Lecture 15: Sexual Selection: Moose Antlers, Visual & Acoustic Advertisements, and Lekking Displays

Scenic Introduction: McNamara Nature Trail

  • Weekend anecdote used to prime interest in natural history

    • Trail options: 4.7 km main loop; up to 7km with side loops

    • Trail equipped with QR-codes for interpretive content

  • Ecological vignette: decomposition on fallen logs

    • Mosses, fungi (esp. turkey-tails) catalyse breakdown of woody tissue

    • Illustrates interconnectedness of forest organisms and nutrient cycling

Moose Field Observations (Early-Morning Algonquin Outing)

  • Tracks → sighting on abandoned rail bed

    • Young bull: “tiny spikes”, bent ear (individual identifier)

    • Mature bull deeper in woods; both alert to each other

  • Pond encounter at sunrise

    • Trio feeding on aquatic plants to replenish sodium post-rut

    • Ear orientation toward sound source (backswept for auditory focus)

    • Courtship vignette: young bull attempts to sniff female’s rump (pheromone sampling)

    • Female body language: arched back, ears back—clear rejection signal

    • Mature bull monitors but shows limited aggression given rut has ended

Advertising for a Mate – General Framework

  • Two primary channels

    1. Non-vocal/visual (morphological structures, movements)

    2. Vocal/auditory (songs, calls, mechanical sounds)

  • Evolutionary driver: sexual selection (Darwin); female choice often paramount

Advantages & Disadvantages of Acoustic Signals

  • Pros:

    • travel long distances,

    • especially low frequencies in forest

  • Cons:

    • Predation/parasitoid risk

    • Crickets & grasshoppers parasitised by phonotactic flies

    • Intraspecific eavesdropping

      • “satellite males” or “sneaker” males exploit caller’s effort

    • Risk of retaliation if territory holder detects cheater

Moose Rut: Acoustic Advertising in Mammals

  • Rut: cow Moose call to attract bulls

  • Rut timing (Eastern NA): late Sept–Oct; contrasts with Alaska

  • Female (‘cow’) call: long, low-frequency wail ending in grunt; mostly nocturnal

    • Propagates far; bulls may use antlers as parabolic reflectors

  • Calling arena: semi-open beaver meadows, wetlands; females stationary, bulls roam

  • Bull responses

    • Guttural grunts (“giant frog stepped on”)

    • Antler thrashing of shrubs

      • audible & visual signal

        • claim territory and female

        • alert female he is coming

      • can snag branches

Visual Advertising & Sexual Dimorphism

  • Typical pattern: males colorful/ornamented, females cryptic (nest concealment)

  • Examples

    • Wood Duck, Scarlet Tanager, Willow Ptarmigan, Northern Flicker

  • Reversed sexual dimorphism & polyandry

    • Wilson’s Phalarope: female brighter; multiple mates; males solely incubate

    • polyandry:

      • females court male and lay eggs

        • male will incubate the eggs

      • female will find more several partners and repeat pattern

    • sexual dimorphism: differences in physical characteristics between males and females of the same species

      • reversed sexual dimorphism: when females are more colorful than males

  • Color as honest indicator

    • Mallard: greener head + more yellow bill → higher mating success

      • reveals age and health

    • House Finch: carotenoid-rich red; brightness correlates with foraging skill → better provisioning of young

      • based on diet

Ornaments as Badges of Status or Maturity

  • Atlantic Puffin bill plates & grooves

    • Badges of status or maturity

    • Grow seasonal keratin “plates”; each groove ≈ 2-year increment → age marker

    • Females prefer males with ≥2 grooves (≥4 yrs) → proven survival & feeding ability

    • a specific characteristic can give a female information on a male’s foraging ability

    • older means survivor that is a good fisherman and great provider

Antlers: Structure, Growth & Significance

  • Composition: dead bone when velvet shed

  • Parts

    • Tine: pointed projection

    • Palm: broad, flattened area

  • Age signatures

    • Prime bulls: 6.5-9.5 yrs (largest palms + long tines)

