| Putting the Mamms onto Mammals
PLEASE NOTE: THIS PROJECT IS NOT SCIENTIFIC. IT IS A HOBBY.
The following is based upon my reading of: Oftedal OT (2002), The mammary gland and its
origin during synapsid evolution, Journal of Mammary Gland Biology and Neoplasia, 7(3),
p.225-245.
One characteristic common to all mammals is that babies
are fed on milk. Surprising as it may seem, a few birds also produce supplementary food
for their off-spring, (p.235). Pigeons, flamingos and emperor penguins all
produce nutritious secretions called crop milks. Certainly in the case of pigeons, this is
high quality stuff and new hatchlings thrive on it. Fathers tend to provide it for
longer than mothers. Members of further animal groups also feed their young with
secretions. Some sharks even employ placentas, and salamander and skink species produce
nourishment, (p.225). But none of them have mammary glands. Like Olav Oftedal I've spent
much time attempting to study these organs. Although none of my efforts have been in the
least scientific, they've been satisfying. The soft, warm skin against my cheek. The
sensations as my fingers… Excuse me. I was momentarily distracted.
As such breasts are known only from mammals, Linnaeus used them as a defining
characteristic for Mammalia in 1758, thus the name. This was something of a
corset-breaking move. He united four-limbed land-lubbers with creatures then more usually
considered as fish; dolphins and whales.
Of course, given their nature, the fossil record of mammary glands is none too voluptuous.
The oldest specimens available are presumably from Siberian mammoths. As that only takes
us back a few tens of thousands of years at the most, they provide little information on
the evolution of breasts and / or lactation. I say and / or because, as the above cited
non-mammals demonstrate, paternally produced baby food isn't restricted on the basis of
breast is best.
The paper offers a historical review of considerations concerning the origins of lactation,
(milk production). Various hypotheses have been offered, several of which were based upon
sweat glands, (p.228). Secretions for keeping eggs warm have been proposed, as have
secretions for keeping them cool, (sticky or moistened fur respectively). Another
possibility is that milk is derived from antimicrobial secretions. It's also been pointed
out that, as mammary glands share features with various types of other gland, the structure
could be the result of teamwork. A difficulty with knowing which factor or factors were
involved is the limited availability of direct evidence.
Synapsida
A bit of linguistic luggage is mentioned on page 231; "mammal-like reptiles".
When that term was coined, Reptilia was held to include any land vertebrate, (or their
aquatic descendants), which weren't amphibians, mammals or birds. However, rather than
being reptiles, synapsids and Reptilia are now seen as sister-lineages descended from
non-reptilian amniotes. This apparently subtle difference is significant. For example,
reptile skin has scales, (even on chicken legs). It was widely assumed this was also the
case for "mammal-like reptiles". Such an assumption is actually supported by no
substantial evidence whatsoever.
On land, synapsids were dominant during the Permian and much of the Triassic. At about 225
million years ago they were deposed, (or superseded), by dinosaurs. The surviving lineages
were restricted to small animals or smaller. Some were, depending upon definitions, either
extremely mammal-like and / or mammals. Many transformations occurred during synapsid
history, and the evolution of lactation was one of them. An intriguing question is when?
Milk roots
"A dependence on lactation may be indicated by an osteological character, the
epipubic bones. The paired epipubic bones articulate with
the pubic bones and project forward and ventrally into the abdominal cavity." A
hypothesis states these bones may help support the weight of pouched youngsters (or
suckling babies in pouchless marsupials). They do no
such thing in 50% of the epipubed population, (males). However, they do tend to be longer
and / or wider in females. The presence of epipubic bones in non-mammalian
tritylodontids might indicate a similar purported
function, which could suggest the presence of pouches and / or milk-producing glands. But
these bones seem to be involved in leg locomotion, and the usage of words such as 'may',
'might' and 'could' should not be overlooked.
The loss of epipubic bones in eutherians, (they were
present in basal representatives and placentals are
eutherians), clearly indicates obsolescence. Should the primary function have been to
support the weight of hungry babies, then they would've have been rendered unemployed by
prolonged periods of inter-maternal gestation, (p.235). (Their presence in all male
marsupials makes me sceptical, but I'm ready to admit to possessing nipples.)
