Is there differences in components of saliva among species

Is there differences in components of saliva among species

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Are there differences in the components of saliva among species? Also, do the functions of saliva differ between species?

It could be better answered if you specified which species you were interested in. Broadly from Wikipedia, snakes have venom, some swifts have gummy saliva to help in the building of nests, spiders produce their web from salivary glands etc. This is too broad a topic. The human salivary composition is found on the Wikipedia page I have linked. Salivary compositions can change with habits which have been detailed in this article. To compare, I am also providing the link to an article on the saliva composition of an assassin bug.

Snake venom

Snake venom is a highly modified saliva [1] containing zootoxins that facilitate the immobilization and digestion of prey, and defense against threats. It is injected by unique fangs during a bite, and some species are also able to spit their venom. [2]

The glands that secrete the zootoxins are a modification of the parotid salivary glands found in other vertebrates, and are usually situated on each side of the head, below and behind the eye, and encapsulated in a muscular sheath. The glands have large alveoli in which the synthesized venom is stored before being conveyed by a duct to the base of channeled or tubular fangs through which it is ejected. [3] [4]

Venoms contain more than 20 different compounds, mostly proteins and polypeptides. [3] A complex mixture of proteins, enzymes, and various other substances with toxic and lethal properties [2] serves to immobilize the prey animal, [5] enzymes play an important role in the digestion of prey, [4] and various other substances are responsible for important but non-lethal biological effects. [2] Some of the proteins in snake venom have very specific effects on various biological functions including blood coagulation, blood pressure regulation, and transmission of the nervous or muscular impulses, and have been developed for use as pharmacological or diagnostic tools, and even useful drugs. [2]

Is there differences in components of saliva among species - Biology

Insect Biology and Ecology: A Primer

For the reader who is unfamiliar with the biology or ecology of insects, this primer will provide needed background information.

This segment is comprised of several paragraphs of general insect information and five subsections:

Insects are the dominant life-form on earth. Millions may exist in a single acre of land. About one million species have been described, and there may be as many as ten times that many yet to be identified. Of all creatures on earth, insects are the main consumers of plants. They also play a major role in the breakdown of plant and animal material and constitute a major food source for many other animals.

Insects are extraordinarily adaptable creatures, having evolved to live successfully in most environments on earth, including deserts and the Antarctic. The only place where insects are not commonly found is the oceans. If they are not physically equipped to live in a stressful environment, insects have adopted behaviors to avoid such stresses. Insects possess an amazing diversity in size, form, and behavior.

It is believed that insects are so successful because they have a protective shell or exoskeleton, they are small, and they can fly. Their small size and ability to fly permits escape from enemies and dispersal to new environments. Because they are small they require only small amounts of food and can exist in very small niches or spaces. In addition, insects can produce large numbers of offspring relatively quickly. Insect populations also possess considerable genetic diversity and a great potential for adaptation to different or changing environments. This makes them an especially formidable pest of crops, able to adapt to new plant varieties as they are developed or rapidly becoming resistant to insecticides.

Insects are directly beneficial to humans by producing honey, silk, wax, and other products. Indirectly, they are important as pollinators of crops, natural enemies of pests, scavengers, and food for other creatures. At the same time, insects are major pests of humans and domesticated animals because they destroy crops and vector diseases. In reality, less than one percent of insect species are pests, and only a few hundred of these are consistently a problem. In the context of agriculture, an insect is a pest if its presence or damage results in an economically important loss.

The adage "know your enemy" is especially appropriate when it comes to insect pests. The more we know about their biology and behavior, including their natural enemies, the more likely we will be able to manage them effectively.

Insects and closely related organisms have a lightweight, but strong exterior skeleton (exoskeleton) or integument. Their muscles and organs are on the inside. This multi-layered exoskeleton protects the insect from the environment and natural enemies. The exoskeleton also has many sense organs for detecting light, pressure, sound, temperature, wind, and odor. Sense organs may be located almost anywhere on the insect body, not just on the head.

