Animal Diversity and Behavior: Study Guide

Animal Diversity

Key Characteristics of Animals

Animals are efficient consumers due to five key characteristics that distinguish them from other life forms:

• Multicellularity: Enables development of specialized organs and systems for locating, capturing, and processing food.

• Heterotrophy: Animals must consume other organisms for energy, driving evolution of diverse feeding strategies, mouthparts, and digestive systems.

• No Cell Walls: Lack of rigid cell walls allows flexibility, movement, and development of specialized tissues for seeking and capturing food.

• Nervous & Muscle Tissue: Nervous tissue enables environmental sensing and rapid response; muscle tissue allows movement.

• Sexual Reproduction & Developmental Stages: Sexual reproduction increases genetic diversity; complex life cycles allow exploitation of different food sources and habitats.

Origin and Evolution of Animals

• First Animals: Emerged 600–700 million years ago.

• Closest Living Relatives: Choanoflagellates are the closest living relatives to animals.

• Cambrian Explosion: Occurred ~541 million years ago, greatly increasing animal diversity and setting the stage for modern ecosystems.

Animal Classification Criteria

Animals are categorized based on four main criteria:

• Symmetry:

  • Asymmetrical: No symmetry (e.g., sponges).

  • Radial Symmetry: Body parts arranged around a central axis (e.g., jellyfish).

  • Bilateral Symmetry: Left and right sides are mirror images (e.g., humans).

• Tissues:

  • No true tissue: Simplest animals (e.g., sponges).

  • Diploblastic: Two tissue layers (ectoderm and endoderm).

  • Triploblastic: Three tissue layers (ectoderm, mesoderm, endoderm).

• Body Cavities:

  • Acoelomate: No body cavity.

  • Pseudocoelomate: Body cavity not fully lined with mesoderm.

  • Coelomate: True body cavity completely lined with mesoderm.

• Developmental Patterns:

  • Protostome: Mouth develops first from embryonic opening.

  • Deuterostome: Anus develops first from embryonic opening.

Protostome vs. Deuterostome Development

Feature	Protostome	Deuterostome
First opening	Mouth	Anus
Cleavage	Spiral	Radial
Development	Determinate	Indeterminate
Examples	Mollusks, Arthropods	Echinoderms, Chordates

Major Evolutionary Events

• Vertebrates: First evolved in the Cambrian Period (Paleozoic Era).

• Mammals: Major diversification occurred in the Cenozoic Era, after dinosaur extinction.

Invertebrates

Definition and Diversity

Invertebrates are animals without a backbone. They differ from vertebrates by lacking an internal skeleton and exhibit vast diversity in form and adaptation.

Major Invertebrate Groups

Group	Key Traits
Porifera	No true tissues/organs, asymmetrical, filter feeders
Cnidaria	True tissues, radial symmetry, diploblastic, gastrovascular cavity, stinging cells
Lophotrochozoa	Bilateral symmetry, triploblastic, protostomes, diverse body plans
Ecdysozoa	Bilateral symmetry, triploblastic, protostomes, grow by molting exoskeleton/cuticle
Echinodermata	Deuterostome development, radial symmetry as adults, endoskeleton of calcareous plates, water vascular system, marine

Arthropods: Unique Ecdysozoans

• Unique Features: Segmented bodies, jointed appendages, chitinous exoskeleton.

• Subgroups:

  • Chelicerates: First pair of appendages are fangs/pincers (e.g., spiders).

  • Myriapods: Body segments with one/two pairs of legs per segment (e.g., centipedes).

  • Crustaceans: Mainly aquatic, two pairs of antennae (e.g., crabs).

  • Insects: Three main body regions, three pairs of legs, two pairs of wings (e.g., butterflies).

Echinodermata: Deuterostome Invertebrates

• Radial symmetry as adults, bilateral symmetry as larvae.

• Endoskeleton of calcareous plates.

• Water vascular system with tube feet for movement and feeding.

• Exclusively marine.

Vertebrates

Chordate Characteristics

All chordates share five key characteristics:

• Notochord: Flexible rod for support and structure.

• Dorsal Hollow Nerve Cord: Develops into central nervous system.

• Pharyngeal Sites/Clefts: Openings in pharynx; gills in aquatic chordates, various head/neck structures in terrestrial vertebrates.

• Endostyle/Thyroid Gland: Produces mucus for trapping food; develops into thyroid gland in vertebrates.