Given that basal mammals were small, the babies were probably born at a relatively immature
stage, (as is the case for small birds). Well-developed endotherms would presumably have
required prohibitively sized eggs for even the most ambitious of small mothers. The
off-spring would've needed parental feeding, especially when the dental details are
considered. The highly specialized teeth of mammals require time to become effective. The
eruption of milk teeth is inevitably delayed until there's jaw-length enough to receive
them. It's highly improbable that mammalian dentition would be viable without parental
feeding, and safe to say that milk would at least have been extremely useful for
Morganucodon & Co.
Among existing endotherms, (mammals and birds), parental feeding of babies is general
practice. Mammals use milk and birds tend to use their beaks. It doesn't seem too
outrageous to accept these differing methods of achieving a similar goal may well be
results of different descent; in short, milk producing mammals are descendants of
proto-milk producing synapsids.
Ancestral skin
Trace fossils show that dinosaurs had scaly, sauropsid skin. Despite assumptions to the
contrary, there's no convincing evidence of reptile-like scales for synapsids.
Estemmosuchus is a Lower Permian genus from which fossilized skin's available.
"Skin from the head of this dinocephalian reveals a dense pattern of concave
lens-like structures." This suggests glandular skin. "Mammal-like
amphibians" is perhaps less inapt than "mammal-like reptiles".
Mammalian glands
Mammal skin has various types of gland, (sebaceous, eccrine and apocrine), and glands are
frequently associated with hair follicles. In the early stages of growth, the hair
penetrates down into the skin and lateral buds can develop into apocrine and sebaceous
glands. These produce modest amounts of secretions of various ingredients; for example,
lotions and coatings for skin and fur. Some develop into large glands useful for
scent-marking, (p.238).
Eccrine sweat glands aren't connected with hairs, and are rarely widespread. Generally,
they're restricted to contact surfaces; palms, soles and the underside of prehensile tails.
The platypus has some on its beak but it's a
fanatical non-conformist. These glands produce relatively bland, diluted secretions. Only
a few primates have gone in for having such sweat glands all over the body as a method of
cooling.
And then, of course, there are mammary glands. These could be directly derived from one of
the three main types, or from their protoforms. They share features with each group, but
none is especially similar. Another possibility, (p.239), is that breasts are hybrids.
Oftedal coldly states: "Sebaceous glands bear little resemblance to mammary
glands". He's a bit warmer when it comes to eccrine sweat glands, but finds them
unlikely as ancestors. Apart from anything else, their generally restricted range (contact
surfaces) would make a transitionary scenario easier to accept if mammary glands were found
on paws. While hands have definitely been observed on breasts… Nevertheless,
improbability and impossibility aren't synonymous.
The author finds far more similarities with apocrine glands and lists some. This is not an
exercise I feel like attempting to replicate but, should somebody be looking for clues, the
Harderian gland may be worth a visit, (p.241). I feel it's important to point out there
are arguments against the author's position, and at least some are addressed on that same
page. His view seems fairly summarized with: "The most likely scenario is that
mammary glands evolved from an apocrine-like skin gland and that was ancestral to both
mammary glands and contemporary apocrine glands."
Mammals
Monotremes
Marsupials
Placentals
An evolutionary scenario
Mammalian traits emerged at various stages among synapsids from the Carboniferous onwards.
Lactation presumably also evolved uninstantaneously. Where available, synapsid skin is
known to be glandular in contrast to the sauropsid condition. All living egg-laying
synapsids (monotremes) reproduce with parchment-shelled eggs. Hard shells are restricted
to a few aberrant sauropsids, (eg. crocs and birds). This suggests basal synapsids most
probably also laid parchment-shelled eggs, which enjoy comparatively less protection from
drying out. These factors imply those early synapsids were animals with potentially
moist skin and the eggs would've benefited from moisturisation. In those circumstances,
secretions for protecting the eggs would have been very useful. Although not as yet
carefully studied, some existing amphibians tend their eggs in possibly similar ways. A
few seem to even provide food for hatchlings from secretions and / or maternal skin, and
some caecilian young appear equipped with specialized teeth for the purpose, (unpublished
studies).
In mammals, mammary and apocrine glands may be descendents of a common gland type, which
was presumably involved in moisturising eggs. At some stage, the ancestral glands became
associated with hair follicles, (which post-date glandular secretions). While hair plays
an important role in keeping mammals warm, that wasn't necessarily the original function.
