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38

Number of legs in terrestrial vertebrates Not only do mammals have four legs but actually all terrestrial vertebrates (which include mammals) have four legs. There are slight exceptions though of lineages that have lost their legs. Typically snakes have no legs anymore. Apesteguia and Zaher (2006) discuss the evolution of snakes legs reduction and report a ...


26

I'll focus on whales and dolphins (cetaceans) as you mention them by name and they are representative for other marine mammals such as seals or manatees. The evolution of cetaceans was one of the fascinating evolutionary mysteries. Clearly, they were mammals, but which mammals were their closest relatives? Clues to solve this mystery began to appear in the ...


23

1. Sheep are fearless 2. English common names are misleading when it comes to the genetic differences between goats and sheep You posted a picture of Mountain Goats (Oreamnos americanus), which are a different genus than Domestic Goats (Capra aegagrus). Both Capra and Oreamnos are members of the Subfamily Caprinae, as are Domestic Sheep (Ovis aries). ...


15

For humans, our blood and other tissues have a salinity which is less than that of sea water. There is an old chestnut that our blood has a comparable salinity to sea water - its just a classic example of textbook error. The major reason we need to drink and excrete (sweat, breath and urine) water is to rid the system of waste- i.e. water soluble chemicals ...


15

There is some evidence that fetal development under zero gravity conditions might be problematic. Wakayama S, Kawahara Y, Li C, Yamagata K, Yuge L, et al. (2009) Detrimental Effects of Microgravity on Mouse Preimplantation Development In Vitro. PLoS ONE 4(8): e6753. doi:10.1371/journal.pone.0006753 The paper is here. These authors studied aspects of ...


9

In mammals there are only two species (known, there may well be others) where males lactate. They are both species of bats and this paper discusses the evolutionary mechanisms that could underlie male lactation. I have ignored human cases because they are more of an unusual occurrence (often brought on by severe dietary stress) rather than an evolved ...


8

You should also bear in mind that the fact that they are great climbers does not make them fearless. For example, if I were to find myself floating 500 meters above the ground, I would be terrified. The fact that birds do not appear to be scared in the same situation does not make them fearless, it just makes them fliers. Similarly, I am sure a fish would ...


8

Among the great apes, chimpanzees and gorillas live in very hierarchical, male-dominated clans that are often in violent conflict with other clans. Bonobos, on the other hand, lead very peaceful lives, and are female-dominated, using sexual contact as a manner of communication to reduce tension within and between groups. Orangutans are largely solitary ...


8

At the very least, I know that male primates also have nipples like female, though they are very close relatives to human. On the other hand, in some of my dissection labs, I noticed that male pigs also have nipples just like the female ones. It seems to be the case that most male mammals have nipples, which probably has to do with mammals being ...


7

Well, technically yes, but most adult dolphins do not have hair: Unlike most mammals, dolphins do not have hair, except for a few hairs around the tip of their rostrum (beak) which they lose shortly before or after birth. The only exception to this is the Boto river dolphin, which has persistent small hairs on the rostrum. Whales do, and depending on ...


7

I know of an ER doctor who had a patient who removed (by biting and pulling) every last part of her tongue, she survived that (although, her ability to speak was probably shot). The stump of her tongue clotted before she could even go into shock (at least where it was really serious). Here is a quote from the book Angels in the ER (this is a paramedic ...


7

It is probably true that toilets and other resting-ish area are always a great place to think about biology, I agree $\ddot \smile$. In short, urine contains the waste from our blood while defecation is just the stuff that we haven't digested. Kidneys are the organs responsible for draining wastes (mostly nitrogen-containing, or nitrogenous, wastes) from ...


7

Until someone has a better method to determine this, and I'm willing to go with idea of that this question is not answerable, I put forward the two likely forms of selection criteria: Which mammal has the highest percentage of keratin in it's body for the highest density, and which has the highest percentage of gas (over the day) for the lowest. I also ...


7

How did they evolve from their original form to their superficially ichthyoid appearance today? This is an example of convergent evolution. Fish appear as they do (streamlined body shape, wide tail, fins, etc.) since these are adaptations to the underwater environment they're living and evolving in. These features are only "ichthyoid" or "fishy" because ...


7

I think I might interpret your question as asking, not just why don't mammals have more than four limbs, but why arthropods have more variety. Insects have six, but others have eight, ten, or more. Partly there are just many more species of arthropods. 80% of animal species are some sort of arthropod, and some lineages of arthropods are distantly related ...


