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Lecture 1 – Amniote Diversity and Reproduction
Amniote features
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A desiccation resistant (yet permeable) cleidoic egg
Extra-embryonic membranes
Internal fertilisation
No metamorphosis or aquatic larval stage
Desiccation-resistant skin
Two or more vertebrae in sacrum
Single astragalus in ankle

Amniote diversity
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The sister group to the amniotes is thought to be the diadectomorphs
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o Lepidosauromorpha – breakable tails, palatal teeth
 Squamata
 Sphenodon
o Archosauromorpha – elongate hindlimb, reduced pedal digits
 Crocodilia
 Aves
o Anapsida – thought to be separate from diapsida and synapsida as they lack
fenestrae, but have now been placed within the diapsida
 Extant anapsids share an ossified plastron (fused bony pieces) and carapace
(expanded ribs and skin plates), have a scapula ventral to the ribcage, and
lack teeth
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 The abrupt design is thought to have been due to Hox genes
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 Many species of tropical frog, such as the tungara frog, lay their eggs in a foam matrix
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 The spadefoot toad is found in arid conditions such as the Sonoran desert, where it hides
underground during the dry season and emerges in the wet season
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The eggs are deposited into the
pool
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e
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Environmental conditions in the north were harsher and
more variable, and extended egg retention in northern populations meant that the
egg remained in stable conditions for longer
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 Placental mammals have an extended period of development within the
uterus, at the end of which they give birth to an infant with functional
internal organs and a skeleton
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 Marsupial mammals have a short intrauterine gestation period before giving
birth to an infant without a developed brain, kidneys, lung or hindlimb, but
with a developed forelimb, shoulder girdle and facial skeleton
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Further development then
occurs in the pouch
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Parturition = the act of giving birth
Oviposition = the act of laying eggs
Lecithotrophic = embryos draw nutrients from the yolk of the egg
Matrotrophic = embryos draw nutrients from maternal sources (placenta, oviducal secretion)
Why are crocodiles, birds and turtles never (ovo)viviparous while squamates are?
 The developing embryos of crocodiles, birds and turtles rely mainly on the eggshell to obtain
minerals such as calcium, while squamates rely more on the yolk
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Tuan et al developed chick embryos for 19 days in either a shell-less
culture or inside an egg during normal incubation, and then fixed and stained the embryos
with Alizarin red to show the calcified bone matrix
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 Another potential reason for a lack of viviparity in birds is their dependence on flight
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g
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 Turtle embryos enter a pre-ovipositional developmental arrest, involving cessation of cell
division and growth in the oviduct
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o A study on three species of freshwater turtle and one marine turtle found that preovipositional developmental arrest was prolonged in a hypoxic environment, and that if
the eggs were exposed to normoxia after a short period of time, development

o
o

recommenced
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pO2 was low in the oviduct of the turtles, and mean pO2 in oviducal secretions was
considerably lower than in saline
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Evolution of amniotes
The earliest fossil evidence of amniotes is from the Carboniferous (about 315Ma)
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The extra-embryonic membranes of the amniote egg do not fossilise, and fossil evidence cannot be
used to ascertain when the amnion, chorion and allantois first appeared
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The typical amphibian life cycle
 Numerous, externally fertilized eggs laid in aquatic environment
 Aquatic larval stage
 Terrestrial, post-metamorphosis adult
Diversity in amphibian life cycles
 Some cave-dwelling plethodontids including Eurycea sp
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 Some amphibians, such as the Surinam toad and some plethodontids, have direct
development with adult-looking young hatching directly from eggs
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The
eggs have large yolk sacs and the offspring show direct development with terrestrially
competent adult-like salamanders emerging from the eggs
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The eggs are laid on land and hatch as small adult with no metamorphosis
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The young are small and the adults do not exceed 10cm in length
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g
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Terrestrially deposited eggs may have relieved the pressure of predation
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Lecture 2 – Constraint and Circulation
Constraint
Gould and Lewontin use the spandrels in King’s College Chapel as an example of an architectural
constraint
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Gould and Lewontin argued that biology also has spandrels – features that cannot be explained in a
functional way, and exist due to developmental constraints
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g
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The archetype described by Geoffroy is the
development of the organism
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However, if Cuvier’s idea was correct we would expect whales, who like turtles frequently dive to
great depths, to display the same ability to bypass the pulmonary circulation while diving
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Whales have other adaptations to deep diving:
 Reduced rate of pumping while diving (bradycardia)
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 High concentration of oxygen-binding red blood cells per unit of blood than other mammals
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 High tolerance for lactic acid (by-product of anaerobic respiration)
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Circulation

