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20

Sexes (male and female) are generally defined in terms of Anisogamy, which means that there are size differences between the gametes (i.e. the reproductive cells that fuse at fertilization). The sex with smaller gametes is defined as male and the sex with larger gametes is defined as female (individuals that can produce both types of gametes are called ...


16

Very short answer: No! You might want to have a look to the wikipedia page on sex-determination systems. Diversity among the species that reproduce sexually Not all species that have sexual reproduction have genders. Yeasts for example have mating types but no gender. Diversity among the species that have genders Sex is determined by both genetic and ...


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 ...


12

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 ...


11

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 ...


7

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 ...


7

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 ...


7

No. There are many sex-determination systems. Mammals and fruitflies use the XX/XY sex-determination system — except for the platypus, which has 10 sex chromosomes. ZW sex-determination system is used by birds and some reptiles. It's similar but with the male having two of the same chromosome (ZZ) and the female being the heterogametic sex (ZW). There's ...


5

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 ...


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

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

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 ...


5

Males and females mosquitoes are morphologically quite different. The majority of individuals you see are females. The males are not attracted to humans and so we see them less often. The main obvious difference is the size of the antenna. When you see the large feather-like antennas of a male, you'll have no doubt; it is a male. The male also tend to be ...


4

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, ...


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

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 ...


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 ...


4

So if you're interested in the detailed anatomy of the glans penis, I recommend this as nice article to go to after you've checked general references. Opening two sentences are worth quoting here: The human glans penis is covered by stratified squamous epithelium and a dense layer of connective tissue equivalent to the dermis of typical skin. Rete ...


4

It's more than just sperm count that matters for a pregnancy if you are considering normal methods of insemination such as intercourse or assisted methods such as artificial/intrauterine insemination or in vitro fertilization. For couples having trouble getting pregnant, most OB-GYNs and/or Reproductive Endocrinologists/Infertility physicians do a semen ...


4

Barnard's wonderful text "Animal Behavior: Mechanism, Development, Function and Evolution" offers some enlightening summary to the discussion of homosexuality in a biological context! Brief Summary to ch 12, page 606: What we know so far: homosexual behavior is recorded all throughout animal kingdom there are evidence for genetic basis to ...


3

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 ...


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 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

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

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

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

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 ...


3

The sphincter of the bladder is under voluntary control (that's what toilet training is about). That's why you can hold your urine even when the stretched bladder is sending messages to your brain that you really have to go. That's not to say there aren't involuntary effects on the bladder during orgasm. Orgasm is a separate entity. It is a complex ...



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