Download Mammalogy Lecture 5 -- Disparity in Diversity between Marsupials

Mammalogy Lecture 5 -- Disparity in Diversity between Marsupials and
Placentals
So, as we’ve seen over the last couple weeks, Eutheria is much more diverse than Metatheria.
This is true in terms of the numbers of species; there are over 4350 eutherian species and ca. 270
metatherian species.
It’s also true with regard to diversity in form. For example, there are no fully aquatic marsupials
and there are no marsupials with powered flight.
So today, we’ll discuss two hypotheses regarding the primary reason for this, and both have to
do with reproduction.
A. Trophoblast Hypothesis - The principle difference is the trophoblast, part of the embryonic
contribution to the placenta that is well-developed in eutherians,
but not metatherians.
Embryoblast & Trophoblast
- Trophoblast implants into the uterine wall; it develops fingerlike
projections into the uterine wall.
- It has many functions, but we’ll worry about one.
- Now all mammals reproduce sexually and , so there is a genetic
contribution of the father.
- Because of this, the developing embryo will be recognized as
foreign tissue by the immune system of the mother and
potentially trigger an immune response.
- The trophoblast actually provides a barrier that separates the embryonic tissues
from the immune system of the mother, yet still allows for the passage of nutrients.
- In many eutherians, this is known to be accomplished by production of chorionic
gonadotropins, which (among other things) maintain the trophoblast and suppress
maternal immune reaction.
- Therefore, the developing eutherian embryo can “hide” from the mothers immune system,
and this allows for a relatively long period of intra-uterine development in eutherians.
- These long gestation times, then result in more fully developed (precocial) young.
- In metatherians, there appears to be little or no suppression of the maternal immune
response. There is no barrier between the developing embryo mother’s immune system.
- Metatherians therefore lack the ability for the embryo to “hide”.
- Thus, to avoid attack from the mother’s immune system, metatherians have a very short
gestation. For example Macropus typically has a gestation period of around 30 days and
Didelphis has a gestation period of around 12 days.
- Metatherians give birth to very poorly developed young (altricial). Neonates attach to
nipples and complete development.
- This has two effects:
1) Metatherians have a much longer overall period from conception to weaning.
2) In metatherians, a higher percentage of time is spent lactating. Lactation is about
twice as energetically costly as gestation.
- Therefore, eutherians experience a lower overall energy cost of reproduction and have a
competitive advantage over metatherians. Figure 9.15 in text illustrates the difference in
investment in reproduction.
- This competitive advantage explains the difference in species diversity between
metatherians and eutherians.
- Also, this longer gestation in eutherians allows for increased diversity in morphology,
especially with regard to the forelimb.
- When metatherians are first born, because of such short gestation times, they are
remarkably poorly developed. Hind limbs are simply limb buds & organogenesis has
only just begun.
- In spite of this, they actually have to crawl from the birth canal to the mammae, which may
or may not be inside a pouch, where they attach and complete development.
- Almost no assistance in offered by the mother. The most she may do is lay a saliva trail
from birth canal to marsupium.
- Because of this, the forelimbs of all marsupials are rather constrained. That is not free to
evolve into such things as the wings of bats or the flippers of marine mammals.
- Summary: the presence of a well-developed trophoblast in eutherians, and the production of
chorionic gonadotropins, allows the embryo to hide from mother’s immune system, and
thereby allows greater gestation time. This reduces overall cost of reproduction relative
to metatherians and allows for greater diversity in morphology especially of the forelimb.
Therefore, the trophoblast hypothesis explains both the disparity in species numbers and also the
disparity in morphological diversity.
B. Basal Metabolic Rate Hypothesis - The fundamental difference is not the trophoblast, but
differences in the relationships of the following: Diet, Basal Metabolic Rate,
Reproductive Capacity.
In Eutherians, BMR is directly coupled to Reproductive Capacity:
increase BMR = increase Reproductive Capacity.
Metatherians lack this direct coupling (for whatever reason):
increase BMR = no change in Reproductive Capacity.
- So under this hypothesis then, in situations in which eutherians can evolve high BMR,
expect that they will out-compete metatherians because eutherians can translate high
BMR to higher reproductive rates.
Conversely, where a high BMR can’t be supported, there should be no competitive difference
between eutherians and metatherians & they should be able to coexist.
This is where diet becomes relevant.
High BMR incurs high energetic cost. Therefore it can evolve only where food is either
very high quality or very abundant.
- We expect that eutherians grazers and carnivores will out-compete metatherian
grazers and carnivores.
High BMR cannot evolve in situations in which food is either low in abundance and or
quality. This is the case for frugivores and opportunistic feeders.
- We expect there to be no competitive advantage for eutherians.
These ideas seem to be supported by fossil records and distributions.
- Marsupials radiated in both Australia and South America, in relative isolation from
placental mammals.
- In South America, there is fossil evidence that there were marsupial carnivores, but when
North and South America came back into contact 3 MYA, eutherians invaded South
America, marsupial carnivores went extinct, and frugivores and opportunistic feeders
persist.
- The majority of marsupial diversity is in Australia, where there were no eutherian
competitors. In fact we see both carnivorous and grazing marsupials only here.
- Furthermore, introduction of the dingo into Australia resulted in the extinction of the
Thylacine or marsupial wolf, and the restriction of the Tasmanian devil Sarcophilus, to
Tasmania.
There was an argument in the literature over which of these was the principle reason for the
disparity in diversity that we see between marsupials and placentals. Both these ideas have
merit and are probably contribute to some degree to the patterns of diversity that we see
today.
If I were forced to choose one of these hypotheses, I would choose the Trophoblast Hypothesis
because it has greater explanatory power. It explains both the difference in species numbers
and the difference in morphological diversity. The BMR hypothesis only explains species
numbers.