Animal Physiology

Gas exchange is vital for animals that use oxygen and produce carbon dioxide. In animals, different strategies have been selected through evolution.

Aquatic and terrestrial animals do not face the same problems to survive. Water is a viscous habitat with poor oxygen concentration when air is rich in oxygen but fails to counterbalance gravity.
Evolution has selected systems that allow gas exchanges in both habitats. Aquatic animals share gills that float and water streams renew gas availability. Terrestrial animals breathe through lungs (mammals for example) or trachea (insects for instance). Both systems have in common reinforced and stiffened tubes that cannot collapse and allow air renewal.
Some animals live in a habitat but breathe in another. Cetacean are aquatic animals but breathe air through lungs…
Others, like frogs, evolve and breathe in water when they’re young, then in the air when they’re adults. They do have gills as tadpoles and after lungs develop.

Nitrogenous waste has to be eliminated from the animal body. Excretion is the key to do so but that function differs between animal species.

Metabolic waste can be dangerous to the animal body. It has to be eliminated. Nitrogenous waste is a fine example of this function and can be achieved through various mechanisms according to the habitat. We can distinguish animals that excrete:

1. Ammonia that is highly toxic and requires a lot of water to be diluted (fresh water fishes)
2. Urea that is less toxic and can be concentrated allowing a less important loss of water (mammals)
3. Uric acid that is not toxic and can be eliminated in a solid form (birds)

Those mechanisms have been selected through evolution as they ensure nitrogenous elimination allowing in parallel osmoregulation (intern solutes concentration) and preventing dehydration for terrestrial animals.

Most of animals do move in their environment. This characteristic comes from muscles and some type of skeleton.

Animals share muscular cells that can contract. In those cells, protein architecture (actin and myosin) leads to filaments sliding when cells are excited through an ATP consuming process. Motion is only possible because those cells are regrouped into tissues and linked to a type of skeleton.
That skeleton can be made of water (earthworm’s hydroskeleton) or stiffened material.
We can distinguish animals with:

• An internal skeleton such as vertebrates (bones and cartilages)
• An external skeleton such as insects (chitin)

Animals share sexual reproduction and thus produce sex cells. However, some animals can be hermaphrodites.

Animals produce gametes through a precise process in their gonads, testes and ovaries. Spermatogenesis produces male sex cells. Diploid cells undergo mitosis continuously to keep their great number and a part of the pool undergoes meiosis to produce haploid cells. Those cells go through differentiation to acquire their mobility for fertilization.
Oogenesis is a cyclic process that selects a few cells to undergo first stages of meiosis and differentiate increasing cell food stock needed for the beginning of embryonic development.
In most animal species, male gametes are produced in greater quantity than female gametes. Some animals can produce both male and female gametes (snails for example). However, they cannot self-fertilize.

Fertilization requires the encounter between male and female gametes. This process is based both on sperm cells motility and female guiding.

The encounter between gametes depends on where animals reproduce. If it occurs in aquatic conditions, most of the time unfertilized eggs are released in water and sperm cells too. In that case oocytes produce a chemical substance that attracts male gametes and fertilization is external (sea urchins).
If it occurs in terrestrial conditions, different processes lead male and female individuals to get close together and choose their partners (peacock spiders do mating dances, birds take part in singing contests, deers fight…). Mating ends up with male gametes being released inside the female body, closed by female gametes. This type of fertilization is internal.
In both cases, sperm cells swim till they reach an oocyte and one and only one can fuse with it thanks to specific interactions inducing the end of female gamete meiosis and joining both haploid nuclei to form the embryo nucleus.   

Embryos can grow inside or outside their mother : oviparous animals lay eggs when viviparous animals embryos develop inside the female body.

Oviparity is shared by species with external and internal fertilization and living in aquatic or terrestrial conditions. Different strategies have been selected with many fragile eggs produced or fewer but stronger eggs. In any case, the embryo feed on his own food stock till the egg hatches.
Viviparity is also shared by aquatic and terrestrial animals but all have an internal fertilization. After that fertilization, the egg stays inside the female where it develops. Nutrition is at one point insured by the mother. In mammals, the embryo implants into the uterine wall and feeding will be possible thanks to the placenta. This placenta brings maternal and foetal bloods close by and allow substances exchanges.
Sex determination can be due to genetics (XX/XY in Humans), to the environment (incubation temperature in some reptiles) or a mix of both…