• Post-Anal Tail: Extension past anal opening; propulsion in aquatic species, reduced in terrestrial species but present in embryos.

Vertebrate Cladogram (Names and Traits)

Group	Traits
Jawless vertebrates (Agnathans)	Vertebral column
Jawed vertebrates (Gnathostomes)	Jaws
Cartilaginous fishes (Chondrichthyes)	Cartilage skeleton
Bony fishes (Osteichthyes)	Bony skeleton
Tetrapods	Lobed fins/limbs
Amphibians	Lungs/lung derivatives
Amniotes	Amniotic egg
Reptiles (including birds)	Scaly skin, feathers (birds)
Mammals	Hair, mammary glands

Main Groups of Fish

Group	Jaw	Skeleton	Fins
Myxini	No	Cartilage	No paired fins
Petromyzontidae	No	Cartilage	No paired fins
Chondrichthyes	Yes	Cartilage	Paired fins
Actinopterygii	Yes	Bone	Ray-finned
Sarcopterygii	Yes	Bone	Lobe-finned

Tetrapods and Amphibians

• Tetrapods: Adapted for terrestrial life with limbs and lungs.

• Amphibians: Transitional taxa bridging aquatic fish and fully terrestrial amniotes; show both aquatic and terrestrial adaptations.

Amniotic Egg

• Protective membranes and shell.

• Self-contained nutrients and waste management.

• Allowed tetrapods to reproduce away from water.

Major Groups of Reptilia

Group	Unique Traits
Turtles	Shell
Squamates	Flexible jaws, diverse body forms, some give live birth
Crocodilians	Four-chambered heart, parental care
Birds	Feathers, flight adaptations, hollow bones

Derived Traits of Birds

• Feathers

• Hollow Bones

• Beak Without Teeth

Derived Characteristics of Mammals and Lineages

• Mammary Glands: Produce milk for offspring.

• Hair/Fur: Insulation and protection.

• Three Middle Ear Bones: Enhance hearing.

• Specialized Teeth: Adapted for various diets.

Lineage	Development
Monotremes	Lay eggs
Marsupials	Birth to underdeveloped young; development in pouch
Eutherians	Young develop in uterus, nourished by placenta

Human Evolution

• Humans are primates, closely related to chimpanzees and bonobos.

• Modern humans (Homo sapiens) evolved in Africa 200,000–300,000 years ago.

Animal Behavior

Tinbergen's Four Questions

• Causation: Immediate physiological or environmental triggers for behavior.

• Development: How behavior develops over an animal’s lifetime.

• Function: Evolutionary advantages or purpose of behavior.

• Evolution: Evolutionary history among related species.

Types of Animal Behaviors

• Fixed Action Pattern: Sequence of innate behaviors, unchangeable, carried to completion once triggered.

• Sign Stimulus: Specific external cue triggering a fixed action pattern.

• Circannual Rhythm: Yearly behavioral cycles, often in response to seasonal environmental changes.

• Migration: Long-distance, usually seasonal movement between regions.

Proximate vs. Ultimate Causation

• Proximate Causation: Immediate triggers and mechanisms (e.g., hormonal changes).

• Ultimate Causation: Evolutionary advantage or function (e.g., increased reproductive success).

Example: Painted Bunting Mating Behavior

• Migration timing to breeding grounds.

• Territorial defense.

• Song and visual displays.

• Courtship behaviors to attract mate.

Innate vs. Learned Behaviors

Type	Definition	Example
Innate	Inherited, performed without prior experience	Baby grasping a finger
Learned	Shaped by experience and environment	Ducklings following first moving object

Culture and Social Learning

• Allows transmission of knowledge and behaviors.

• Shapes interactions with environment and other individuals.

• Increases behavioral flexibility and adaptability.

Mating Systems: Trade-Offs

System	Features	Trade-Offs
Monogamy	One male, one female	Both parents invest in care; fewer mating opportunities
Polygyny	One male, multiple females	Males father many offspring; less parental care by males
Polyandry	One female, multiple males	Increased genetic diversity; females invest more energy
Promiscuity	Multiple partners, no strong pair bonds	Maximizes genetic diversity; little/no parental care

Altruism, Inclusive Fitness, and Kin Selection

• Altruism: Behavior reducing individual fitness to increase another’s fitness.

• Inclusive Fitness: Evolutionary success includes helping relatives’ offspring survive and reproduce.

• Kin Selection: Natural selection favoring behaviors that increase relatives’ reproductive success, even at a cost to the individual.