Behav Ecol Sociobiol (2010) 64:827–833 DOI 10.1007/s00265-009-0899-y

ORIGINAL PAPER

Perceived predation risk and mate defense jointly alter the outcome of territorial fights Tsunenori Koga & Satoshi Ikeda

Received: 29 September 2009 / Revised: 14 December 2009 / Accepted: 16 December 2009 / Published online: 21 January 2010 # Springer-Verlag 2010

Abstract Virtually all animal conflicts occur over access to mates or resources that affect survival, the two key components of fitness. In this paper, we report that predation risk and mate defense jointly affect the outcomes of contests between male sand crabs (Scopimera globosa) for burrows in which crabs mate and take shelter from predators. We observed the contests under three different conditions: (1) the natural condition of low predation risk and without the presence of a female; (2) the first experiment in which we imposed upon only intruding males the perception of predation risk—by digging them from their burrows, capturing and handling them, and placing them into other males’ burrows—to increase the value of the burrows for the intruders as shelter, and (3) the second experiment in which we repeated this treatment but increased the resource value of the burrow to the resident by placing a female in his burrow. The difference in body size between contestants was the main determinant of victory in all analyses. However, perceived predation risk also partly affected the outcomes of the fights: The motivated intruders were likely to win even when they were a little smaller than the residents. In addition, defense of a female had a significant effect on the outcomes of fights: The motivated residents won more fights than the motivated intruders, indicating that these two treatments caused asymmetric increases of the resource value. This is the first report of two external factors simultaneously raising resource value, affecting motivation of contestants, and altering the outcome of fights. Communicated by M. Jennions T. Koga (*) : S. Ikeda Faculty of Education, Wakayama University, Wakayama 640-8510, Japan e-mail: [email protected]

Keywords Contests . Fights . Resource value . Motivation . Perceived predation risk . Mate acquisition

Introduction Survival and mating are the two most important components of an organism’s fitness. Virtually all animal contests are over access to mates or resources that affect survival (Andersson 1994; Endler 1986). For example, territories, which are defended against conspecific and/or heterospecific intruders (Bradbury and Vehrencamp 1998), typically provide food, shelter, and a protected site for mating and breeding. Many factors determine the outcome of resource competition, including the residency effect (Davies 1978; Fayed et al. 2008; Krebs 1982; Olsson and Shine 2000), body size (Gherardi 2006; Jennings et al. 2004), weapon size or performance (Lappin and Husak 2005; Sneddon et al. 1997), age (Kemp et al. 2006; Koga and Hayashi 1993), energy reserve or stamina (Briffa and Sneddon 2007; Hazlett et al. 1975), aggressiveness mediated by hormone levels (Huber et al. 1997; Oliveira et al. 2001; Rohwer and Rohwer 1978), winning in the past (Kemp and Wiklund 2004; Whitehouse 1997), and resource value (RV; Brown et al. 2007; Enquist and Leimar 1987; Gherardi 2006; Lindströn and Pampoulie 2005). Many of these factors affect an individual’s resource holding power, and differences in this trait between competitors often explain the outcome of most contests (Gherardi 2006). According to Humphries et al. (2006), in the absence of RV asymmetry, a contest is predicted to be won by the competitor with the higher resource holding power; in the absence of a difference in this trait, the competitor that places a greater value on the resource is expected to win. Variation in RV for escaping the potential risk of predation

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or obtaining a mate has been shown to affect motivation, fighting behavior, and the outcome of contests. In the case of escaping predation, hermit crabs (Pagurus longicarpus) that are in shells too small for their bodies—thus making them more vulnerable to predation—tend to initiate more fights and are more likely to win than crabs in suitably sized shells (Gherardi 2006; see also Elwood and Neil 1992; Hazlett 1970, 1981). In the case of mate acquisition, male crickets (Acheta domesticus) that are motivated to acquire a mate tend to win more fights due to increased aggressiveness (Brown et al. 2007). In another study of this species, the aggressiveness of resident males toward the intruding males increased with the number of potential mates they perceived by smell (Buena and Walker 2008). Few studies of other animals (but see Jonart et al. 2007) have demonstrated that aggressiveness increases with the existence of or the number of defended mates, that is, that aggression increases with the benefit of winning a fight. In addition, to our knowledge, no study has determined how perception in the risk of predation to a challenger and acquisition of a mate by a defender affect aggressiveness and the outcome of contests for a resource that provides both protection and a mating site. Most ocypodid crabs (fiddler crabs, ghost crabs, sand crabs, and their allies) burrow in intertidal sand and mud flats and are active on the surface during low tide. These typically small crabs are the prey of many shorebirds from which they escape by running into their burrows (e.g., Backwell et al. 1998; Koga et al. 2001). Hence, the burrows serve as shelters and are an important resource for the crabs escaping from avian predators. In many ocypodid species, crabs also mate in burrows that are defended by males and used by females during incubation (e.g., Backwell and Passmore 1996; Christy 1982; Christy and Salmon 1984; de Rivera 2005; Koga et al. 1993). These crabs provide an excellent opportunity to test the effects of perception of predation risk and mate defense on fights between intruders and residents for burrows. Predation risk and courtship or mate searching have been shown to be trade-offs in a fiddler crab (Koga et al. 1998). In addition, under elevated predation risk, some male fiddler crabs become bolder in response to the availability of potential female mates (Pratt et al. 2005; Reaney 2007). Previous studies of competition between males for burrows indicated that body size differences and residency both affected the outcome of fights (Hyatt and Salmon 1978; Jennions and Backwell 1996; Koga et al. 1999; Pratt et al. 2003; Takahashi et al. 2001; Wada 1986, 1993). However, RV was not artificially manipulated in these studies. Recently, Fayed et al. (2008) tested whether different food conditions (i.e., relative RV) affected the outcomes of fights for burrows, but this result was not significant and residency was the most important determinant of winning. Thus, it is still unclear how