Insects have three body regions: head, thorax, and abdomen. The head functions mainly for food and sensory intake and information processing. Insect mouthparts have evolved for chewing (beetles, caterpillars), piercing-sucking (aphids, bugs), sponging (flies), siphoning (moths), rasping-sucking (thrips), cutting-sponging (biting flies), and chewing-lapping (wasps). The thorax provides structural support for the legs (three pairs) and, if present, for one or two pairs of wings. The legs may be adapted for running, grasping, digging, or swimming. The abdomen functions in digestion and reproduction.

The internal anatomy of insects is characterized by an open circulatory system, a multitude of breathing tubes, and a three-chambered digestive system. With the exception of a heart and an aorta, there are few blood vessels insect blood simply flows around inside the body cavity. Air enters the insect through a few openings (spiracles) in the exoskeleton, and makes its way to all areas of need by way of branching tubes, which permeate the body. The insect digestive system is long and tube-like, often divided into three sections, each with a different function. The insect nervous system transports and processes information received from the sense organs (sight, smell, taste, hearing, and touch). The brain, located in the head, processes information, but some information is also processed at nerve centers elsewhere in the body.

Knowledge about the structure and function of the insect exoskeleton has proven critical in developing insecticide formulations that are able to penetrate this multi-layered protective covering. Studies of insect communication have led to the discovery of chemical compounds used by insects to locate each other or host plants, and many of these have now been identified and produced synthetically. For example, pheromones are very specific compounds released by insects to attract others of the same species, such as for mating. Synthetic pheromones are now widely used to bait insect traps for detecting the presence of a pest, to determine its abundance, or for control. Control may involve the use of many traps to "trap out" the pest or the pheromones can be dispersed throughout the crop to "confuse" insects, making it more difficult for them to find a mate.

As simple as it may seem, knowing what type of mouthparts an insect has can be very important in deciding on a management tactic. For example, insects with chewing mouthparts can be selectively controlled by some insecticides that are applied directly to plant surfaces and are only effective if ingested contact alone will not result in death of the insect. Consequently, natural enemies that feed on other insects, but not the crop plant, will not be harmed.

Since insects obtain oxygen through their spiracles, plugging these openings causes death. That is how insecticidal oils control insects. Components of the microbial insecticide Bacillus thuringiensis enter the digestive system and break down the gut lining. Knowledge of the nervous system of insects has led to the development of several types of insecticides designed to disrupt normal nerve function. Some of these are effective simply by contacting the insect.

Most species of insects have males and females that mate and reproduce sexually. In some cases, males are rare or present only at certain times of the year. In the absence of males, females of some species may still reproduce. This is common, particularly among aphids. In many species of wasps, unfertilized eggs become males while fertilized eggs become females. In a few species, females produce only females.

A single embryo typically develops within each egg, except in the case of polyembryony, where hundreds of embryos may develop per egg. Insects may reproduce by laying eggs or, in some species, the eggs may hatch within the female which shortly thereafter deposits young. In another strategy common to aphids, the eggs hatch within the female and the immatures remain within the female for some time before birth.

Insect Growth and Development (Metamorphosis)

Insects typically pass through four distinct life stages: egg, larva or nymph, pupa, and adult. Eggs are laid singly or in masses, in or on plant tissue or another insect. The embryo within the egg develops, and eventually a larva or nymph emerges from the egg. There are generally several larval or nymphal stages (instars), each progressively larger and requiring a molt, or shed of the outer skin, between each stage. Most weight gain (sometimes > 90%) occurs during the last one or two instars. In general, neither eggs, pupae, nor adults grow in size all growth occurs during the larval or nymphal stages.

Complete Metamorphisis : Life Cycle of the convergent lady bug

The two types of metamorphosis typical of insect pests and natural enemies are gradual (egg > nymph > adult) and complete (egg > larva > pupa > adult). In gradual metamorphosis, the nymphal stages resemble the adult except that they lack wings and the nymphs may be colored differently than the adults. Nymphs and adults usually occupy similar habitats and have similar hosts. Gradual metamorphosis is typical of true bugs and grasshoppers complete metamorphosis is typical of beetles, flies, moths, and wasps. The immatures of these latter species do not resemble the adults, may occupy different habitats, and feed on different hosts. Some moth and wasp larvae weave a silken shell (cocoon) to protect the pupal stage in flies, the last larval skin becomes a puparium that protects the pupal stage.

