World Weaving

First of all, what is a mammal?

The following characteristics make an animal a mammal:

Vertebrate (has a spine)

Warm-blooded (generates its own heat)

Hair/fur

Three middle ear bones

Mammary glands (glands that produce milk, hence the name)

Possess a neocortex (a region of the brain reptiles and birds don’t have)

Four chambered heart

Give birth to live young (except in the case of monotremes, which are egg-laying mammals such as the platypus. There are only five species of these though)

That’s the basics of it, and although they share these traits, there is huge diversity among mammals on Earth. Just think about it - an elephant and a mouse are both mammals, but they are still wildly different. You can be really creative when you brainstorm a mammal, because they are one of the most diverse animal groups. So, what can vary among mammals, and how?

Size and Metabolism

As you probably already know, mammals can vary wildly in size. It’s important to keep in mind, though, that the bigger the animal is, the more it will need to eat. The African elephant, one of the biggest mammals on Earth, can eat up to 200-600 pounds a day. In comparison, a rabbit is not going to eat nearly that much.

Habitat also is a factor when considering size, at least on the larger end. Mammals can get fairly large on land, but they can be even larger in the water, because the buoyancy of the water supports spinal structures that otherwise couldn’t withstand the full force of gravity on land.

Also, the bigger an animal is, the slower a metabolism it will have. Smaller animals have very fast metabolisms (they have to work harder to generate more heat). This occurs because the surface area to volume ratio increases as the animal gets smaller. Because smaller animals have more surface area relative to their mass, they lose heat faster, and therefore have to work harder to generate it. The faster the metabolism, the faster the heartbeat, and the animal will likely seem more hyperactive/energetic too.

Hair/Fur and Thermoregulation

Animals can range from being nearly hairless like humans, to possessing fur of Chewbacca proportions. This generally depends on habitat (humans developed in a very warm area first, so they didn’t need fur. When we moved to colder areas, we had a level of intelligence that allowed us to make clothes to help). The hotter the area, the less fur a mammal is going to have, since hair/fur is primarily for thermoregulation.

If an animal does have fur in a hot area (like a jackrabbit), it will need a way to shed excess heat. In the case of rodents/rabbits that live in the desert, they tend to have huge ears with lots of blood vessels and minimal fur, which allow the heat to dissipate through the ears’ large surface area. Some animals like dogs and other canines will pant to release heat from their tongues. In the case of cats, furless areas in addition to the ears, such as paw pads and the nose, help to release heat. Basically, if you are going to brainstorm a furry animal in a hot area, it’s the most realistic if the fur is short, and if it has a way to prevent itself from overheating. 

In contrast, an animal that lives in a cold area will likely have thick fur. Sometimes, there will be multiple layers of fur. Down-like fur that is softer tends to be closer to the skin, and coarser fur is closer to the outside. In extremely cold areas, a mammal may also have a layer of thick fat called blubber to keep itself warm. This is often the case with arctic marine mammals. In the case of some whales, they often only have blubber, and don’t possess fur other than a few whiskers or sparse hair.

Whiskers

Speaking of whiskers, not all hair-like structures on mammals are used for thermoregulation. Also called vibrissae, whiskers are specialized hairs that animals used to help interpret the environment around them. They possess a connection to the brain, and can pick up sensory data the animal otherwise wouldn’t be able to receive (vibrations in air/water, physical objects close by, etc). Whiskers can grow in many different places on animals, but are usually on the face. All animals, including primates, possess functional whiskers, except humans (sadly, a beard and mustache cannot pick up sensory vibrations).

Feet

The kind of feet a mammal has depends on what kind of mammals they are. Ungulates have hooves. Hooves are basically like enhanced toenails - they are surrounded by a thick layer of keratin. They are often worn down by running, which ungulates do a lot of (since they tend to be prey mammals).

Most other mammals have claws, which can be used for attacking, tearing apart food, or digging. How big/sharp the claw is depends on what it’s used for. A fine, sharp claw like a cat has wouldn’t be very practical for an animal like a badger, which digs heavily. Badger claws are blunt and tough, whereas cat claws are sharper and meant for tearing apart flesh.