Functions of the circulatory system:
 Transports gases between the sites of internal and external respiration
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 Gathers or dissipates heat to control body temperature
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 Transports hormones to target organs
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 Carries cells and chemicals of the immune system
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The developmental basis of the vertebrate circulatory system is the segmented pattern of vertebrate
embryology
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These contribute to juvenile and adult circulation in different ways
depending on the group
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Mandibular arch
2
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Glossopharyngeal arch
In amniotes:
 Arches 1-3 contribute to the circulation between the embryo and its nascent heart
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o The carotid arteries derive in part from arch 3
o The aorta derives from arch 4
o The pulmonary artery derives from arch 6
Other vertebrates:
 The lungfish has arches 2-6 contributing to adult circulation with arch 6 still contributing to
pulmonary circulation
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o In the Australian lungfish, arches 2-6 all have fully functional gills which are supplied
by four aortic arches
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 Amphibians have contributions from arches 3-6 in their larval and adult circulatory systems

and again arch 6 contributes to pulmonary circulation, with arches 3-5 carrying gills
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Endothermic animals have different circulatory requirements to ectothermic animals:
 Ectotherms have a lower metabolism and resting body temperature than endotherms
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 Endotherms require high pressure delivery of blood to the body, and mammals and birds
have therefore lost the 2nd systemic arch to facilitate this delivery
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o In mammals, the left system arch remains
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# systemic # chambers Endothermic or
 As ectotherms do not require
Group
arches
in heart
ectothermic?
a continual supply of O2 rich
Mammals
1
4
Endothermic
blood, they can go long
Birds
1
4
Endothermic
periods without breathing,
Crocodiles
2
4
Ectothermic
and can shut off the
Squamates
2
3
Ectothermic
pulmonary circulation
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 Oxygenated blood returns to the heart through the inferior vena cava into the right atrium
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Any blood reaching the right ventricle passes into the pulmonary artery, but is diverted into
the aortic arch via the ductus arteriosus
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The ligamentum arteriosum, a
small ligament joining the pulmonary artery and the aortic arch, remains from the ductus arteriosus
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Some cardiac defects of adult humans resemble adult morphologies of other vertebrates:
 Atrial septal defect – the foramen ovale has not closed, leaving a ‘hole in the heart’, similar
to the heart of some adult salamanders
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However, sometimes it does not,
resulting in a persistent truncus arteriosus similar to the common trunk for systemic and
pulmonary circulations in an adult frog exiting from a single ventricle
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Components of the ear:
 Outer ear – between the pinna and the tympanum (tympanic membrane held in otic notch)
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 Inner ear – between the oval window and the brain, contains the cochlea (for hearing) and
three semicircular canals (for balance), as well as the round window, a pressure outlet to
compensate for input from the oval window
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It
consists of an indented tube (short in reptiles but elongated in mammals and birds) called the
external auditory meatus which opens to the surface through the external orifice
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The irregular shape of the pinna allows localisation of sound from different directions
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Vibrations from the tympanum are conducted by one or more delicate ear ossicles in the middle ear
to the oval window in the inner ear
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Impedance matching in various vertebrates:
 Frogs have the most sensitive impedance-matching ears of amphibians, with a high size-ratio
of the tympanum to oval window similar to amniotes (very large tympanum) and a long,
delicate columella (the bone in the middle ear)
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They have a much lower sensitivity to sound
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 Lizards have a pronounced tympanic membrane but no pinnae
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An elongate columella connects the tympanum to the oval window
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 Birds have a single bone in the middle ear (columella) and an uncoiled but elongate cochlea,
and have great sensitivity to high frequency sounds
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Evolution of the stapes:
 The hyomandibula (homologous to the stapes) was primarily responsible for jaw suspension
in fish, and evolved a secondary function in some fish of sound transfer to the inner ear
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In tetrapods, the stapes gradually became lighter and responsible for sound transmission
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It is likely that sound was
transmitted via the jaw or shoulder girdle
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The lower jaw of mammals is comprised of a single bone, the
Non-mammals Mammals
dentary
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The
Columella
Stapes
Quadrate
Incus
jaw joint in mammals is dentary-squamosal, while other amniotes
Articular
Malleus
have a quadrate-articular jaw joint
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 Movement out of the jaw facilitated their role in transmitting sound to the inner ear
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Primitive members of each group had larger post-dentary bones relative to the size of
the dentary bone, and there is a progression towards mammals where the post-dentary bones
decrease in size and the dentary increases
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Brasilodon, Brasilitherium and Pachygenelus are advanced cynodonts that have both a
dentary-squamosal jaw joint and a quadrate-articular jaw joint
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 He also noticed that:
o In the pig embryo, the malleus and incus derived from the first branchial arch along
with the jaw
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o In the pig embryo, the stapes derived from the second arch along with the dorsal
part of the hyoid bone
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 The muscle that inserts on the malleus (the tensor tympani) is innervated by branches of
cranial nerve V (trigeminal nerve), which is generally associated with derivatives of the 1 st
branchial arch
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Lecture 4 – Heterochrony and Feeding
Heterochrony
Heterochrony is a source of developmental phenotypic variation upon which selection may act
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However, this idea only deals with one kind of
heterochrony, namely terminal addition where a species extends the ontogeny of their ancestors
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Von Baer cited examples that did not conform to the biogenetic law, such as cases where the order
of appearance of traits differs
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However, it was pointed out the
human embryos develop gill slits and a tail which are not specific to the species
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 May be a result of pleiotropy, where a single gene has multiple effects on different traits
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However, there are several kinds of heterochrony:
 Terminal addition – a descendent species extends the ontogeny of its ancestor
 Terminal deletion – a descendent species matures early
 Nonterminal addition – a descendent species skips stages in the ontogeny of its ancestor
 Nonterminal deletion – a descendent species adds stages to the ontogeny of its ancestor
 Substitution or reversal – developmental stages are switched or reversed
Alberch et al described two categories of heterochrony:
 Paedomorphosis – adult descendent resembles a juvenile ancestor
o There are three types:
 Hypermorphosis – ontogeny is longer as offset is late
 Acceleration – development is more rapid
 Predisplacement – ontogeny is longer as onset is early