Gradual Metamorphisis : Life cycle of the insidious flower bug

Insects are cold-blooded, so that the rate at which they develop is mostly dependent on the temperature of their environment. Cooler temperatures result in slowed growth higher temperatures speed up the growth process. If a season is hot, more generations may occur than during a cool season.

A better understanding of how insects grow and develop has contributed greatly to their management. For example, knowledge of the hormonal control of insect metamorphosis led to the development of a new class of insecticides called insect growth regulators (IGR). The insect growth regulators are very selective in the insects they affect. Based on information about insect growth rates relative to temperature, computer models can be used to predict when insects will be most abundant during the growing season and, consequently, when crops are most at risk.

Insect Classification and Identification

It is necessary to classify insects so that we can organize what we know about them and determine their relationships with other insects. For example, all members of a particular species will feed on similar foods, have similar developmental characteristics, and exist in similar environments. Most often, insect species are classified based on similarities in appearance (morphology). The flies, for example, can be distinguished and classified separately from all other winged insects because they have only one pair of wings. The hierarchy used to classify the diamondback moth, a worldwide pest of crucifers, is as follows

  • Phylum - Arthropoda
  • Class - Insecta
  • Order - Lepidoptera
  • Family - Plutellidae
  • Genus - Plutella
  • species - Plutella xylostella

This universal method is used to prevent confusion among geographic regions of the world. Consequently, Plutella xylostella refers to the same insect species in the United States as it does in Asia or anywhere else in the world. Common names, however, can vary from one location to another.

Ecology is the study of the interrelationships between organisms and their environment. An insect's environment may be described by physical factors such as temperature, wind, humidity, light, and biological factors such as other members of the species, food sources, natural enemies, and competitors (organisms using the same space or food source). An understanding or at least an appreciation of these physical and biological (ecological) factors and how they relate to insect diversity, activity (timing of insect appearance or phenology), and abundance is critical for successful pest management.

Some insect species have a single generation per season (univoltine), while others may have several (multivoltine). The striped cucumber beetle, for example, overwinters as an adult, emerges in the spring, and lays eggs near the roots of young cucurbit plants. The eggs hatch, producing larvae that emerge as adults later in the summer. These adults overwinter to start the cycle again the next year. In contrast, egg parasitoids like Trichogramma overwinter as immatures within the egg of their host. During the summer they may have several generations.

Insects adapt to many types of environmental conditions during their seasonal cycle. To survive the harsh winters, cucumber beetles enter a dormant state. While in this dormant state, metabolic activity is minimal and no reproduction or growth occurs. Dormancy can also occur at other times of the year when conditions may be stressful for the insect.

It is often better to consider insects as populations rather than individuals, especially within the context of an agroecosystem. Populations have attributes such as density (number per unit area), age distribution (proportion in each life stage), and birth and death rates. Understanding the attributes of a pest population is important for good management. Knowing the age distribution of a pest population may indicate the potential for crop damage. For example, if most of the striped cucumber beetles are immatures, direct damage to the above ground portions of the plant is unlikely. Similarly, if the density of a pest is known and can be related to the potential for damage, an action may be required to protect the crop. Information about death rates due to natural enemies can be very important. Natural enemies do nothing but reduce pest populations and understanding and quantifying their impact is important to effective pest management. This is all the more reason to conserve their numbers.


Our editors will review what you’ve submitted and determine whether to revise the article.