Marine mammals, of course, tend to have fins. Some, like otters, have claws, but the less time the animal spends on land, the less likely it is to have claws. Mammals that live primarily in the water tend to use their teeth for attacking instead. 

In the case of primates, their extremities tend to be closer to hands and feet (since that is what humans have, after all). The nails on their hands and feet can grow sharp enough to be claws in some species, but in humans, they are generally not that tough and serve more of an ornamental feature nowadays.

Diet

Generally, mammals eat plant matter (herbivorous) or other animal matter (carnivorous). If they eat only insects, such as the anteater, they are insectivorous. Some mammals, like humans, can eat both plants and animals (omnivores).

Herbivorous animals are the lowest mammals on the food chain, and eat plants directly. Because plant matter is tough to digest (plant cell walls contain cellulose, a tough, fibrous material), herbivorous mammals usually have flat teeth to help grind plant material. They often have multiple stomachs to help digest plant matter completely.

Omnivores, which can eat both animal and plant matter, can digest plant matter, but not as well as herbivores can. In the case of humans, we only have one stomach, and it can’t digest all kinds of plant matter, just some. This is why things like celery strings and the tougher parts of vegetables sometimes don’t get digested. For the rest of their nutritional needs, omnivores tend to eat animal matter as well. An omnivore can survive on only plant matter with the right forethought (vegetarians/vegans are an example of this), but this is most common in humans. If you are brainstorming another sentient mammal species that can consider these things, though, having them be vegetarian despite being omnivores is perfectly plausible, though. 

Carnivores are animals that can’t digest plant matter at all (please, please never feed your cat a solely plant diet for this reason. They are carnivores and need meat). They rely on herbivores to break down the carbohydrates that plants produced via photosynthesis, and then they eat the herbivores (or other carnivores) to gain the nutrition they need. Carnivores tend to have sharp teeth that assist in tearing flesh of other animals to break it down.

Some carnivores, like the anteater, are specialized insectivores. They often have specialized mouths for hunting insects. For instance, anteaters have long snouts that can poke into insect tunnels, and they also possess long, thin tongues that can scoop the insects out. Since they don’t need to tear the insects apart, they don’t have teeth, and they use their sharp claws to break apart ant and termite mounds instead (and for defense).

Reproduction and Birth

Aside from egg-laying mammals such as echidnas and platypuses, mammals generally give birth to live young. Animals engage in sexual reproduction, and the female of the species becomes pregnant and gestates the young until it’s ready to be born. Gestation periods vary among mammals, but the longer it stays in the womb, the more developed the baby will be.

Some animals are born more independent than others, and this varies on the species. Ungulates (hoofed mammals) can often run and walk soon after birth, but the giant panda is born tiny and helpless, and needs a lot of protection after birth before it’s able to move around on its own. Animals that are more likely to be preyed upon tend to be more independent upon birth, to increase their chances of survival. If a mother wild horse had to carry its foal around because it was unable to walk, they would both be at a disadvantage, and a carnivorous predator could pick them off much more easily.

Mammals also possess mammary glands, which produce milk to feed their young. Even if the animal can walk shortly after birth, it usually stays with its mother to take advantage of this. Milk provides nutrients, and it assists the new baby’s immune system by carrying antibodies from the mother to help protect against illness.

Behavior/Social Structure

Mammals vary wildly when it comes to social structure, just like birds do. They can range from being completely solitary to forming huge groups, so any level of sociability is plausible for a mammal. There are pros and cons to different sociability levels.

Pros of Groups

Protection from predators (safer in numbers)

Assistance with care of young

Cons of Groups

Must share resources (groups function better when there are abundant resources)

More visible (predators will be attracted to large groups of prey animals)

A lot of prey animals form groups, but not all do. Sometimes it’s safer to be solitary and have other defense mechanisms against predators, like staying hidden and minding your own business. Most larger predators do not form groups (with some exceptions like lions), because otherwise, they would have to share prey, and all the effort that went toward a kill wouldn’t have a decent payoff anymore.