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An example of paedomorphosis is facial prognathism
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show less marked
facial prognathism and a large braincase, similar to juvenile Pan sp
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Peramorphosis – adult ancestor resembles a juvenile descendent
o There are three types:
 Progenesis – ontogeny is shorter as offset is early
 Neoteny – sexual development overtakes slow somatic development
 Postdisplacement – ontogeny is shorter as onset is late
o An example of peramorphosis is the separation of the ear ossicles from the jaw
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o Another example is the development of the heart
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Paedomorphosis and peramorphosis can be quantified using three variables:
 α = onset of growth
 β = offset of growth
 κ = rate of growth
Heterochrony cannot always be described as either paedomorphosis or peramorphosis
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For example, the
sequence of developmental events varies between placental mammals and marsupial mammals,
with marsupials having earlier ossification of the face and shoulder girdle and later development of
the brain and sensory organs than placental mammals
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Feeding begins with two key stages:
1
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Transfer to the oesophagus
Amphibian feeding strategies:
 Salamanders commonly use suction feeding
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Excess water then
leaves through the gills
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At close range, muscles catapult
the tongue over parted mandibles and into contact with the prey
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Retraction of the tongue returns food to the mouth
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Two main strategies are used to achieve
prey capture:

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Ram feeding - the predator moves forward with its mouth open, engulfing the prey and
surrounding water
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Hydraulic transport is used to transfer the food to the oesophagus, and the hyoid apparatus provides
the skeletal basis for the generation of hydraulic suction
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The tongue
compensates for this lack of hydraulics
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The pattern of mouth opening and hyoid protraction and retraction is similar in amphibians and fish
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Just after the point of widest gape (greatest jaw opening), the hyoid begins to protract
towards its original position to ‘reset’
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The key differences between feeding in amniotes and non-amniotes are:
 In amniotes, the hyoid protracts during the slow-opening phase of the gape cycle prior to
the fast-opening phase, whereas in non-amniotes the hyoid retracts during the fast-opening
phase and protracts after the point of widest gape is reached
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 Amniotes show inertial feeding or bolting, which involves using rapid head movements with
an open gape to transport prey items to the oesophagus
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 Amniotes show extensive intraoral food processing (slicing and chewing), and the
interlocking dental occlusion present in mammals plays a large role in this
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Cranial kinesis is used by snakes to enable them to ingest prey larger than their head
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Lizards have lost their lower temporal bar
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Woodpeckers also use cranial kinesis to enable it to repeatedly bash its head against a tree
at a high frequency
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Crocodiles have both temporal bars present and no evidence of kinesis
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Metakinesis