Saliva, a thick, colourless, opalescent fluid that is constantly present in the mouth of humans and other vertebrates. It is composed of water, mucus, proteins, mineral salts, and amylase. As saliva circulates in the mouth cavity it picks up food debris, bacterial cells, and white blood cells. One to two litres of fluid are excreted daily into the human mouth. Three major pairs of salivary glands and many smaller glands scattered in the surface tissue of the cheeks, lips, tongue, and palate contribute to the total amount of saliva. Small amounts of saliva are continually being secreted into the mouth, but the presence of food, or even the mere smell or thought of it, will rapidly increase saliva flow.

The functions of saliva are numerous. Primarily, it lubricates and moistens the inside of the mouth to help with speech and to change food into a liquid or semisolid mass that can be tasted and swallowed more easily. Saliva helps to control the body’s water balance if water is lacking, the salivary glands become dehydrated, leaving the mouth dry, which causes a sensation of thirst and stimulates the need to drink. Saliva reduces tooth decay and infection by removing food debris, dead cells, bacteria, and white blood cells. It also contains small amounts of the digestive enzyme amylase, which chemically breaks down carbohydrates into simpler compounds.

This article was most recently revised and updated by Michele Metych, Product Coordinator.

Vertebrate Digestive Systems

Vertebrates may have a single stomach, several stomach chambers, or accessory organs that help to break down ingested food.

Learning Objectives

Differentiate among the types of vertebrate digestive systems

Key Takeaways

Key Points

  • Monogastric animals have a single stomach that secretes enzymes to break down food into smaller particles additional gastric juices are produced by the liver, salivary glands, and pancreas to assist with the digestion of food.
  • The avian digestive system has a mouth (beak), crop (for food storage), and gizzard (for breakdown), as well as a two-chambered stomach consisting of the proventriculus, which releases enzymes, and the true stomach, which finishes the breakdown.
  • Ruminants, such as cows and sheep, are those animals that have four stomachs they eat plant matter and have symbiotic bacteria living within their stomachs to help digest cellulose.
  • Pseudo-ruminants (such as camels and alpacas) are similar to ruminants, but have a three-chambered stomach the symbiotic bacteria that help them to break down cellulose is found in the cecum, a chamber close to the large intestine.

Key Terms

  • peristalsis: the rhythmic, wave-like contraction and relaxation of muscles which propagates in a wave down a muscular tube
  • proventriculus: the part of the avian stomach, between the crop and the gizzard, that secretes digestive enzymes
  • cellulose: a complex carbohydrate that forms the main constituent of the cell wall in most plants

Vertebrate Digestive Systems

Vertebrates have evolved more complex digestive systems to adapt to their dietary needs. Some animals have a single stomach, while others have multi-chambered stomachs. Birds have developed a digestive system adapted to eating un-masticated (un-chewed) food.

Monogastric: Single-chambered Stomach

As the word monogastric suggests, this type of digestive system consists of one (“mono”) stomach chamber (“gastric”). Humans and many animals have a monogastric digestive system. The process of digestion begins with the mouth and the intake of food. The teeth play an important role in masticating (chewing) or physically breaking down food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The esophagus is a long tube that connects the mouth to the stomach. Using peristalsis, the muscles of the esophagus push the food towards the stomach. In order to speed up the actions of enzymes in the stomach, the stomach has an extremely acidic environment, with a pH between 1.5 and 2.5. The gastric juices, which include enzymes in the stomach, act on the food particles and continue the process of digestion. In the small intestine, enzymes produced by the liver, the small intestine, and the pancreas continue the process of digestion. The nutrients are absorbed into the blood stream across the epithelial cells lining the walls of the small intestines. The waste material travels to the large intestine where water is absorbed and the drier waste material is compacted into feces that are stored until excreted through the rectum.

Mammalian digestive system (non-ruminant): (a) Humans and herbivores, such as the (b) rabbit, have a monogastric digestive system. However, in the rabbit, the small intestine and cecum are enlarged to allow more time to digest plant material. The enlarged organ provides more surface area for absorption of nutrients.


