At first glance, that might sound like a stupid question that is excluded by the very definition of ‘mammal’, but I’m talking about ‘mammal’ in the taxonomic, not everyday sense.
I suppose humans could almost be included, since we have found artificial substitutes for mother’s milk (let’s not get into an argument about their merits and pitfalls).
My sister’s guinea pigs gave birth the other day and that’s what started me thinking; unlike many other small mammals, infant guinea pigs are very quickly independent and very quickly weaned - I’m just wondering if there’s an animal species out there somewhere that is technically a mammal, but either produces offspring that can eat normal food, or that exclusively provides some other form of infant sustenance (such as pre-chewed or regurgitated food).
I believe the answer is No – all newborn mammals are nourished by milk for at least a short time before weaning. What may have led you astray is that IIRC the monotremes do not have mammae – nipples – but the females secrete milk from mammary glands that are modified sweat glands onto the belly fur, which is then lapped up by the infants.
No, I’m not confused about the monotremes; I was just wondering if there’s any mammal that has evolved away from the necessity of nursing - which wouldn’t stop them being mammals in the taxonomic tree (it’s hard to see how this would happen, but it’s not logically impossible) - in the same way that some bird species have become flightless, for example.
Keep in mind, however, that “flight” is not a diagnostic character for birds. So not flying doesn’t make one less a bird than being able to fly. For mammals, the taxonomic definition is pretty much the same as the “everyday” definition (in the sense that the two describe the same groups of animals). None to my knowledge have evolved to the point where nursing is completely unnecessary, though there are several which wean very quickly (hooded seals, for example, nurse for only about four days).
I’m sure the taxonomists would have a field day if such an animal were ever discovered, but there ain’t one. All mammals supply milk to their young; all mammals are designed to live on mother’s milk until weaning.
We can feed other species artificially as well as humans, but that means nothing to a taxonomist.
That’s a good question, and I supsect the answer would be “Yes, it would”. If it lost that ability, but still had hair and mammalian type teeth, then it might need a new classification. If it retained the ability to nurse, but didn’t use it, then I suspect it’d still be a mammal.
(the question was, if a mammal evolved to lose the ability to nurse, would that bump it out of the class Mammalia.)
I disagree, because such a creature would still be closer genetically to all the mammals than to anything else. Besides, if you declared such a critter not to be a mammal, you’d have to come up with a whole new class for it, coeval not only with Mammalia but with all the other vertebrate classes as well.
Some women can’t nurse their babies, but that doesn’t make them mammals any less…not to mention the three billion or so men who also can’t nurse.
I agree. I think such an animal would still be considered a mammal. It would probably be a new group of mammal, but losing only one characteristic of the group would not be sufficient to kick it out of the group, especially if its lineage is clearly mammalian.
However, this:
doesn’t really have anything to do with it. Classifications are based on the species level. Individuals who don’t meet the norm really don’t count. And, of course, NO mammalian male can nurse. To be really specific, you wouldn’t say that all mammals nurse their young, you’d say that all mammalian species have females that nurse their young.
Like what we’ve done to monotremes for failing to use a placenta? Maybe, but MAM seems to be the name of the group and thus THE defining characteristic. I’m quite likely wrong though.
Actually, the definitions require the presence of mammary glands, not the behavior of nursing. So guys still get to be mammals. And with the presence of the correct hormones, dudes CAN lactate though as a general rule they leave this up to the wimmin.
Well, we’re straying into GD territory here, not QG, but…
I still think it depends. If one, isolated species were discovered, you’d probably be right. How many species are in class by themselves? If a whole set of species were discovered, a new class migh be in order (no pun intended).
This will inevitably evolve into one of those discussions about whether dinosaurs are birds. If you follow the cladistic convention whereby a valid taxon must include a common ancestor and all of its descendants, all descendants of present-day mammals will be mammals until the end of time.
How does this work with respect to fossil mammals, Finch? I presume the defining characteristics of mammals must concern bone structure, so that we can classify fossils. Is there a belief that nursing evolved contemporaneously with whatever these skeletal features might be?
Just a thought. As far as evolving beyond the need to nourish with milk, please do not forget the immune system’s role here. Milk is ingested not only for calories, but also as a way of passing large protein molecules (antibodies). Without early ingestion of colostrum, failure of passive transfer often leads to death.
A mammal species that lost the ability to produce milk could only be removed from the class Mammalia if you followed Linnean classification, and even then it would be a subjective call by the classifier. Under more modern cladistic analysis, we would define mammals as something like “all descendents of the most recent common ancestor of Ornithorynchus and Rattus”, which would include any non-lactating descendents of any extant mammal.
Of course we aren’t sure exactly when lactation evolved, which is what makes it a lousy diagnostic characteristic for mammals, since lactation organs don’t get fossilized. We can be pretty sure that every true mammal since the Cretaceous period lactated, since marsupials, monotremes and placentals all lactate, it is an ancestral characteristic inherited from our most recent common ancestor.
But what about all the Permian mammal-like reptiles? Does lactation go back that far? We aren’t really sure that hair goes back that far. No one knows. And that’s why things like having three ear bones and one jaw bone are used as diagnostic characteristics of mammals, rather than mammary glands.
While mammals were originally named on the basis of their mammary glands, one key diagnostic character is actually the structure of the inner ear. Other identifying features include the structure of the jaw (which is closely tied with the structure of the inner ear), as well as the form of the teeth and dentition and other features of the skull (fusion of assorted bones from the ancestral condition, for example). So, assuming an animal had these features it would likely be classified as a mammal whether it was found to nurse its young or not (or, in the case of fossils, they are classified as mammals entirely based on these characters, since mammary glands and nursing behaviors don’t fossilize).
I’m with the people who say that a lineage that lost lactation would not be–or should not be–excluded from Mammalia. That sort of classification would be silly. Sensibly, we consider plant lineages that have lost the ability to photosynthesize to be plants.
Another interesting factual question is whether there are any mammals in which males lactate. There are certainly cases of large paternal investment (e.g., humans), so why not go all the way?
We’re chordates, despite having evolved beyond two of the three defining characteristics of Phylum Chordata – presence of a notochord (replaced by the backbone) and of pharyngeal gill slits (since we’re not aquatic, their function has been assumed by the lungs and they occur only briefly in the embryo, if at all). Nonetheless, we are derived from forms which possessed a notochord and pharyngeal gills, so we are validly part of Chordata.
Likewise an animal descended from mammals which had ceased nursing its young because of their highly precocial development at birth would remain a mammal, since it would be descended from mammals which exhibit the class characteristic of nursing young.
Because fossil forms can only be classified based on hard-parts classification, the jaw joint characteristic (along with the presence of the malleus and incus as auditory ossicles) is the key point. This site discusses the evolution of the jaw joint and other mammalian characteristics at some length. Another characteristic is the definition of a variegated dentition, with incisors, canines, biscuspids, and molars present in some form, though possibly modified or lost in development, in contradistinction to the reptilian homogeneity of dentition.
As I recall, the hypothetical transition from reptile to mammal recognizes seven characteristics, including (in no particular order) the jaw joint, variegated dentition, lactation, hair/fur, viviparity, and two others – and the jaw-joint transition falls square in the middle of the seven, and therefore, as a petrifiable character and the mark that a majority of the distinguishing characters have been achieved, makes an excellent break-point between “reptiles tending towards mammalian characteristics” and “mammals preserving reptilian characteristics.”