Mesokinesis

Prokinesis
Streptostyly

Hinge across the back of the skull
allowing rotation between the
neurocranium and outer
dermatocranium
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Hinge in the dermatocranium
passes in front of the orbits
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Found in most modern lizards
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Monokinetic mesokinesis is rare
but found in some burrowing lizards
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Found in most lizards, snakes and birds
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Therefore, separation of feeding and
breathing passages is essential
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 The fully ossified elongate palate divides the two cavities, requiring a solid connection
between parts of the skull and limiting the capacity for joints besides the dentary-squamosal
(jaw) joint to exhibit mobility
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 Mammals also have precise tooth occlusion (coming together of upper and lower teeth in
alignment) which requires a firm skull, and would not be possible with cranial kinesis
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Animals can be classified according to their body temperature:
 Poikilotherm – changeable body temperature
 Homeotherm – constant body temperature
 Ectotherm – body temperature dependent on external heat
 Endotherm – body temperature dependent on internal heat

Ectotherm

Endotherm

Poikilotherm
Body temperature changes
with the environment
Body temperature is cyclically
lowered as permanently high
body temperatures cannot be
maintained

Homeotherm
Maintain optimal
temperatures by tracking
through stable environments
Maintain optimal
temperatures through internal
processes

Most vertebrates are ectothermic
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g
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g
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g
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Heat regulation (thermoregulation) can occur in several ways:
 Via blood circulation
o Tuna possess a rete mirabilis for countercurrent heat exchange in red muscle, the
vessels of which can enlarge or constrict depending on whether the fish wants to
conserve or lose heat
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The limb
is close to the ambient temperature, reducing heat loss
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o Dogs have a rete mirabilis in their rostrum and ventral to the brain, to protect the
brain from overheating during periods of high metabolic activity such as hunting
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o Blood flow to capillary beds is controlled by smooth muscles
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 Via insulation
o Feathers and hair increase the thickness of the stationary air layer closer to the skin
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Oil has a lower surface tension than
water and reduces the effectiveness of fur and feather insulation
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Subcutaneous body fat is found in mammals and birds, with blubber being especially
common in aquatic mammals
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Via size
o Larger animals lose less heat as they have a smaller surface area to volume ratio,
and are inertial homeotherms – they can maintain a constant body temperature
without having to generate heat internally, as they lose so little
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Weaning takes place after thermoregulatory mechanisms
have developed
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Basking can be facilitated by anatomical
specialisations, for example the Early Permian synapsid Dimetrodon had a large sail
on its back to collect heat when orientated perpendicular to the sun and dissipate
heat when parallel to the sun or in the shade
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DVM has a thermoregulatory function in some juvenile fish including
the juvenile sculpin (Neverman and Wurtsbaugh, 1994)
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Stomach analyses
ruled out the possibility that they could be ascending to feed, as their diet is mainly
composed of benthic organisms and the fullness of their gut increases through the
day and decreases at night
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It may occur
multiple times in 24 hours and the length of torpor may change seasonally or
increase prior to migration or pregnancy
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o Seasonal torpor occurs in mammals
Seasonal
Daily
and one species of bird with median
torpor
torpor
mass of 85g
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Hibernators can rely on stored fat alone for 5-7 months, and
hibernation occurs in bouts of days and weeks throughout the cold season
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At the
same body temperature, hibernators generally consume less oxygen than daily
heterotherms
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Emperor penguins survive for months in the Antarctic winter without feeding in
temperatures reaching -50⁰C
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They slow down their metabolic rate to an
extent to minimise resource consumption without compromising their core body
temperature, and conserve heat by huddling together
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o Some pythons use shivering thermogenesis to incubate their eggs, but are still
classified as poikilothermic ectotherms
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o The primary motor centre for shivering lies in the dorsomedial part of the posterior
hypothalamus and is normally inhibited by signals of warmth from the preoptic area
of the hypothalamus
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Although these impulses are
not rhythmic, they increase muscle tone, and consequently metabolic rate
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 Via non-shivering thermogenesis
o Aerobic respiration
 The hypothalamus produces thyrotropin-releasing hormone (TRH) which
stimulates the pituitary gland to produce thyroid-stimulating hormone
(TSH), which in turn leads to the secretion of thyroid hormones T3 and T4
into the bloodstream
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 Aerobic respiration produces heat as a by-product
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 Endotherms use more energy and therefore need to eat more than
ectotherms
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 Cells that form part of internal organs have a greater mitochondrial density
than other cells
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o Na+/K+ ATPase
 The Na+/K+ ATPase removes excess Na+ from the cell, producing heat
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o Brown adipose tissue
 Brown adipose tissue or brown fat is fat tissue that uses glucose and
produces heat
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Human infants possess dense patches of brown fat in their neck and back
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Adult humans tend to
have more brown fat if they:
 Have a low BMI
 Are female
 Are young
It is found in mammals but not birds, which may reflect their ability to
hibernate seasonally
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Exhalation is achieved by passive relaxation of the thorax and lungs
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Use of an aspiration pump enables the posterior ribcage to ventilate while
the head (including the hyoid apparatus) is otherwise engaged, for example in feeding
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Amphibians use an intermediate mechanism, where the buccal pump is used for inhalation and the
axial muscles drive exhalation
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 In crocodiles, the liver separates the pleural and peritoneal cavities, and a piston-like
movement of the liver powers inhalation, supplemented by movement of the ribcage
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 Turtles are unable to move their ribs as they have expanded to form the carapace
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 Birds have a flow-through breathing system powered by the axial musculature of the ribcage
in addition to the motion of their wings, which drives respiration by moving the sternum
relative to the ribcage
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The extremely efficient gas exchange systems found in mammals and birds reflects their higher
demand for oxygen as endotherms
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In every 1cm3 of lung there is 300cm2 of alveolar surface area in a human compared
to 20cm2 in a frog
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Their lungs are small and attached to the ribcage, and there are
many accessory air sacs that facilitate the constant movement of air across the lungs
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o Approximately half of the air inhaled enters the posterior air sacs, and the other half
passes across the lungs into the anterior air sacs
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Air flows into air vesicles (atria) which
project radially from the parabronchi, and then into air capillaries
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o