Birds face special challenges when it comes to obtaining nutrition from food. They do not have teeth, so their digestive system must be able to process un-masticated food. Birds have evolved a variety of beak types that reflect the vast variety in their diet, ranging from seeds and insects to fruits and nuts. Because most birds fly, their metabolic rates are high in order to efficiently process food while keeping their body weight low. The stomach of birds has two chambers: the proventriculus, where gastric juices are produced to digest the food before it enters the stomach, and the gizzard, where the food is stored, soaked, and mechanically ground. The undigested material forms food pellets that are sometimes regurgitated. Most of the chemical digestion and absorption happens in the intestine, while the waste is excreted through the cloaca.

Bird digestive system: The avian esophagus has a pouch, called a crop, which stores food. Food passes from the crop to the first of two stomachs, called the proventriculus, which contains digestive juices that break down food. From the proventriculus, the food enters the second stomach, called the gizzard, which grinds food. Some birds swallow stones or grit, which are stored in the gizzard, to aid the grinding process. Birds do not have separate openings to excrete urine and feces. Instead, uric acid from the kidneys is secreted into the large intestine and combined with waste from the digestive process. This waste is excreted through an opening called the cloaca.


Ruminants are mainly herbivores, such as cows, sheep, and goats, whose entire diet consists of eating large amounts of roughage or fiber. They have evolved digestive systems that help them process vast amounts of cellulose. An interesting feature of the ruminants’ mouth is that they do not have upper incisor teeth. They use their lower teeth, tongue, and lips to tear and chew their food. From the mouth, the food travels through the esophagus and into the stomach.

To help digest the large amount of plant material, the stomach of the ruminants is a multi-chambered organ. The four compartments of the stomach are called the rumen, reticulum, omasum, and abomasum. These chambers contain many microbes that break down cellulose and ferment ingested food. The abomasum, the “true” stomach, is the equivalent of the monogastric stomach chamber. This is where gastric juices are secreted. The four-compartment gastric chamber provides larger space and the microbial support necessary to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient absorption, while the large intestine aids in the elimination of waste.

Ruminant mammal digestive system: Ruminant animals, such as goats and cows, have four stomachs. The first two stomachs, the rumen and the reticulum, contain prokaryotes and protists that are able to digest cellulose fiber. The ruminant regurgitates cud from the reticulum, chews it, and swallows it into a third stomach, the omasum, which removes water. The cud then passes onto the fourth stomach, the abomasum, where it is digested by enzymes produced by the ruminant.


Some animals, such as camels and alpacas, are pseudo-ruminants. They eat a lot of plant material and roughage. Digesting plant material is not easy because plant cell walls contain the polymeric sugar molecule cellulose. The digestive enzymes of these animals cannot break down cellulose, but microorganisms present in the digestive system can. Since the digestive system must be able to handle large amounts of roughage and break down the cellulose, pseudo-ruminants have a three-chamber stomach. In contrast to ruminants, their cecum (a pouched organ at the beginning of the large intestine containing many microorganisms that are necessary for the digestion of plant materials) is large. This is the site where the roughage is fermented and digested. These animals do not have a rumen, but do have an omasum, abomasum, and reticulum.