Please consult a tropical tarantula for discussions upon other uses for hair-like
structures, (but bear in mind its 'hairs' are formed by very different processes, and
arachnid vocabulary is notoriously limited).
The proposal here is that hair was originally used to spread fluid onto eggs. The fluid
came first and the follicles developed at the production sites. Subsequently, the hair
become involved with insulation and may pre-date loosely defined mammals by a long time, as
might a nutritious fluid known as milk, (p.246)
Pointing out that this scenario involves speculation would be stating the obvious, seeing
as the author refers to it explicitly as a hypothesis. However, it's certainly not
groundless and its plausibility can be tested. If mammary secretions and hairs played a
role in egg care, then it may well be that this still occurs in monotremes. Presently,
it's unknown whether female monotremes lactate during incubation, but indirect information
suggests they perhaps do.
1. Monotreme eggs take up fluid and obviously enough increase radically in size in the
uterus, which suggests fluid permeates the shell. Most of this process seems to occur
before the external layer of shell is in place.
2. When laid, the eggs are covered with a coating of organic material containing foreign
particles. These must've got there somehow. (During incubation platypus eggs are glued
together, while echidna eggs are held in a pouch and secured with hairs.
3. Echidna mammary glands: "develop a tubular grade of organization late in the
breeding season, even before eggs have been laid." They secrete a milk-like substance
containing 12% dry matter.
If it can be shown that monotremes produce milk in order to care for the eggs, (rather than
only as food for the hatchlings), this would support the thrust of the hypothesis. It
might also help explain why monotremes don't have nipples, as coating eggs with liquid is
perhaps more effectively done with a furry brush. If so, then nipples presumably arose
when that purpose was rendered obsolete by live-birthing.
Mammary hair in marsupials may be an evolutionary hangover, and the vestigial 'egg teeth'
in some newborn marsups, (eg. brush-tailed possums and koalas), can hardly be cited as
evidence for 'intelligent design'. A structure to assist baby marsupials to hatch from
eggs which were never laid is gloriously useless. Both that and the mammary hair would be
in line with egg-laying ancestors.
If you'll excuse me, I'm now going to return to my favoured methods of studying mammary
glands.
FINAL CAUTION
Postscript
With thanks to David L. Nathan M.D.
Trevor Dykes, June 2004
PLEASE NOTE: THIS PROJECT IS NOT SCIENTIFIC. IT IS A HOBBY.
The following is based upon my reading of: Oftedal OT (2002), The origin of lactation as a
water source for parchment-shelled eggs, Journal of Mammary Gland Biology and Neoplasia,
7(3), p.253-266. This study is the 'sister' article of Oftedal's evolutionary scenario on
mammary glands.
Parchment-shelled eggs have advantages over hard-shelled ones, but they also have
drawbacks. One of these is a vulnerability to drying out, (p.253). Then again, if a shell
is prone to allowing a loss of liquid, then it's also more likely to absorb the stuff from
outside. Characteristics which pose potential problems can offer potential solutions.
Oftedal's hypothesis is that lactation began as a method of supplementing the water supply
for eggs. It could also have provided nutrients such as sodium and ionic calcium as well.
Inevitably, there's some overlap between his 'sister' papers, but there's much of interest
in both.
The basal amniote approach to eggs
The fossil record also points to the original eggs having had parchment-shells, as they're
poor candidates for preservation. Presently, no fossilized shell has been positively
identified from earlier than the Middle Triassic. There was a report of a Permian egg,
but it was a case of mistaken identity. Various structures can look superficially similar.
As the fossil record progresses through the Mesozoic, fragments of shell and even complete
eggs become available on a worldwide basis. Furthermore, there's no such supply of the
soft-shelled variety. They're rotten candidates for fossilization.
A derived amniote approach
Letting off steam
Assuming surrounding conditions provide for 99% relative humidity and a temperature of
28°C, five grammed soft-shelled eggs would have about two weeks of viability, (p.257).
500 grammers might manage sixteen weeks. Hard rigid shells can prolong life considerably.
Using the same sizes and conditions, losing a third of the water would take over seven to
twenty years respectively. If vulnerability to evaporation were the only issue, that
advantage would be overwhelming.
