.................... KINGDOM METAZOA III. VERTEBRATES III. The mammal

Introduction. When we see a bitch suckling its young and how she defends them from possible enemies, we see the great similarity to the human behavior. When we see how a cat teaches its youngs to hunt we can also see the resemblance. Also, when in a zoo we look into the eyes of a chimpanzee, a gorilla or an orangutan, we find the strange feeling that we too are being watched by a look quite similar to ours. Reality is not that mammals are much like us but that we are the mammals. Also it is not that the big apes or primates closely resemble to us, but it is that we are also the primates.

Our evolution has led us a possesian of a brain that, in proportion to the body, is the largest of all species. Basically this is what allowed us to move in a few million years from collecting fruits and defending us from other species to dominate them and live from them. Later, thanks to the discovery of how to control the fire and how to get the energy from oil, the human population has experienced tremendous growth, so high that we have become a major threat to most of the other species. Today, their survival depends on us. This is why we have to support all the initiatives that are aimed at keeping this world as well or making it better than we have received, for the next generations. We do not know if our intelligence is enough to prevent our own extinction by excessive environmental destruction. What is clear is that the way forward begins with better understanding of the other species and then finding out what we have to do so that all species can live on this planet, the only known place that living things can accomodate.

Activities to do. Read the explanations on the general character of the Mammals and perform "Multiple Choice Test 20". Then read the text on reproduction and classification of Mammals and do the exercises "Relate pictures with names 20" and "Crosswords 20".

1 . The Mammals. Are the vertebrates that usually have their body covered with hair, they give birth to the youngs (viviparous) and then they suckle them, and they maintain a constant internal temperature (homeotherms). The hair are the epidermic structures, unique to mammals, that protect them from cold and prevent from attacks to the skin.

1. External morphology. There are three parts in thier body: head, trunk and tail.

• The head. It generally presents two nostrils, two ears, two eyes fitted with lids, mouth provided with lips to suckle milk during lactation, and four types of teeth (incisors to cut, canine teeth to tear, premolars and molars to chew) each one located in a different dental socket.

• The trunk. It generally presents four limbs formed by a long bone, then two long parallel bones and a set of small bones that form five fingers. These are the bones forming the skeleton of the flat part of the fore-foot of a tetrapod vertebrate. They can be adapted to the march (as for example the legs of a horse), excavation (like the legs of a mole), apprehension of objects (like hands of a chimpanzee), fly (like the wings of a bat) or swim (like the fins of a whale or a seal). The walking legs may end up with nails (unguligrade mammals like the horse or pig) in toes (digitigrade mammals like the dog) or the sole of a foot (plantigrade mammals such as bears or humans). Females have the mammary glands to give milk to infants (lactation).

• The tail. It may be missing (as in the gorilla) or can form a flipper (as in whales). It usually acts as a rudder to maintain or change the direction in quick races. In some species, like spider monkeys, it is prehensile and is used to catch the branches.

2. Internal anatomy. The heart of mammals has two atrias and two ventricles as birds and therefore have complete double circulation. Breathing is through lungs. In some groups, the digestive system has special features, such as ruminants have the belly for storing food and then return it to the mouth and ruminate (chew again). Most of them have the cloaca but the excretory orifice and anus separate have the seperated end.

The stomach of a ruminant, as a cow

3 . Reproduction. The males have two testicles from which get two sperm ducts that open into the urethra that passes inside the copulatory organ or penis. Fertilization is internal. The females have two ovaries and two oviducts that go to the muscular cavity called uterus or womb that communicates with the outside by an expandable conduit called vagina, which is where the male penis lodge during the intercourse. Embryonic development can be done in three different ways:

• inside the egg as happens only in Prototherias, like the platypus.

• inside the rudimentary uterus for a short time and then finished in the marsupial bag, where are breast, as it happens only in Metaterios as kangaroo.

• inside the uterus throughout all the development due to the structure called placenta, which allows feeding embryo from maternal blood to the time of delivery, as in Eutherias, as the cat and the majority of mammals.

5 . The human specie. The humans are the primates mammals characterized by having a brain much more developed than the other species. This allows them to reflect, plan the future and with the assistance of the larynx, articulate complex language to express ideas. They also have a standing position very well adapted to the hours of march, two hands with the thumb opposite to other fingers, which allows them to manipulate objects with great precision, and a very long infant stage, which gives them a long period of learning. All these features, among which the intelligence, gace our specie the ability to decide about the distribution of territory and, with this, we have taken the responsibility to protect, or not, the other forms of life. Today the survival of the planet basically depends on our specie.