Monotremes and marsupials Flashcards
What are the major similarities between monotremes and reptiles?
- cloaca = single urogenital opening
- seperate uteri into urogenital sinus
- semi-permeable shell deposited in uterus
- no nipples
- pectoral girdle: corocoid, precoracoid, interclavicle
- no vibrissae
- cervical ribs present
- lack ear pinnae
- body temperature and metabolic rate lower than other mammals
- forelimb bones are heavy and elaborate (more sprawling gait)
Milk secretion in monotremes
- no nipples – suck from tufts of fur
- platypus – pores on belly
- echidna – glandular lobes in pouch
Which of the skeletons below represents the pectoral girdle of a monotreme? How is it similar to the reptilian skeleton on the left?
pectoral girdle of monotremes (middle) is elaborate, bony, lots of different bones (like reptile pectoral girdle) vs. higher mammals like muskrat (right) which is very simplified
reflection of change from splayed to more upright posture
What are some major derived characteristics of monotremes?
- spur on ankle (sometimes with venom, i.e. platypus)
- electroreceptors in bill/beak – help to detect prey
- edentate (no teeth) but may have grinding plates
Duck-billed platypus
Ornithorhynchus anatinus
S.C. Prototheria
O. Monotremata
F. Ornithorhynchidae (bird + beak)
- Semiaquatic (webbed feet, flat tail)
- semifossorial (builds burrows)
- Australia
- molar-like teeth lost in adulthood
- adults have grinding plates in mouth
Echidna
S.C. Prototheria
O. Monotremata
F. Tachyglossidae
- Australia, New Guinea
- spines (modified hairs)
- panniculus carnosus muscle (used to roll into a ball)
- no teeth in any development stage
- pouch to hold young
- no true nipples
Marsupium
- abdominal pouch in females
- echidnas and most marsupials
- found in animals that show small maternal investment prior to birth, and high investment in lactation
What kind of reproductive tract is characteristic of marsupials?
- paired reproductive tract
- short gestation, parturition occurs through pseudovaginal canal
- 2 lateral vaginas
What is the difference between marsupial and eutherian placentas?
Marsupial placenta (top)
- Choriovitilline placenta derived from yolk sac (larger yolk sac)
- membranes less developed
- less nutrient exchange
- most nourishment from yolk = rely on yolk sac, can’t stay in mother for long
- gestation must be short (no villi)
Eutherian placenta (bottom)
- Chorioallantoic placenta derived from allantois and chorion
- villi strengthen attachment and increase surface area for efficient exchange of nutrients and wastes
- a lot of nutrient exchange = offspring can be inside mother for longer
- yolk sac smaller - once yolk sac is used up, begins to exchange nutrients with mother via villi
How can this graph be used to tell the difference between a metatherian and a eutherian?
- marsupials have a very short gestation period and a relatively long lactation period
- eutherians have a longer gestation period and a shorter lactation period
- eutherian offspring are much more developed at birth than marsupial offspring (altricial)
- when born, climb up mother using deciduous claws on forefeet, attach to teat
What is embryonic diapause?
- can be caring for 3 different levels of offspring at the same time
- young kangaroo drinking from teat
- joey in pouch on teat, arresting development of embryo in uterus
- embryo in diapause in uterus
Polyprotodont
long dentary, small unspecialized I’s
Diprotodont
short dentary, 1st lower I’s lengthened to meet upper I’s
Didactyly
unfused toes, each in own skin sheath
Syndactyly
bones of 2nd and 3rd toes fused, together in a common sheath
Marsupial mole
s.C. Theria
I.C. Metatheria
S.O. Australidelphia
O. Notoryctemorphia
- the only completely fossorial marsupial
- vestigal eyes covered by skin
- “swim” through sandy soil