Pros of Solitary

Easier to hide

Don’t need to share

Can establish a territory (which can be important re: reproduction. Keeping others of your same sex out of your area increases an animal’s ability to mate with the opposite sex)

Cons of Solitary

You’re all on your own in this, pal

Nothing else around to distract predators

No assistance with care of young

Mammals can also have very simple social structure (i.e. hanging around in a group for protection reasons), or they can have complex, family-like structures. Family-like social structures tend to be seen in more intelligent animals like primates and cetaceans (whales/dolphins) where multiple generations of the same family tend to hang around together and help each other out. This is called “kin selection.” It is thought that by helping relatives survive, an animal helps to ensure that its genes, or genes closely related to it, are the ones that survive into future generations.

Courtship

The last thing to think about when creating a mammal species is courtship and mating. This varies wildly among mammal species, and some processes are more flamboyant displays than others. 

Usually, though, an animal that wishes to mate will display certain behaviors that are meant to attract the opposite sex. Mammals can also give off pheromones that signal that the animal is ready to mate, and other mammals of that species will be able to pick up on the pheromones.

Often, males of a species will fight to try and impress an individual it wishes to mate with. It’s not always males who do this, but that is what’s most common among Earth mammals. Ungulates are especially noted for doing this, often fighting with their antlers (if they have them). Often, a group of females (or just one) will watch a group of males fight, and will choose the one she wants to mate with.

If an individual animal is choosing who it wants to mate with, it can decide based on a number of factors:

Who wins the fight

Who looks the healthiest

Who has the biggest antlers/mane/other trait designed to attract mates

Gifts given/who is best at finding food

Who has been able to push all other contenders out of their territory

The following characteristics make an animal a bird:

Vertebrate (has a spine)

Feathers

Wings

Warm-blooded (generates its own heat)

Lay hard-shelled eggs

High metabolic rate

Light, airy skeleton

Air sacs connected to the lungs

That's the basics of it, but obviously, there's huge diversity among birds on Earth, and it makes sense that there also would be a fair amount of bird diversity in any world you might brainstorm. So, what can vary among birds, and how?

Feathers

Feathers do a lot of things than just look pretty (although looking pretty is one of their jobs, in some cases). Obviously they are also used for flight, but they are also important for insulation (preserving body heat), and can be used for communication as well.

Feather function determines how feathers look. Birds tend to have body feathers, flight feathers, and down feathers under the body feathers that are used for insulation. Body and flight feathers tend to be the feathers you see, and down feathers are a lot softer and less stiff.

In most cases (although there are exceptions where the females are more flamboyant, or both sexes are equally flamboyant), male birds tend to be more flamboyant than females. Because of this, they tend to have flashier feathers with brighter colors, and they also can have other interesting features like tail ornaments or crests. There is a disadvantage to this, though - if an ornament, crest, or bright colors impede movement or attract predators, that might be bad for the bird. However, some bird species still look like this, because they manage to reproduce before a predator manages to get them. Healthier birds tend to have brighter and fuller plumage, which lets females know that they are good males to pick, so they have the best chance of producing offspring.

Here's a few fairly flamboyant birds:

Blue-crowned motmot

Cockatoo

Fiery throated hummingbird

Beaks

All birds have beaks, but there are many different kinds of beaks! It all depends on what the bird eats, since beaks directly manipulate food. Here are some of the many varieties of beak, and there's more than you might think!

Birds can eat a variety of things, depending on the species:

Fruit

Seeds/grain

Fish

Carrion

Insects

Live mammalian prey

Human food

Nectar

Small pond organisms

Leaves (generally only the hoatzin and mousebird)

Birds who are generalists tend to eat a mixture of many of these, and these are often the birds who find it easiest to survive, especially in urban environments. Birds who only have one thing they eat are called specialists, and they are easier to make extinct. If their food supply disappears, they will too, and they don't have any other options.

