Discussion on the humble cow to being distracted by echidna testicles (again)
Where else should I begin but discussing the uniqueness and complexity of the Australian wildlife. Most Australian primary school students are taught that animals are divided into: birds, mammals, marsupials, amphibians and reptiles. Then university complicates matters and everything you thought was true is a myth….Today I am going to start by introducing you to: mammals, marsupials, eutherians, placentals and monotremes. Before I begin I must warn you that several books have been filled with this subject so I am glossing over some of the finer points to provide you with a general overview. If the need arises I can go into depth explaining the differentiation at a genetic level or why it is particularly fascinating that the stylopodia (humerus and femur) form post zeugopodia (radius/ulna, tibia/fibula) ossification in monotremes but not in placentals except for the european mole.
Now a classic example of a mammal is the cow:
It eats grass; reproduces with another member of its species of differing gender (male and female required; produces milk for its live-born young (does not lay eggs) and it has a beautiful hairy coat (extremely huggable).
This is a marsupial:
It produces live-young like the cow and feeds its offspring milk. An Australian primary school student would be able to tell you that all marsupials have pouches for their young which is what makes them different to cows. This is true in some instances such as the brush-tailed possum, common wombat and the red kangaroo. However, right when everything seems nice and simple tricky species such as bandicoots come into play…. Bandicoot babies are fully developed when born and therefore the females do not require a pouch for their young to develop in but despite this are still considered marsupials. If you don’t need a pouch to be a marsupial then why can’t the humble cow be a marsupial? The answer unfortunately is that the definition for marsupial and mammal is far more complex than what you may have learnt in primary school.
I will talk soon on what is a mammal and what is a marsupial precisely but first I need to introduce you to the eutherians. Eutherians are members of the Class ‘Eutheria’. There are three groups within Eutheria that can be found alive today. They are:
- Placentals (apes and sheep)
- Marsupials (kangaroos and possums)
- Monotremata (echidnas and platypus)
I am going to begin with explaining what is a placental because they are the simplest. The characteristics of a placental are:
- pregnant females of the species nourish their young with a placenta
- lacking epi-pubic bones
- viviparous (give birth to live young)
- young when born are fully developed
The most basic requirement of a placental is that you need to have a placenta. Before any males reading this become too concerned I should add that the female of the species during pregnancy needs to have a placenta in order for the species to be considered placentals. No males of any species has a placenta. The placenta is the source of nourishment for the unborn child within the mother’s uterus. It is commonly referred to as the afterbirth.
Epi-pubic bones are often referred to as marsupial bones because they have shown to aid in supporting the pouch of the marsupial female. However, the original reason for the existence of epi-pubic bones is unknown. No placental can have epi-pubic bones as it would too greatly restrict the abdomen during pregnancy and parturition (giving birth).
Placentals are viviparous which means that they give birth to live young. ‘Live young’ is a bizarre turn of phrase but when it comes down to it simply means that they don’t lay eggs.
Now onto Marsupials…. The classic example of a marsupial the red kangaroo. It has a pouch; gives birth to live young the size of a jelly bean; gestating female lacks a placenta; has fur and the young are nourished with milk. Many marsupials do fall within these guidelines but then there are those species that must of banded together and decided to be difficult for future taxonomists. Bandicoots I am looking at you…. In the bandicoot the embryos are attached to the uterine wall by a placenta like organ that allows the young to develop fully within the uterus. The function of this organ is similar to that of the true placenta but is much smaller and less refined and therefore the bandicoot is not considered to be a placental.