Mosquito Bites and the Effect of Saliva on Viral Replication and Dissemination

The comparison between virus infection dynamics in animals infected through various inoculation routes, has permitted to observe the impact that mosquito-aided viral entry can have on the host. Mice infected with WNV via mosquito bite show higher viral loads and earlier dissemination from local inoculation sites to neighboring tissues than mice infected by needle injection, including earlier breach of the nervous system (Styer et al., 2011). In rhesus macaques, inoculation of DENV through infected mosquito feeding leads to higher and longer viremia than inoculation via subcutaneous, intradermal, or intradermal + SGE needle injection. In addition, macaques infected via mosquito feeding and intradermal + SGE injection show skin inflammation and cellular infiltration at the site of inoculation and higher levels of liver aminotransferase than those infected through other inoculation routes (McCracken et al., 2020). In a similar study, investigators found that macaques infected with ZIKV via mosquito bite developed systemic infection and altered tissue tropism to the virus, which was disseminated mainly in the hemolymphatic tissues, female reproductive tract, liver, and kidneys, whereas the virus was also detected in the cerebrum of one animal and the eyes of the two animals inoculated via subcutaneous needle injection (Dudley et al., 2017). No significant difference was observed in the peak viral load between the two groups, but the time to reach the viral load was different, with the subcutaneous group reaching the viremia peak faster than the mosquito-infected group. In the case of RVFV, intradermal infection of mice in conjunction with SGE leads to a significant increase in viral titers in blood, brain, and liver, and more severe thrombocyto/leukopenia (Le Coupanec et al., 2013). Additionally, mice intradermally infected with RVFV and exposed to non-infected mosquito bites show shorter survival. Similarly, a mouse model for infection using an avirulent strain of the Semliki Forest virus, a virus closely related to CHIKV, showed that mice exposed to the virus after A. aegypti feeding showed higher viral RNA levels at the inoculation site than unbitten mice, as well as earlier dissemination to the brain and evolution to a lethal outcome in some of the mice (Pingen et al., 2016). Although not explored exhaustively, the effect that varying the localization of the saliva inoculum induces in the host has also been investigated. Mice that receive WNV and SGE inoculated together show significantly higher viral titers than those that receive the virus and SGE separately in distal locations, highlighting the local effect of salivary factors on enhancing viremia (Styer et al., 2011).

Replication and Latency

Replication of all herpesviruses is a multi-step process. Following the onset of infection, DNA is uncoated and transported to the nucleus of the host cell. This is followed by transcription of immediate-early genes, which encode for the regulatory proteins. Expression of immediate-early gene products is followed by the expression of proteins encoded by early and then late genes.

Assembly of the viral core and capsid takes place within the nucleus. This is followed by envelopment at the nuclear membrane and transport out of the nucleus through the endoplasmic reticulum and the Golgi apparatus. Glycosylation of the viral membrane occurs in the Golgi apparatus. Mature virions are transported to the outer membrane of the host cell inside vesicles. Release of progeny virus is accompanied by cell death. Replication for all herpesviruses is considered inefficient, with a high ratio of non-infectious to infectious viral particles.

A unique characteristic of the herpesviruses is their ability to establish latent infection. Each virus within the family has the potential to establish latency in specific host cells, and the latent viral genome may be either extra-chromosomal or integrated into host cell DNA. Herpes simplex virus 1 and 2 and varicella-zoster virus all establish latency in the dorsal root ganglia. Epstein-Barr virus can maintain latency within B lymphocytes and salivary glands. Cytomegalovirus, human herpesvirus 6 and 7, Kaposi's sarcoma herpesvirus and B virus have unknown sites of latency.

Latent virus may be reactivated and enter a replicative cycle at any point in time. The reactivation of latent virus is a well-recognized biologic phenomenon, but not one that is understood from a biochemical or genetic standpoint. It should be noted here that an anti-sense message to one of the immediate-early genes (alpha-O) may be involved in the maintenance of latent virus. Stimuli that have been observed to be associated with the reactivation of latent herpes simplex virus have included stress, menstruation, and exposure to ultraviolet light. Precisely how these factors interact at the level of the ganglia remains to be defined. It should be noted that reactivation of herpesviruses may be clinically asymptomatic, or it may produce life-threatening disease.

Is there differences in components of saliva among species - Biology

The placentas of all eutherian (placental) mammals provide common structural and functional features, but there are striking differences among species in gross and microscopic structure of the placenta. Two characteristics are particularly divergent and form bases for classification of placental types:

  1. The gross shape of the placenta and the distribution of contact sites between fetal membranes and endometrium.
  2. The number of layers of tissue between maternal and fetal vascular systems.

Differences in these two properties allow classification of placentas into several fundamental types.