The anterior air sacs empty air directly into the trachea from which it is then
expelled, and the posterior air sacs empty air into the lungs
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Respiratory turbinates are delicate branched bone structures located in the nasal
fossa which increase the surface area inside the nose
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They regulate the temperature of the air passing through the nasal fossa
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This minimises the
loss of body heat through breathing
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They regulate the water content of the air by removing water from exhaled air and
humidifying inhaled air
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However, birds consistently have three times the surface area
for the trachea that mammals do, and they rely on their elongate and convoluted trachea to
minimise water and heat loss while breathing
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It is thought that at least some Mesozoic dinosaurs were endothermic, and there are
several lines of evidence contributing to this theory
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o The ratio of herbivores to carnivores can be estimated from fossil remains
...

o However, there are many factors that affect preservation and sampling at fossil
localities, such as:
 Averaging long periods of time
 Sampling only one facies of an otherwise diverse paleoenvironment
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o Impressions in rock indicate that pterodactyls had hair
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 Body size of dinosaurs
o Mesozoic dinosaurs were located in temperate regions
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o When winter arrived in temperate climates, small ectotherms could hide in deep
crevices and keep warm until the spring; however large ectotherms would not have
been able to do this
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 Size and morphology of nasal passages

o

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If dinosaurs were endothermic, they would need to retain heat and moisture while
breathing, and would have possessed nasal turbinates
...

o Triassic fossil synapsids such as Glanosuchus have scars within a large nasal fossa
suggesting turbinates were present
...

o However:
 The nasal fossa is not the only site for heat and water exchange
...

 The dinosaurs examined by Hillenius and Ruben were larger than modern
birds and would have therefore been more resistant to heat and water loss
due to their small surface area to volume ratio, making them potential
inertial homeotherms
...

o There are two types of feathers,
plumulaceous and pennaceous, both
of which develop from a simple
tubular structure originating from
the dermis in a chick
...

o Velociraptor has notches along its forearm bones similar to those in the featherbearing bones of modern birds, however the distribution of the feathers and the size
of Velociraptor make it extremely unlikely that they could fly
...

o While the presence of feathers in Mesozoic dinosaurs does not prove they were
endothermic, it is consistent with the hypothesis that they were
...

o O’Connor and Claessens (2005) found a similar pattern of diverticula in the vertebral
columns of terrestrial dinosaurs such as Majungatholus and argue that these
dinosaurs possessed accessory air sacs connected to a flow-through system
...

 As the chick grows, this spreads throughout the neck and is followed by
growth of diverticula more caudally in the vertebral column
...

 More derived dinosaur species have more caudal pneumatisations
...

o The presence of air sacs and a flow-through breathing system suggests that the
dinosaur possessed a high metabolic rate and needed large amounts of oxygen to
support high levels of aerobic respiration, which is consistent with endothermy
...

o Ectotherms have growth rings in their bones, as they grow in seasonal spurts, while
endotherms grow constantly and do not
...

o However, it has been found that some early birds did have growth rings

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