PSY 2364 Animal Communication Territorial signals Territory in ecology, any area defended by an organism or a group of similar organisms for such purposes as mating, nesting, roosting, or feeding. Home range Area occupied by an animal which is not necessarily defended Why defend a territory? Benefits: exclusive access to important resources abundant or sufficient supplies of food, a mate, safe hiding places, shelter, nest sites Costs: loss of time, loss of energy resources devoted to signaling energy lost patrolling and defending territory boundaries risk of predation when exposed cost of an injury if escalated fighting breaks out. Design rules for territorial signals Design Feature Range marks Locatability Identification level Rule Visual Auditory Olfactory Territory boundary Territory and/or owner Species, individual Posture, color patch Presence on territory Color pattern Presence on territory Loud calls Amplitude cues, signal degradation Frequency, temporal pattern Scent deposit along border Marks along boundary; sender not locatable Chemical mixture Bird song and territory defense Competition for limited resources Signal to keep intruders out Long-distance threat signal Effectiveness of a threat depends on the likelihood of escalated aggressive behavior if not heeded advertisement, threat, and attack. Neighbors vs. strangers Neighbors: territory holders with demonstrated fighting potential, with established track record Strangers: Some are floaters or non-owners searching for a vacant territory An owner has more to lose from an invading floater than the other way around. Important to distinguish neighbors from floaters 1
The ability to determine the level of threat posed by an intruder allows territory owners to modulate their territorial responses in a cost-effective way. Reduced aggression towards familiar adjacent neighbors, termed the dear-enemy effect, has been shown in numerous species. Do territory owners perceive distant neighbors from the same neighborhood as strangers because of their unfamiliarity, or as dear-enemies because of their group membership? http://www.youtube.com/watch?v=9di0lh7_r_u To investigate this question, the authors recorded songs of male skylarks (Alauda arvensis) and played them back to adjacent neighbors, distant neighbors a few territories away in the same microdialect area, and strangers. They also measured how far skylark songs propagated in their natural habitat, and repertoire similarity between adjacent neighbors, distant neighbors and strangers. considered to be dear enemies? PLoS ONE 5(8): e12428. Skylark #1 Adjacent neighbor Distant neighbor Skylarks responded less aggressively to songs of distant, unfamiliar neighbors than to stranger songs. The analysis revealed that individuals share a large proportion of syllables and sequence types with both adjacent and distant neighbors, but only a few syllables and no sequences with strangers. 2
Conclusions: Reduced aggression toward distant neighbors probably results from their familiarity with the vocal group signature shared by all members of the neighborhood. In skylarks, dearenemy-like relationships can occur between unfamiliar individuals who share a common acoustic code. Ranging Ranging is the ability to tell how far away the source of the signal is. Cues include: amplitude decline (inverse square law) degradation in signal characteristics (distortion with increasing distance) scattering, interference and echoes caused by obstacles, blurring of signal onsets and offsets by reverberation, etc. Songbird repertoire size Why do birds differ in repertoire size? Anti-habituation hypothesis Anti-exhaustion hypothesis Beau Geste hypothesis ranging hypothesis escalation-level hypothesis Song mimicry Northern Mockingbird (Mimus polyglottos) 200 or more different songs Repertoire increases annually Audible up to 400 meters Territory: 100 m x 100 m Audience: up to 100 birds (Kroodsma, 2005) Northern Mockingbirds Botero CA, Boogert NJ, Vehrencamp SL, Lovette IJ. (2009). Climatic patterns predict the elaboration of song displays in mockingbirds. Curr Biol. 19(13), 1151-1155. Mockingbirds (and related species) sing more elaborate songs in variable, unpredictable climates. Song elaboration is accompanied by "larger brain sizes, higher foraging innovation rates, higher reproductive flexibility, and higher sociality." Assessment Theory Parker (1974), Maynard Smith & Parker (1976) Selection should favors individuals who are able to assess the physical and behavioral traits of rivals and use this knowledge to adjust their behavior to the costs and benefits of fighting and the probability of winning. 3
