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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 as some lineages have lost their legs. Typically snakes have no legs anymore. Apesteguia and Zaher (2006) discuss the evolution of snakes legs reduction and report a ...
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Short answer
Technically, only mammals lactate. To lactate means to produce milk from the mammaries to feed a baby or young animal. Milk, in turn, is defined as the secretions from mammary glands. However, there are animals other than mammals that produce milk-like substances to feed their young.
Background
Both male and female pigeons (Columba livia) ...
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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 ...
29
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).
...
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For humans, our blood and other tissues have a salinity which is less than that of sea water. The osmolarity of blood is 275-299mOsm/L, while the osmolarity of seawater is ~1000 mOsm/L.
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 or heat we do not need. The ...
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Producing milk, per se, is a strictly mammalian phenomenon. There are functionally similar processes in non-mammals (e.g., 'crop milk', produced by some birds), but because these processes evolved independently from the evolution of mammalian milk, they are not considered to truly be milk.
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The average domesticated dairy cow produces far more milk than would be required to feed their calf. All cows, wild and domesticated, will only lactate in the period between their calf's birth and weaning. Milk is calf-food, and when there's no calf, there's no evolutionary advantage in producing milk.
On dairy farms, cows are milked twice daily, from ...
25
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 ...
24
I'm almost certain that your question is based on the press that Patricia J Yang's research is receiving (e.g., here and here).
Yang and her co-authors examined the structure and mechanics of some dead wombats to investigate this question further. They found that varying degrees of pressure in the latter portion of the wombat's intestines (in conjunction ...
answered Nov 19 '18 at 22:43
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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 ...
15
It is probably true that toilets and other resting-ish area are always a great place to think about biology, I agree $\ddot \smile$.
Why do we urinate?
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 ...
15
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 ...
14
Male mice lack nipples too. Mice are frequently used for embryonic research as they are small and reproduce quickly. It is thought that male mice do develop nipples, but that they regress during development (Wysolmerski, 1998).
In general, it is thought that mammalian organisms develop as females by default when there is no male (Y) chromosome present (...
13
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 ...
13
I agree with you that the question is ambiguous, and also that the most sensible answer would be C. However, one could make a more or less reasonable argument in favor of several other answers, too.
a. The common ancestor of whales and fish possessed genes for fins.
Technically, this statement is true. At least some of the fins of whales and fish are ...
12
It's probably a dugong, based on the location, lack of a dorsal fin, split tail, lack of a blowhole, and narrowing of the snout.
The prominent vertebral column looks unusual, but that might come from the strange posture it is being held in or from malnutrition.
This publication from the Australian government shows necropsy of several dugongs (note, this ...
11
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 ...
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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 ...
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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 ...
11
Tree jumping.
The obvious answer is that having a balancing ballast is incredibly handy for some of the death defying jumps squirrels perform. The tail is needed for that.
A big bushy tail might be a good counter to predation.
You make the assumption that an inconspicuous tail would be bad for predation. I would challenge that assumption, but I am making ...
10
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, melanins ...
10
Dairy cows are bred, or selected to give milk. So they do produce excessively. The normal bovine wild type, like other mammals, not only produce less milk, but also will tend to stop lactating when the calf is not nursing.
I would be interested to know if dairy cows stop giving milk if they are not milked every day. I understand they can get very ...
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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 ...
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This is a white morph red fox, not an arctic fox.
As noted in the question, this fox has larger, more pointy ears than an arctic fox, and the second picture shows it to have a longer muzzle as well.
Another clue is that these pictures were taken in the springtime or early summer (which you can tell from the new growth on the plants in the picture), and ...
10
Sloths have long, thick, sticky tongues covered in a carpet of tiny, rear-ward pointing spikes that they can pull leaves in with.
So the tongues are quite different from human tongues and likely much less "soft" to touch and more "rough".
According to scanning electron microscopy studies on the topography of a sloth's tongue, the following results were ...
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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 breast-...
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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 ...
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The answer really depends on what aspect of the urine and feces one is considering.
On the atomic level, no, urine and feces are composed entirely of atoms taken from our environment. As one would expect, as there is no "Humanium" on the periodic table. In fact, all the atoms in urine and feces were originally created by stars.
On the molecular level, yes. ...
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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 ...
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