Classification Based on Placental Shape and Contact Points

Examination of placentae from different species reveals striking differences in their shape and the area of contact between fetal and maternal tissue:

  • Diffuse : Almost the entire surface of the allantochorion is involved in formation of the placenta. Seen in horses and pigs.
  • Cotyledonary : Multiple, discrete areas of attachment called cotyledons are formed by interaction of patches of allantochorion with endometrium. The fetal portions of this type of placenta are called cotyledons, the maternal contact sites (caruncles), and the cotyledon-caruncle complex a placentome. This type of placentation is observed in ruminants.
  • Zonary : The placenta takes the form of a complete or incomplete band of tissue surrounding the fetus. Seen in carnivores like dogs and cats, seals, bears, and elephants.
  • Discoid : A single placenta is formed and is discoid in shape. Seen in primates and rodents.

Classification Based on Layers Between Fetal and Maternal Blood

Just prior to formation of the placenta, there are a total of six layers of tissue separating maternal and fetal blood. There are three layers of fetal extraembryonic membranes in the chorioallantoic placenta of all mammals, all of which are components of the mature placenta:

  1. Endothelium lining allantoic capillaries
  2. Connective tissue in the form of chorioallantoic mesoderm
  3. Chorionic epithelium, the outermost layer of fetal membranes derived from trophoblast

There are also three layers on the maternal side, but the number of these layers which are retained - that is, not destroyed in the process of placentation - varies greatly among species. The three potential maternal layers in a placenta are:

  1. Endothelium lining endometrial blood vessels
  2. Connective tissue of the endometrium
  3. Endometrial epithelial cells

One classification scheme for placentas is based on which maternal layers are retained in the placenta, which of course is the same as stating which maternal tissue is in contact with chorionic epithelium of the fetus. Each of the possibilities is observed in some group of mammals.

Type of Placenta Maternal Layers Retained Examples
Epitheliochorial + + + Horses, swine, ruminants
Endotheliochorial - - + Dogs, cats
Hemochorial - - - Humans, rodents

In humans, fetal chorionic epithelium is bathed in maternal blood because chorionic villi have eroded through maternal endothelium. In contrast, the chorionic epithelium of horse and pig fetuses remains separated from maternal blood by 3 layers of tissue. One might thus be tempted to consider that exchange across the equine placenta is much less efficient that across the human placenta. In a sense this is true, but other features of placental structure make up for the extra layers in the diffusion barrier it has been well stated that " The newborn foal provides a strong testimonial to the efficiency of the epitheliochorial placenta. "

Summary of Species Differences in Placental Architecture

The placental mammals have evolved a variety of placental types which can be broadly classified using the nomenclature described above. Not all combinations of those classification schemes are seen or are likely to ever be seen - for instance, no mammal is known to have a diffuse, endotheliochorial, or a hemoendothelial placenta. Placental types for "familiar" mammals are summarized below, with supplemental information provided for a variety of "non-familiar" species.

Type of Placenta Common Examples
Diffuse, epitheliochorial Horses and pigs
Cotyledonary, epitheliochorial Ruminants (cattle, sheep, goats, deer)
Zonary, endotheliochorial Carnivores (dog, cat, ferret)
Discoid, hemochorial Humans, apes, monkeys and rodents


Comparative Placentation is an excellent and comprehensive site for obtaining information on placental structure for a large number of domestic and wild animals

Conclusions and Perspectives

B. burgdorferi produces a number of products that allow it to colonize and persist in its natural mammalian and tick hosts. Although the functions of only a few B. burgdorferi products have been clearly defined, some (such as OspC) are required for the bacteria to survive the initial attack of the mammalian innate immune system, while others (like VlsE) contribute to resisting the subsequent acquired immune response. Bacterial factors such as RpoS and RpoN are components of signaling cascades regulating gene expression for survival in different environmental conditions. With the powerful genetic techniques now available for manipulating the spirochete, the mouse, and even the tick, the interactions among these three that lead to infection and disease are beginning to emerge.

Charles Darwin and Natural Selection

Charles Darwin and Alfred Wallace independently developed the theories of evolution and its main operating principle: natural selection.