6

I don't think, there is a precise answer about the evolutionary mechanisms, but "mechanically": mammals have principally just two types of pigments: eumelanin and pheomelanin, both of which have their color variants, but within a known range. Bird pigments, besides melanins, include carotenoids and porphyrins. Arthropods generally have carotenoids, ...


6

It sounds pretty much like a fox squirrel (or better: as masked face fox squirrel which are found in Alabama). They look like this (taken from this website): There seems to be quite some variation in terms of fur color, some animals are more greyish: Taken from here, this website also contains some additional information.


6

The answer by Remi.B is excellent, I'll just attempt an explanation by way of gene networks: In genetics we see new genes "linking" to the older genome by regulation pathways and by being "fit" only in the context of the existing genome. This has the effect of making the older genes indispensable. Change them and you rupture the whole mesh. If you want to ...


5

Most mammalian males have nipples. The duck-billed platypus does not have nipples but you begin to see development of nipples in marsupials (Park and Lindberg 2004) like the opossum and kangaroo. Development of a complete nipple begins in the eutherian (placental) mammals. The mammary glands develop early in the embryo along a pair of ridges called the ...


5

The number of mammary glands a species has is related to litter size. The relationship generally follows the "one-half rule," which states that the average litter size is equal to half the number of mammaries. The number of mammaries also tends to put an upper limit on litter size. It's not necessarily a hard limit, but survival tends to drop noticeably when ...


5

Because of that, I assume botulinum toxin is more dangerous to humans than many animals. Couldn't find too many examples but, there are some things to consider (according to [1]): there are seven distinct types of toxin with variable action among animals different dose / effect intensity ratio between toxin types toxins A, B, E and F cause disease in ...


4

I'm wondering if it is safe to assume that the approximate number of cells per unit mass in a mammal will remain fairly constant throughout its lifespan. Not exactly. When a tissue is put under stress, it can respond in four main ways: Hypertrophy - individual cells get larger. E.g. stressed muscle cells get bigger. Atrophy - invidivual cells get ...


4

For what concerns amino acids, mice rapidly reject meals that are not balanced in essential amino acids and continue to look for other kind of foods. This behavior is called aversion response and it is an adaptive phenomena that can be observed already 20 minutes after exposure to the unbalanced food. The mechanism involves brain sensing of uncharged tRNAs. ...


4

From Wikipedia: Sloths go to the ground to urinate and defecate about once a week, digging a hole and covering it afterwards. They go to the same spot each time and are vulnerable to predation while doing so. The reason for this risky behaviour is unknown, although some believe that it is to avoid making noise while defecating from up high that would ...


4

GULO encodes L-gulonolactone oxidase which catalyses a step in the biosynthetic pathway to ascorbic acid (vitamin C). I did a BLAST search at NCBI using the mouse GULO protein as probe, excluding mammals, and I saw highly significant hits in reptiles, birds, sea urchins, fungi and bacteria. I conclude that this enzyme, (or very close relatives) is ...


3

These poisons prevent recirculation of vitamin K and thus formation of prothrombin which is essential for coagulation of blood and are required for maintaining the integrity of capillaries. The depletion of vitamin K is slow and in a couple of days internal hemorrhage occurs extensively and the animal dies of shock. Early recognition of poisoning in humans ...


3

As it happens someone has just published a theory about this. To save you following the link, I reproduce the abstract below. I must admit that I had never realised that lactose is synthesised within the organelles of the secretory pathway (β4-galactosyltransferase is a Golgi enzyme and α-lactalbumin is, of course, a whey protein, so is in transit through ...


3

Whether using quantitative models, or "animal models", I think this is a useful quote to keep in mind: A model is a lie that helps you see the truth. -- Howard Skipper As for evidence that using mice models for human nutrition is justified -- I believe there has been a good deal of research that has provided useful insight on the influence of ...


3

Bats distinguish day and night the same way that other animals do, with an internal circadian clock and by environmental cues (dawn and dusk). Most bats (over 1000 different species total) are nocturnal, meaning that they are out of their roost and foraging at night. This includes that vast majority of bats, which also echolocate. Some bats, notably ...


3

This is not my field so I am sure there are other examples, but certain neurons will definitely be larger in adulthood than in infancy. There are motor neurons that connect the spine to, for example, the toes. These will grow in length as an animal grows. So, in a human infant they will be a few centimeters long and can reach lengths of over a meter in an ...



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