The following are typical characteristics of a marsupial:
- when born the young are scarcely more than an embryo
- in males the penis presents behind the testicles
- the sperm’s nucleus is flattened along the dorsi-ventral plane changing the outward appearance
- epi-pubic bones present
- they lack an anus but instead have a cloaca
- lack of correlation between basic metabolic rate and brain size
- strong sexual dimorphism (the males and females are easily differentiated)
- in females:
- ‘vaginal apparatus’ that opens to the urogenital sinus
- urogenital sinus also contains the urethra
- urogenital sinus and the gut empty into the urogenital opening
- two uteri separated by individual cervices
- short gestation period
When attempting to identify an animal in the field it would be considered unusual to collect and analyse a sample of sperm to see if the nucleus was flattened on the dorsi-ventral plane. However, it is a defining characteristic of a marsupial.
When compared to placentals, marsupials may be misconceived to be small brained. However, the inclusion of primates in the placental group greatly increases their overall average. Marsupials also lack a partial correlation between BMR (basic metabolic rate) and brain size. Previous hypothesis have stated that large brain size requires a higher metabolic rate. An alternate hypothesis states that placentals have increase BMR and brain size due to their extended gestation whereas marsupials achieve an increased brain size through extended lactation.
Marsupials never have a single uterus. They always have multiple uteri. The other main distinction between marsupials and placentals is the stage of development of their young when born. Placentals give birth to highly developed young whereas marsupials give birth to under-developed young. Just ignore the bandicoots…
Now onto monotremes… There is a strong argument for designating monotremes as the strangest animals in Australia and I am about to reveal why…
Living species of monotremes include: short-beaked echidna, long-beaked echidna and duck-billed platypus. Monotremes are descended from the evolutionary step between reptiles and birds to mammals. They lay eggs but they also provide milk for their young. The discovery of the monotreme has caused many generations of headaches in the scientific community. Monotremes lack breasts and nipples but instead excrete milk from mammary glands beneath the skin. Fascinatingly enough the milk of monotremes exhibits a relatively high presence of anti-bodies to aid in the development of the young’s immune system. This is possibly in compensation for the non-sterile environment in which the milk is delivered.
Monotremes also operate at a lower metabolic rate than marsupials as indicated by their average body temperature of 31°C compared to marsupials at 35°C and placentals at approximately 37°C.
In my opinion one of the most remarkable characteristics of the monotreme is only present in the males. They have testicles in their abdominal cavity! Excuse my excitement but after a science degree you find joy in the smallest of things. This means that their testicles are located in their stomach region. This does not seem that impressive until you remember why testicles are external in mammal species. Scientists believe the most likely reason for testicles being external to the body is for maintaining the perfect temperature for the creation and housing of sperm. Testicles in placentals have a 6°C temperature gradient which means that the blood entering the testicles may be 39°C but blood returning from the testicles to the body is only 33°C. This is just one of the many ways of how the testicles are kept cool. If the testicles are inside the body then how is it possible for the sperm to remain viable? Birds also have abdominal testicles but they are located in a pouch which subsequently lowers the temperature by 4°C ensuring the safety of the sperm. Anyway I am getting distracted by echidna testicles again…. back to the differences between monotremes and every other animal in existence.
One of the most stark differences between the monotreme and other eutherians is its method of sex determination. In humans sex (in this instance I am discussing the genitalia that a person is born with) is decided genetically by the sex-determining region Y (SRY) protein. Females are born with the chromosomes XX and males XY. In reality it is more complicated but for now let’s just pretend the world is simple. Reptiles however do not determine their sex through chromosomes in the same way that placentals do. For example, crocodile sex is determined by temperature (TSD). Monotreme sex determination is an unknown. There are a number of theories discussing the roles of GATA4 and DMRT1 but the process remains largely a mystery. There is one known however, monotremes do not determine their sex in a way similar to any other known species.
Overall, Australian animals are bizarrely unique and their strangeness is not limited to what you may have learnt in the primary school classroom or the facts gleaned from a libre pad. Topics up for discussion next time may include: how a kangaroo joey migrates from the uterus to the pouch; embryonic diapause- hitting the pause button on pregnancy; how echidnas communicate through smell or maybe even how sperm is able to traverse the treacherous environment that is the female reproductive system.