Defensive Strategies in Biology

Defensive Strategies-1

• Animals, plants, and even bacteria must defend themselves to survive and reproduce.
• Defensive strategies are diverse, including camouflage, poisons, venoms, and various physical adaptations.

Primary Principle

• The primary principle behind defensive strategies is that organisms need to protect themselves from predators, herbivores, and parasites (pathogens).
• Parasites live on or in other organisms for part of their life cycle.
• Pathogens cause diseases in the host after infecting it.

Defensive Strategies Continued

• Camouflage is a strategy used by many organisms to blend in with their surroundings, helping them avoid detection.
• Highly venomous bites, such as those from the Gaboon Viper (Bitis japonica), are effective defense mechanisms.
• Some animals, like skunks, use foul-smelling secretions as a defense.
• Other animals, like the Fulmar petrel, use projectile regurgitation as defense.
• Structural adaptations, such as spines on hedgehogs and porcupines, are also used for defense.
• Some animals use poison glands for defense.

Lectures Overview

• The lectures cover basic defenses in plants and animals.
• Poisons and venoms are discussed as defensive mechanisms.
• Mimicry, a strategy of using resemblance to another organism for protection, is a topic of the lectures.
• Immune system defenses, innate immunity, and the adaptive immune response are also part of the lecture material.
• Epidemiology, R0, herd immunity, and novel pathogens are included.

Outline of Issues

• Evolutionary context is important for understanding defenses.
• Biological fitness refers to the relative ability of an individual to survive and reproduce within a population.
• Darwin emphasized survival, but neo-Darwinism (modern synthesis) – from 1930 onward – places greater emphasis on reproductive output.
• Lifetime Reproductive Success (LRS) is an important measure of evolutionary success.

Predators and Parasites

• Predators and parasites significantly affect Lifetime Reproductive Success (LRS) and fitness.
• Plants and animals, including bacteria, must defend against predators and parasites.
• Parasites are organisms that at least part of their life cycle is on/in another organism.
• Pathogens are organisms that cause disease to the host after infection.
• Defense can be physical, chemical, and/or behavioral.

Clutch Size and LRS

• Lifetime Reproductive Success (LRS) is determined by counting eggs and tracking offspring survival.

Parasites and Predators Continued

• Parasites are typically smaller than their hosts, and have a shorter life expectancy.
• Many parasites infect only one or a few hosts in their lifetime; rarely do they kill.
• Parasites are categorized as Micro and Macro forms partially based on immune response.
• Predators are typically larger than their prey and have a longer life expectancy.
• Predators often attack many different prey in their lifetime.
• Predators typically kill their prey.

Relationship Between Survival and Reproductive Success

• Correlation data shows a relationship between survival and reproductive success.

Mosquitoes

• Specifically, Aedes aegypti mosquitoes act as vectors for various diseases, including yellow fever.
• They also transmit malaria, a parasitic disease that causes high mortality among humans.

Micro-pathogens (Micro-parasites)

• Micro-pathogens, also called micro-parasites, have a very small size and quick replication within their hosts.
• They cause severe morbidity and mortality in large numbers of people.
• Micro-pathogens are commonly viruses, bacteria, and protozoa.

Viruses

• There are different characteristics of viruses, including whether they are enveloped or non-enveloped.
• Examples of families of viruses include herpesviridae, cornavidae, retroviridae, etc.

Influenza Viruses

• Influenza viruses have different types (A, B, and C) and subtypes.
• The taxonomy and classification of influenza viruses are based on their surface antigens (HA and NA).
• The genome is segmented.
• Their structure is enveloped, and potentially pleomorphic ranging from spheres to extremely long filaments with helical capsid.

Macroparasites

• Macroparasites are large in size and do not directly and quickly reproduce within their host.
• They do not typically cause severe morbidity or high mortality in their hosts, however the number of parasites present may cause a health burden.
• Complex life cycles are common, and frequently involve multiple intermediate hosts.
• Examples include worms, ticks, lice, bed bugs, and various types of flys.

Ticks, Lyme Disease, Rabies, and Roundworms

• Ticks can transmit Lyme disease.
• Rabies is a virus that can infect foxes and be fatal.
• There are different types of worms (Roundworms & Tapeworms).

Points to note

• Both attack and defense can be expensive in terms of energy allocation.
• Defense benefits should outweigh the cost of defense.
• Plants and animals invest in various defense mechanisms.
• Defenses can be physical, chemical, and cellular.

Basic Defenses-1

• Bacteria also have "immune" defenses.
• One discovered recently is CRISPR-Cas9.
• Cas9 is an enzyme that snips out parts of invading viral nucleic acid.
• This enzyme acts against phages (viruses that infect bacteria).

Basic Defenses-1 (Animals and Plants)

• Structural defenses include spines, thorns, trichomes, modified skin (e.g., hedgehog, porcupine).
• Other defenses include fowl smells (e.g., skunk, fulmar petrel) and poison (e.g., salamander).

Plant Defenses

• Plants are immobile but are attacked by herbivores.
• The interaction between plants and herbivores dominates terrestrial environments.
• Constitutive and inductive defenses are common in plants.

Plant Defenses Continued

• Constitutive defenses are preformed chemical defenses, like alkaloids and terpenes, that are always present in the plant.
• Inductive defenses are triggered in response to an attack.
• These defenses can involve volatile organic compounds summoned by the plant.

Constitutive Compounds

• These include alkaloids like Curare, Atropine, Cocaine, Strychnine, and Nicotine.
• These compounds negatively affect insect grazers.

Inductive Compounds

• These are volatile organic compounds (VOCs) and are released in response to plant damage or herbivore presence.
• Biogenic VOCs (BVOC) also contribute to defense.
• Herbivore-Induced Plant Volatiles (HIPVs) are summoned by herbivores to deter them or attract their enemies.
• Allelopathy refers to chemicals released by one organism that impact an other organism's growth and reproduction (positive or negative).

Physical and Chemical Defenses in Plants

• Various physical and chemical defenses work together in plants.

Poisons

• Poisons are compounds that are produced or sequestered by animals.
• Examples include cane toads, which produce bufotoxin, resulting in toxicity to predators.
• Amphibians, puffer fish, and other organisms also produce toxic substances.

Venoms

• Venom is a toxic substance delivered via a bite, often causing mechanical damage.
• Venom is commonly used in prey capture.
• Examples of venoms are those from snakes (cobra, pit viper, mamba), spiders, scorpions, and other organisms.

Aposematic Colouration

• Aposematic coloration is a warning strategy used by animals with unpalatable or poisonous substances.
• Bright colors and patterns are used to signal danger to potential predators.

Mimicry

• Two main types of mimicry are Mullerian and Batesian.
• Mullerian mimicry involves multiple unpalatable species developing similar warning signals.
• Batesian mimicry involves a harmless species adopting the warning signals of a harmful one to deceive predators.
• Mimicry can be complex and controversial and may overlap with camouflage.

Camouflage and Crypsis

• Camouflage and crypsis are defensive strategies that allow animals to blend in with their surroundings.
• These strategies help to avoid detection by predators or for ambush hunting by offensively using camouflage.

Summary-1

• Different defensive strategies exist in various organisms.
• Plant defenses can be constitutive (always present) or induced (in response to attack).
• Animals use various defense mechanisms, including venom, poisons, and camouflage.