    • Very young (≤2.5 yrs): small spikes; very old: broad palms but reduced tines

  • Annual cycle

    1. Only for breeding season

    2. Buds form late Apr/early May under vascularised “velvet” skin

      1. feeds blood with nutrients to growing bone beneath

    3. Rapid growth (fastest bone growth in mammals)

      1. complete by Aug (~4 months)

      2. Mass per pair: 10-20KG

    1. Velvet dries;

      1. bulls rub trees → expose bone (often orange-stained)

    2. Dec–Jan: antlers shed (simultaneously or within hours) at pedicle seam

  • Winter sexing tip: bull = black muzzle, cow = brown

Antlers in Behavioural Ecology

  • Display: lateral presentation communicates size; may avert combat (antler display)

  • Indicators of age, health and status

  • Sparring vs. Combat

    • Sparring: when 2 bulls are contesting each other stature by strength

      • a lot sparring happens after the rut as playful

      • establishing strength and dominance with other bulls

    • Off-rut (Nov–Dec): ritualised sparring establishes dominance hierarchy; low injury risk

    • Peak rut: escalated fights; injuries include eye gouges, abdominal punctures; occasional mortality

    • Antler display can defuse aggression by the size

  • Physiological cost: bulls lose up to 10% body mass

    • (approx 100lbs in 1000-lb animal) due to fasting during rut

Female Moose Mate-Choice Dynamics

  • Females will choose a bull in it’s prime if given a choice

  • Female remains “coy” ≈ several days; bull guards but no immediate copulation

  • Allows replacement by larger bull, reinforcing selection for prime males

  • Ear postures

    • Forward: attention/listening

    • Laid back: aggression (toward rivals or humans)

Non-Ungulate Ornaments

  • Dobsonfly male: enlarged “tusks” (mandibles) function as display; likely female-choice trait

Ritualised Visual Displays beyond Morphology

  • Waterfowl head-bob sequences (head display)

    • Mallard, Hooded Merganser: synchronized bobbing/crest raising

  • Ruffed Grouse: (ruff display)

    • drumming + fanned tail display + neck black “ruff” collar

  • Insect aerial swarms: (aerial display)

    • midges form “columns”; mechanism of mate choice poorly understood

  • Ebony Jewelwing (damselfly) (wing display)

    • Male hovers over perched female with wing-flash display

    • Female signals acceptance by clapping wings over dorsum;

      • rejection = wings folded

  • Fireflies (Lampyridae beetles) (aerial light display)

    • Species-specific bioluminescent flash patterns;

      • females (often flightless) answer from vegetation

Choreographed Pair Displays

  • Mute/Tundra Swans: (synchronized display)

    • mirrored head & wing movements (“heart-shaped” neck pose)

  • Sandhill Cranes: (ritualized dances)

    • duet calling, mutual bows, leaps >1m high

Communal Display Grounds – Leks

  • Leks: area where multiple males gather to perform simultaneous courtship for visiting females

  • Taxa highlighted

    • Wild Turkey: caruncles, beard, snood; tail fan provides backdrop for colored head skin; males pause display when deer (only “one buck” joke) cross

    • Sharp-tailed Grouse: inflate yellow-orange neck sacs, raise pointed tails; elaborate foot-stomping dance

      • communal display grounds

    • Few top males secure majority of copulations after days of performance → high sexual selection pressure

Summary: Costs, Benefits & Diversity of Mate-Attracting Signals

  • Acoustic and visual signals evolve under balance of

    • Female preference (driving elaboration)

    • Predation/parasitism risk (limiting exaggeration)

    • Energetic cost (e.g., weight loss in bulls, dancing endurance in lek species)

  • Ornaments and displays serve as honest indicators of

    • Age, health, genetic quality, foraging efficiency, social rank

  • Sexual selection produces rich biodiversity of colors, sounds & behaviours observable in field