Learning Objectives

Explain how natural selection can lead to evolution

Key Takeaways

Key Points

  • Wallace traveled to Brazil to collect and observe insects from the Amazon rainforest.
  • Darwin observed that finches in the Galápagos Islands had different beaks than finches in South America these adaptations equiped the birds to acquire specific food sources.
  • Wallace and Darwin observed similar patterns in the variation of organisms and independently developed the same explanation for how such variations could occur over time, a mechanism Darwin called natural selection.
  • According to natural selection, also known as “survival of the fittest,” individuals with traits that enable them to survive are more reproductively successful this leads to those traits becoming predominant within a population.
  • Natural selection is an inevitable outcome of three principles: most characteristics are inherited, more offspring are produced than are able to survive, and offspring with more favorable characteristics will survive and have more offspring than those individuals with less favorable traits.

Key Terms

  • natural selection: a process in which individual organisms or phenotypes that possess favorable traits are more likely to survive and reproduce
  • descent with modification: change in populations over generations

Charles Darwin and Natural Selection

In the mid-nineteenth century, the mechanism for evolution was independently conceived of and described by two naturalists: Charles Darwin and Alfred Russel Wallace. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world to places like South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, as with Wallace’s later journeys to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador. On these islands, Darwin observed that species of organisms on different islands were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with a unique beak shape. The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the mainland of South America. Darwin imagined that the island species might be modified from one of the original mainland species. Upon further study, he realized that the varied beaks of each finch helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, while insect-eating finches had spear-like beaks for stabbing their prey.

Beak Shape Among Finch Species: Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral species had adapted over time to equip the finches to acquire different food sources.

Natural Selection

Wallace and Darwin observed similar patterns in other organisms and independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, also known as “survival of the fittest,” is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits. This leads to evolutionary change, the trait becoming predominant within a population. For example, Darwin observed that a population of giant tortoises found in the Galapagos Archipelago have longer necks than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could reach more leaves and access more food than those with short necks. In times of drought, when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that could not reach the food source. Consequently, long-necked tortoises would more probably be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.

Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring, although how traits were inherited was unknown. Second, more offspring are produced than are able to survive. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace were influenced by an essay written by economist Thomas Malthus who discussed this principle in relation to human populations. Third, Darwin and Wallace reasoned that offspring with the inherited characteristics that allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over successive generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment it is the only mechanism known for adaptive evolution.

Papers by Darwin and Wallace presenting the idea of natural selection were read together in 1858 before the Linnean Society in London. The following year, Darwin’s book, On the Origin of Species, was published. His book outlined his arguments for evolution by natural selection.

Charles Darwin and Alfred Wallace: Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented together before the Linnean Society in 1858.

Studies of Natural Selection After Darwin

Demonstrations of evolution by natural selection can be time consuming. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important demonstrations of the operation of natural selection. The Grants found changes from one generation to the next in the beak shapes of the medium ground finches on the Galápagos island of Daphne Major.

The medium ground finch feeds on seeds. The birds have inherited variation in the bill shape with some individuals having wide, deep bills and others having thinner bills. Large-billed birds feed more efficiently on large, hard seeds, whereas smaller billed birds feed more efficiently on small, soft seeds. During 1977, a drought period altered vegetation on the island. After this period, the number of seeds declined dramatically the decline in small, soft seeds was greater than the decline in large, hard seeds. The large-billed birds were able to survive better than the small-billed birds the following year.

The year following the drought when the Grants measured beak sizes in the much-reduced population, they found that the average bill size was larger. This was clear evidence for natural selection of bill size caused by the availability of seeds. The Grants had studied the inheritance of bill sizes and knew that the surviving large-billed birds would tend to produce offspring with larger bills, so the selection would lead to evolution of bill size. Subsequent studies by the Grants have demonstrated selection on and evolution of bill size in this species in response to other changing conditions on the island. The evolution has occurred both to larger bills, as in this case, and to smaller bills when large seeds became rare.

Figure (PageIndex<1>): Finches of Daphne Major: A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available to finches, causing many of the small-beaked finches to die. This caused an increase in the finches&rsquo average beak size between 1976 and 1978.