How do animals assess their “fighting How do animals assess their “fighting ability” (which influences the cost of ability” (which influences the cost of

fighting)?fighting)?

Do animal assess each other's fighting ability

in contests?

Mutual Assessment HypothesisMutual Assessment Hypothesis

• Contestants assess each other

• EX. sequential assessment model (Enquist & Leimar 1983)

• a contest: a series of interactions during which individuals gradually assess each other’s fighting ability

• larger differences are more easily detected than smaller

• => contest durationsmaller > contest durationlarger

=> contest: less costly (display) -> more costly (escalation)

• observations: fighting ability => contest duration/intensity used as supporting evidences for mutual assessment in animal contests

• (e.g. Austad 1983; Crespi 1986; Robertson 1986; Crowley et al. 1988; Wells 1988; Englund & Olsson 1990; Rosenberg & Enquist 1991; Enquist et al. 1990; Olsson 1992; Stamps & Krishnan 1994; Hack 1997; Hack et al. 1997; Moya-Laraño & Wise 2000; Hofmann & Schildberger 2001; Pratt et al. 2003).

Do animal assess each other's fighting ability

in contests?

Self Assessment HypothesisSelf Assessment Hypothesis

1. Energetic War of Attrition1. Energetic War of Attrition (Mesterton-Gibbons et al. 1996; Payne & Pagel 1997)

• persistence in contests: own ability (energy reserve)• size of opponents does NOT matter• size of smaller opponent => duration • But, how about the empirical data?• fighting ability => contest duration/intensity • Taylor & Elwood (2003): • random-pairing, self assessment can also generate the

pattern• small : any sized individuals vs. similar-sized opponents• large : smallest individuals vs. the biggest opponents • => negative relationship: "smaller size" vs. " fighting

ability "• smaller size => fighting ability => contest

duration/intensity

Do animal assess each other's fighting ability

in contests?

Self Assessment HypothesisSelf Assessment Hypothesis

2. Cumulative Assessment2. Cumulative Assessment (Payne 1998)

• persistence in contests: own ability

• size of opponents does NOT matter

• a contestant gets into a contest with a preset cost threshold

• cost threshold: NOT influence by the ability of the opponent

• But, stronger opponent inflict more injury cost on the weaker

• => opponent ability => "cost threshold" reached faster

• => the ability of the opp. appears to have influence

• same predictions as the mutual assessment when interactions involve injury costs

Self Assessment

Mutual Assessment

Cumulative Assessment

Energetic War of

Attrition

Random Size Pairings

Smaller size

Larger size

Size difference

Equal Size Pairings

Pair size

Predictions from the 3 Hypotheses

Contest Duration

† † †

†/N

†NN

—

—

—

— —

Which hypothesis do these data match?

General pattern of new findings...

Self Assessment

+ NS

When the size of both contestants were considered...When the size of both contestants were considered...

Mutual AssessmentMutual Assessment

• Wasp (Hemipepsis ustulata; Kemp et al. 2006)• But did not test against the cumulative assessment

Self AssessmentSelf Assessment

1. Energetic War of Attrition

• Jumping spider (Plexippus paykulli; Taylor et al. 2001) • Fallow deer (Dama dama; Jennings et al. 2004)• Amphipod (Gammarus pulex; Prenter et al. 2006)• Cape dwarf chameleon (Bradypodion pumilum; Stuart-Fox

2006)

2. Cumulative Assessment

• Wellington tree weta (Hemideina crassidens; Kelly 2006)

• fiddler crabs (Uca mjoebergi; Morrell et al. 2005) • hermit crab (Briffa & Elwood 2000)