Beaks are also used for grooming/preening feathers and using objects (some birds, like crows, use tools like sticks!),

Size

Birds on Earth range from the tiny bee hummingbird to the enormous ostrich. Generally, smaller birds have a higher metabolism and can move their wings more quickly. Since they are smaller, it's harder for them to keep themselves warm, and they have a higher activity level than larger birds as well (they often need to be faster to avoid predators, since they are smaller). Smaller birds also tend to feed more often than bigger birds. You'll see a hummingbird stop at a flower for nectar multiple times a day, but something like a large bird of prey may need to only hunt once a day, if that.

Eggs

All bird eggs have hard shells, unlike reptiles, which sometimes lay soft eggs. Since birds evolved from reptiles, it's likely that soft-shelled eggs are more primitive than hard-shelled eggs. There are still ways that eggs can vary, though!

First of all, eggs can vary by color and design (like spots). Most eggs are white or some pale color, but this is your world, so go crazy! It would be really neat to see a species with bright colors and ornate designs on its eggs.

Eggs can also vary by how their shell feels on the outside. Most eggs feel a generic sort of smooth when you touch them, but they can also be shiny/slippery, oily (waterproof, in swimming species), or rough/chalky. Also, most eggs are oval shaped, but cliff-dwelling species tend to have eggs that look more like rounded cones. This prevents eggs from rolling off a cliff's edge, because they roll around in a tight circle instead of a straight line.

Feet

First of all, birds walk on their toes, instead of their entire foot like humans do. Their feet are also covered with scales in most cases, and tend to be webbed in species that spend a lot of their time in the water, since webbing helps with swimming.

Bird feet tend to have one claw (even if it's small) on the end of each toe, for grasping things. Toes can also be arranged a variety of ways:

Most birds, especially songbirds, have anisodactyl toes. Woodpeckers, parrots, ospreys, and owls have zygodactyl toes. Only trogons have heterodactyl toes, and kingfishers/hornbills have syndactyl toes. Pamprodactyl toes are pretty much exclusive to mousebirds and swifts.

Wings/Flight

Most birds fly, although there are some exceptions. More primitive bids like the ostrich, who are more closely related to dinosaurs, tend to have wings that haven't evolved enough for flying yet. Or in the case of the penguin, their wings evolved to suit their environment - penguin wings are used for swimming, rather than flying.

Depending on the kind of flight, there are different types of wings. If a bird needs to do a lot of tight manoeuvring and precise flight, it will have elliptical wings, which are rounded and shorter. If a bird needs to move very quickly like a hummingbird or a falcon, its wings tend to be short and pointed. If a bird needs to soar, its wings are usually much longer than they are wide. An extreme example of this is the albatross, whose wings are incredibly long.

Behavior/Sociability

It's also important to think about how your particular species of bird acts. Birds have varying degrees of sociability. They can range from solitary to flocking birds, or somewhere in between. Being solitary helps a bird keep its resources to itself so no one else can steal them, but flocking provides protection from predators (both from sheer number of birds, as well as signals/noise other birds may give off if they see a predator).

Most prey birds try to keep themselves hidden from predators, and aren't very loud except for their courtship songs, although there are some exceptions. The tufted titmouse, for instance, lets out loud warning calls when it sees a predator, which alerts all the nearby songbirds to get out of the way, helping more birds than just the titmouse itself.

Most predatory birds keep to themselves, since they tend to hunt alone and not have many, if any, predators. Some smaller species are exceptions, though - certain species of bee eaters (you can probably guess what they prey on from their name) still form flocks, since they aren't as big as hawks or eagles.

Courtship

The last thing to think about is bird courtship (basically, how they woo a potential mate, who they choose to mate with, and how the eggs are raised). Most birds are socially monogamous, and stick with their mate for either a few years or a lifetime. There are some exceptions, though. In some species, males mate with multiple females, or females mate with multiple males.

Courtship displays often involve special calls or songs, as well as flight displays. Sometimes, females get to choose from a variety of males, all of whom are competing at the same time to try and impress her (either with calls or displays, or by fighting with each other). This is called a lek, and tends to be common in game and groundbirds.

Most of the time, it's the female who guards the nest and sits on the eggs, but this can also be reversed in some cases. Whoever isn't guarding the nest tends to bring food and supplies to the other parent. There are also birds like cowbirds, who lay their eggs in other birds' nests and force them to raise their chicks.