Selectively laying
The masses tested don't reflect the full range of sizes produced by living tetrapods.
Squamate eggs extend from 0.06 - 303 grammes; turtles 2.6 - 107; crocodiles 52 - 113; birds
0.3 - 1,480. Estimated weights for extinct birds go as far as 4 kilos and dinosaurs up to
5.5 kilos. Living egg-laying synapsids (monotremes) content themselves with 1.5 to 2
grammes.
Water supply
Such growth isn't an option for rigid-shelled eggs, as they'd shatter. The protection against
water loss comes with costs and limitations. As the scope for replenishment after the egg
has been laid is minimal, the original provision needs to be generous. That's why fried
eggs have so much white. Birds which nest in wet places tend to produce particularly
impervious eggs; eg. grebes, (p.258).
Even with the best available protection, passionate knights could end stickily, should they
lose control and fall of their mounts in mid gallop. That may sound smutty when read aloud,
but armour can be a death trap.
A couple of fun words
Endotherms
Endotherms typically incubate eggs at higher temperatures, and this serves to speed up
embryological development. Some ectotherms are able to achieve similar effects by exploiting
the environment; nests in sunny spots, depth in the ground, rotting vegetation, sunbathing
by females which retain eggs internally... Some pythons wrap their bodies around their
eggs and generate warmth by shivering.
Fully developed endothermic approaches, (both birdy and mammalian), necessitate some form
of insulation from the ground. Soil is an efficient conductor of heat and can store it in
prodigious quantities. This is a source of potential danger for endothermic eggs, as they
require a stable temperature. Nesting material provides a popular solution, but it's not
the only method. Emperor penguins use their feet and lower stomachs.
Birds
Synapsids
Various mammalian characteristics mark us out as oxygen and energy hungry endotherms, and
many were in place before Mammalia. Oftedal provides a
useful list of some.
# Maxilloturbinals, in the front of the nasal chamber, are associated with high oxygen
consumption. They both warm incoming air and at to hinder water loss upon exhaling. These
have been reported for therocephalians and cynodonts. However, the evidence is actually
indirect. They're inferred from the presence of small ridges in the right kind of position.
The structures themselves haven't been found, (Kemp 2005, p.127), and the ridge may have
had another function, (eg. scent).
# Well vascualarized bone, which has to do with fast growth. It's a condition present in
therocephalians, dicynodonts and cynodonts. (As it can be found in some ectotherms as well,
it would be less than instructive if taken in isolation.)
# A bony secondary palate, which allows for the simultaneous consumption of oxygen and food.
This is found in some therocephalians, dicynodonts and cynodonts. (A similar condition
occurs in birds and crocs.)
# The reduction of lumbar ribs, which is one indication of
the division of the torso into distinct lumbar and thoric regions. Among other things, this
allowed room for a diaphragm. The reduction was underway in non-mammalian cynodonts, though
still further reduced lumbar ribs were still present in some mammals, (eg.
Fruitafossor).
# The transition to a more upright leg posture. This applies to therocephalians,
dicynodonts and especially cynodonts. The straightening carried on among mammals.
Nevertheless, the egg-laying monotremes still have
sprawling habits in comparison to we therians.
# The restructuring of the cynodont jaw and increasingly efficient teeth, which point to
increases in activity and a faster rate of food consumption.
Therocephalians, dicynodonts and cynodonts all survived the worst mass extinction(s) in the
history of the planet; the Permian-Triassic extinction(s). It was the cynodonts which
eventually conquered much of the world. We call the last synapsid survivors mammals.
Taken individually, none of these six points is conclusive evidence of some degree of
endothermy. However, collectively, they suggest some form of endothermy may have arisen by
the Upper Permian. The cynodont Thrinaxodon is known from the lowermost Triassic,
and its anatomy makes little sense for an ectotherm. Its body is convincingly endothermic.
If so, then the eggs would have contained endothermic embryos with all the potential
disadvantages associated with water loss. They would have required protection,
replenishment of supply or a mixture of both.
Tending eggs
As various extant non-mammals moisten eggs with body fluids, it's reasonable enough to
assume that extinct synapsids could also have done so.
Elastic monotreme eggs
Additional sources:
Trevor Dykes (not a paleontologist)
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