Yack JE, Smith ML, Weatherhead PJ. Caterpillar talk: acoustically mediated territoriality in larval Lepidoptera. Proc Natl Acad Sci U S A. 2001 Sep 25;98(20):11371-5. Caterpillar talk: Acoustically mediated territoriality We provide evidence for conspecific acoustic communication in caterpillars. Larvae of the common hook-tip moth, Drepana arcuata (Drepanoidea), defend silk nest sites from conspecifics by using ritualized acoustic displays. Sounds are produced by drumming the mandibles and scraping the mandibles and specialized anal oars against the leaf surface. Staged interactions between a resident and intruder resulted in escalated acoustic duels that were typically resolved within minutes, but sometimes extended for several hours. Resident caterpillars generally won territorial disputes, regardless of whether they had built the nest, but relatively large intruders occasionally displaced residents from their nests. All evidence is consistent with acoustic signaling serving a territorial function. As with many vertebrates, ritualized signaling appears to allow contestants to resolve contests without physical harm. Comparative evidence indicates that larval acoustic signaling may be widespread throughout the Lepidoptera, meriting consideration as a principal mode of communication for this important group of insects. Assessment Theory Parker (1974), Maynard Smith & Parker (1976) Selection should favors individuals who are able to assess the physical and behavioral traits of rivals and use this knowledge to adjust their behaviour to the costs and benefits of fighting and the probability of winning. Costs: Why defend a territory? loss of time, loss of energy resources devoted to signaling energy lost patrolling and defending territory boundaries risk of predation when exposed cost of an injury if escalated fighting breaks out. Benefits: Why defend a territory? exclusive access to important resources abundant or sufficient supplies of food, a mate, safe hiding places, shelter, nest sites. Female poplar aphids defend a single leaf on a cottonwood tree against intruders. Two female aphids may compete for up to two days for control of a large, healthy leaf. Competition includes communication displays and aggressive kicking behavior. Fights can result in death of one combatant. 4
Coyotes defend a territory of up to 10 km 2. Large mammals that do not defend territories tend to occupy large home ranges. Songbirds defend exclusive territories by singing, visual displays, and aggressive interactions with intruders. When birds are removed from territories during the nesting season, the vacated areas are soon re-occupied (Krebs, 1971). Territory size is linked to availability and distribution of food: patchy distribution of food is associated with larger territories. The winner in territorial disputes is almost always the resident or owner. Why are intruders so quick to give up? Game theory models: each animal will adopt an evolutionary stable strategy to resolve conflicts between residents and intruders. (ESS) (ESS) a behavioral strategy that leads to a stable probability of outcome, and stable proportions of each type of strategist in the population. (ESS) Strategies: Hawk (always escalate) Dove (give a display) Bourgeois (escalate if owner, else display) 5
Game Theory Models Payoff Matrix Resident Escalate Display Escalate Opponent Display Resident always wins rule Davies (1978) speckled wood butterflies resident males always defeated intruders, unless they were removed from the site and another male moved in and claimed it. But later studies showed that residents do sometimes lose. On average, residents tend to be larger, strong, faster than rival intruders. But sometimes the winner is not physically stronger or larger. Alternative hypothesis: the payoff from holding a territory may change over time. The longer the occupancy, the greater the benefits (disputes with neighbors are already settled, locations of food are known, etc). Longstanding residents have more to lose. Assessment signals Communication during territorial interactions can be seen as providing information that can be used by the receiver to make a decision. The information conveyed can affect the behavioral response of the recipient. Assessment signals Cichlids (tropical fish) defend a nest site by displaying and/or attacking intruders. If the intruder is larger than the territory owner, he or she may attack and drive the owner away. But if the owner is larger, the intruder may decide to look elsewhere. 6
The handicap principle Zahavi (1975) proposed that reliability is essential for successful communication and signaling. Receivers of signals act in their own best interests by checking the reliability of signals; respond to messages carrying a certificate of reliability or handicap (e.g. the peacock s tail). The handicap principle The investment required for using the signal must be greater than the potential gain a cheater would make from using it improperly. An ESS is a strategy which, if adopted by the members of a population, will not be replaced by an alternative strategy. Concept derived from the mathematical theory of games: the best strategy for any individual depends on what the other players are doing. Example: Male green tree frogs assemble near the shore of a pond and produce song choruses which attract females. Calling increases risk of predation and is energy consuming. Some males adopt the strategy of cheating. These satellites wait near calling males and intercept females as they approach. Cheating is not an ESS, however, because if all males cheated then no callers would remain. But a mixture of callers and satellites can be an ESS. ESS theory can be applied in situations where different competing strategies are possible. Sometimes only a single strategy can be effective; other situations allow multiple strategies to co-exist. Fighting behavior (Maynard Smith): Hawk strategy (attack, retreat only when injured) Dove strategy (display, and retreat when injured) Retaliator (display, but attack if you are attacked) Computer simulations favor the retaliator strategy, and this is the one most commonly observed in nature. Can ESS theory help to explain why animals communicate and/or cooperate? 7