families. I refer you to Matsunuga & Kawano (2001) Sex determination ...


2

My hypothesis is "Yes", that is "Early human beings are far more promiscuous than we are now". So how i came up this? Cos Human came from Chimpanzee right? No. Also, when human evolved, they started to realized that promiscuousness wss not healthy & not good for society I'm sorry, what ancient society are you claiming cracked down on ...


2

Generally, all diploid species pass through a haploid phase in their life. This is called the alternation of generations and the cycle may be presented like this: Commonly we see organisms that spend most of their life in the diploid phase, with greatly reduced haploid phase (e.g. humans "live" in haploid state only as gametes). However, this is not the ...


2

This is not on theoretical grounds, but here is an existence proof: a number of bacteria, which reproduce asexually, are polyploid. Here is a blog I really like informally discussing the concept of ploidy in bacteria. The example I am familiar with is cyanobacteria, which can have 3-4 all the way up to 142 copies of its genome, according to this paper (1). ...


2

Many organisms have multiple sexes. For instance, here is one which apparently has 7 (and each sex, or "mating type" can only mate with the others). The "gotcha" is that, mostly, they're unicellular organisms where mating in itself is weird to begin with. I haven't ever heard of a metazoan which truly has more than one sex (there may be variation within the ...


2

Hermaphrodites and gender changing (M->F F->M) fish I have heard of. There are also occasionally individuals who are neither male or female, which may or may not count here. But 3+ separate genders I haven't seen except for fungi, which might occasionally be multicellular, but probably not what you are thinking of. Fungal genders are so many because they ...


2

In birds and reptiles females are the heterogametic sex with allosome configuration ZW. However it has been reported that a homogametic WW female boa was born by parthenogenesis. So it means that although W is shorter than Z, it can still support life. So I guess that ZW system in general exchanges more sex-limited genetic material than XY system. Platypus ...


2

I'm going to give a tentative answer full of speculation and guesswork, but it's too long to fit in a comment so here goes. Sex duration is possibly a sort of human reproductive handicap. Sexual arousal can soften the cervix, increasing fertility during lengthy sex sessions. Lengthy sex exposes them couple to predators and takes time and energy, so if you ...


2

There are plenty of animals with brief procreative periods, as you suggest. In chimps, sex takes 10 to 15 seconds - much less time than humans. So the question is really why have humans evolved to be different? I don't think any of the answers based on fitness are particularly compelling. It's not like sex feels like running a marathon or less physically ...



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