Bats L8

Bat Overview

  • Only mammals capable of flight.
  • Use complex echolocation strategies.
  • Exhibit long lifespans and slow reproductive rates.
  • Live in complex social groups.

Bat Key Features

  • Nose morphology.
  • Ear morphology including tragus.
  • Tail morphology.

Bat Evolutionary History

  • Date back to the Eocene epoch.
  • Likely evolved in forested environments, leading to poor fossil preservation.
  • Incomplete fossil record often relies on teeth.
  • Rudimentary flight and echolocation evolved by the early Eocene.
  • Enlarged cochleae indicate highly derived echolocation.

Flight vs. Echolocation

  • The bat wing is a patagium.
  • Onychonycteris finneyi had low aerodynamic efficiency but could flutter.
  • Tanzanycteris mannardi had derived echolocation.
  • Several hypotheses exist:
    • Echolocation first: Primitive echolocation used for prey detection evolved before powered flight, then lost in Pteropodidae, with convergent evolution in Rhinolophoidea and Vespertilioniformes.
    • Powered flight first: Echolocation linked to flight.
    • Gliding model: Supported by limb structure and fossils.

Convergent Evolution

  • Convergent evolution in laryngeal echolocation.
  • Mormoopidae also use CF echolocation, but produce it differently.
  • Pteropodidae likely lost laryngeal echolocation.

Cryptic Speciation

  • Tight link between echolocation, morphology, and lifestyle leads to large cryptic variation.
  • Over 1430 known species, but more are possible.
  • Diversity reflects variations in diet, roosting, and social behavior.

Echolocation

  • Allows bats to "see" using sound.
  • Three main forms:
    • Tongue click / wing click.
    • Orally emitted.
    • Nasally emitted.
  • Not all bats echolocate; some fruit bats use vision or olfaction.

Key Features of Echolocation Calls

  • Frequency: Low travels further but gives coarse spatial information; high attenuates quicker but reflects from small objects.
  • Band width.
  • Duration: Longer call travels further with less detail; shorter calls used in cluttered environments.
  • Amplitude: Louder call travels further.

Echolocation and Body Size

  • Increase in frequency = decrease in wavelength.
  • Small bat = small emitter (mouth) = broader beam.

Phases of a Call

  • Search phase: Widely spaced calls to detect distant objects.
  • Approach phase: Shorter and faster calls.
  • Terminal buzz phase: Just before capture.

Echolocation Modes

  • Oral: Frequency modulated calls with broadband sweep; self-deafening; short duration; detect fine details.
  • Nasal: Predominantly constant frequency calls with highly evolved cochleae; long duration calls; adjust frequency for Doppler shift; nose leaf structure creates a stable narrowband beam.
  • Mixing Oral and Nasal: Mouth and nasal emitting can occur simultaneously to improve accuracy and reduce energetic costs.

Evolutionary Arms Races

  • Bats evolve echolocation to hunt moths.
  • Moths evolve "ears" (tympanal organs) and escape behaviors.
  • Bats evolve "whispering strategy."
  • Butterflies move into daylight.

Flight Overview

  • Bat wing morphology and echolocation are tightly linked.
  • Aspect ratio and wing loading are important.
  • Bat flight is more precise than birds due to wing morphology.

Bat Wings

  • Convex curve of wing = air moves faster over rather than under wing = lift.
  • Angle of attack (AoA): Bats finely adjust AoA by changing wing shape, allowing for precise lift production and control.
  • Flexible wings allow AoA adjustments across different parts of the wings.
  • Digits reduce turbulence.
  • Can create thrust on both downstroke and upstroke.

Wing Shape and Flying Mode

  • Wide, short wings with rounded tips = manoeuvrable, slow flying.
  • Long, thin wings with pointed tips = fast, less agile flying.

Terrestrial Movement

  • Some bats, like Mystacina tuberculata, spend most of their time on the ground.
  • Vampire bats (Desmodus rotundus) can walk, run, and jump.

Physiological Impacts of Flight

  • Beat wings 10 times per second.
  • Take a breath on each wing beat.
  • Exhale late in upstroke, echolocate coinciding with wingbeat.
  • Heart rate increases 2-6x resting rate.
  • Metabolic rate during flight increases 14x rest.
  • Increase in body temperature.

Bat Distribution

  • Found on every continent except Antarctica.

Sociality in Bats

  • Exhibit huge diversity of social systems linked to roosting and reproductive ecology.
  • Use specific social calls for aggression, distress, mother-offspring communication, and courtship.
  • Some species recognize individuals through calls and have "friends."

Mating Strategies

  • Promiscuity: Swarming at mines/caves with multiple copulations and no mate guarding; common in large, dispersed populations.
  • Polygyny: Males select a mating roost and sing to attract a harem of females.
  • Lekking: Males come to singing sites to attract females; rare in mammals but used by several bat species.
  • Monogamy: Roost and forage together, highly territorial.
  • Mating disruption: Males guard females by following and being aggressive towards other males.

Roosting Strategies

  • Plants: Exposed roosts, tree cavities, inside carnivorous pitcher plants, large roosts in trees, inside rolled leaves.
  • Caves: Clumped or separate, often in crevices with important microclimatic conditions.
  • Manmade Structures: Houses, bridges, railway tunnels, barns, sheds; maternity colonies prefer warmer sites, hibernation in cooler sites.

Surviving Lean Times

  • Migration and hibernation are common strategies.
  • Hibernation: Reduce metabolic rate, decrease body temperature, reduce heart rate and respiration; involves torpor (short-term hibernation), fat stores, and periodic rousing.
  • Hibernation locations differ by exposure, bat condition, temporal variability, and species-specific variability.

Migration

  • Common strategy for accessing different food sources (tropical bats) or food/hibernation (temperate bats).
  • Tends to be intracontinental movements.
  • Partial migration is common.
  • Eidolon helvum: Longest migration of any African land mammal (~2500 km) with ~8 million individuals.
  • Tadarida brasiliensis: Largest bat colony in the world (15 million bats); climate change is causing earlier migrations.

Feeding Strategies

  • Carnivorous bats: Adaptations for consuming vertebrates include dome-shaped cranium, larger sagittal crest, elongated rostrum, and large bite force.
  • Insectivorous bats: Basal state with echolocation calls, sharp pointed teeth, large ears, and strong jaw musculature.
  • Nectivorous bats: Adaptations include hovering ability, elongated extensile tongues, grooves and brushlike papillae, and fast metabolism.
  • Sanguineous bats: Blood-feeding adaptations include specific gene loss, heat sensors in face, anticoagulants in saliva, and unusually large stretchy stomach.

Ecosystem Services

  • Pollination: Banana, mango, durian (chiropterophily).
  • Seed Dispersal: Figs, pioneer species (Chiropterochory).
  • Nutrient Cycling: Through guano deposition.
  • Invertebrate Pest Control: Eat agricultural pests, reducing the need for chemical pesticides.

Bats and Disease

  • Flight increases body temperatures (hyperthermia) to ~40°C, selecting for viruses that tolerate fever responses.
  • Flight produces high levels of reactive oxidative species (ROS), leading to positive selection on DNA repair genes.
  • Constitutive interferon expression means bat antiviral system is always alert, controlling viruses early and reducing inflammatory response.
  • Bats tolerate viral infection without tissue damage.
  • Bats are long-lived and live in large social colonies, allowing for long-term stable virus persistence and spread.

Threats to Bats

  • Habitat loss including roost destruction.
  • Light pollution.
  • Wind turbines.
  • Pathogens (e.g., white-nose syndrome).
  • Persecution and hunting.
  • Climate change.
  • Pesticides.