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differences in perceived RV may affect the aggressiveness of challenger and defender and the outcomes of fights. In this paper, we report that an increase in the value of a burrow as a refuge to a challenger who has lost his burrow and an increase in the value of the burrow as a mating site to the defender jointly alter aggressiveness and the outcomes of fights for burrows in the sand bubbler crab, Scopimera globosa deHaan 1835 (Brachyura: Ocypodoidea: Scopimeridae). We first observed crabs fight under natural conditions of relatively low predation risk and without a female in the defenders’ burrows. We then observed fights (1) after imposing a predation threat on challengers who had lost their own burrows by digging, capturing, handling, and releasing them into the entrance of the defenders’ burrows and (2) after providing defenders with females in their burrows as potential mates. These two treatments should increase RV of the burrow and thus each male’s motivation to fight for the burrow, leading to escalated contests and perhaps altering their outcomes.

Materials and methods S. globosa lives in burrows in mixed-sex colonies on sandy mud tidal flats in Japan. Crabs feed and interact socially on the surface during daytime low tides. Males and females live alone, each in their own burrow, except when pairs form in males’ burrows for mating (Ono 1965). Under natural conditions, we found that most contests for burrows (86%) occurred between males (male–male, n=43; male– female, n=5; female–female, n=2), probably because males often abandon their own burrows to search for wandering females for burrow mating, whereas females retain their own burrows and tend to ovulate there after surface mating (Koga 1998). Here, we consider only male–male fights for burrows. A contest starts when an intruder approaches and contacts a resident at the entrance of his burrow. If the difference in body size (carapace width) between the two contestants is large, the larger one often strikes the smaller one using his chelae and wins the contest. When the size difference is small, the rivals push each other with their chelae while lifting their first and second ambulatory legs and standing firm with their third and fourth legs. The winner acquires the burrow, and the loser leaves the area. When predatory shorebirds capture the crabs on tidal flats, the birds usually repeatedly catch and release them to remove the chelae and ambulatory legs of the victims for easier consumption (Koga, personal observation). The predatory birds, mostly whimbrels in our study sites, capture S. globosa on the surface while running or in a burrow by inserting their long, fine beaks to grasp the crab. When we walked and approached S. globosa on the tidal

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flats, residents quickly retreated, descended, and hid in their own burrows, whereas wanderers ran about, became motionless (probably because of cryptic coloration of their carapace with the substratum), or hid in burrows regardless of whether they were vacant or occupied by residents. That is, the burrows are a potential predation avoidance resource even for crabs without burrows. We observed fights between male S. globosa under natural conditions on a tidal flat along the Kino River estuary (June–September in 2004 and 2005, n=43) and under experimental conditions at a site on the Waka River estuary (June–August 2007, n =102), Wakayama city, Wakayama Prefecture, Japan. We conducted direct observations on the crabs very quietly, without movements, not disturbing the behavior of the crabs. During observation, avian predators could not access the crabs because of the presence of the observer. In fights under natural conditions with low predation risk, we captured both males after the end of fights and measured their carapace width to the nearest 0.1 mm using a hand caliper. For the first experiment, we artificially motivated intruders to fight hard with resident males for burrows. We randomly chose and dug males out of their burrows, measured them, marked their carapace with a colored pen (Mitsubishi paint marker) for individual distinction, and used these males as intruders (n=50). This handling imposed a perceived predation risk on the intruders and, hence, increased the value of the burrow as a refuge. An intruder was placed into the entrance of a resident’s burrow within a few minutes after his capture. We scared the resident to retreat and descend into his burrow before the intruder was placed; thus, the resident would recognize a risk of predation, although the risk would be much smaller than that perceived by the intruder. After the introduced male descended into the resident’s burrow, we watched that the two males stroke and touched each other with their legs of one side at the narrow shaft of the burrow. Then either one male emerged and walked away without a further fight occurring on the surface (i.e., did not escalate) or both males emerged from the burrow and fought at the burrow entrance and/or on the surface (escalated), i.e., the fights occurred in the burrow and/or on the surface. The loser of the fight then walked away. After the contest finished, we captured both crabs and measured the carapace width of the resident to compare with that of the intruder. In the second experiment, we again captured, handled, and measured the intruder to increase the RV of the burrow as a refuge. In addition, we provided the resident with a potential mate to raise his motivation to fight. As mates, non-ovigerous females were collected in the burrow area of the same tidal flat on the day of the experiment. Each female was either placed in a burrow (n=26) or nearby,

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allowing the resident to capture and drop her into his burrow (n=26). In both cases, after the pairing, the male positioned himself above the female in his burrow to plug the entrance and protect her for burrow mating when she was receptive. When the resident emerged to plug the burrow for mating, we put the intruder in the burrow. As in the first experiment, our activity caused the resident to retreat and descend in his burrows before the intruder was placed; thus, the resident would recognize a risk of predation, although the risk would be much smaller than that perceived by the intruder. This procedure produced a pair of rivals that were both motivated to fight hard for the burrow as a refuge (in the case of the intruder) or as a place for mating (in the case of the resident). Because the resident was positioned between the intruder and the female, we did not think the intruder recognized the presence of the female. After the fight ended, we captured and measured the resident’s carapace width. All individual crabs were used only once and never reused to avoid the effect by pseudoreplication. We used logistic regression analyses to determine the effect of size differences, male status (resident or intruder), and the increases of RV on the outcomes of the fights. To clarify the effects of RV on the outcome of the fights, analyses were conducted for the natural conditions versus the first experiment and for the first experiment versus the second experiment in addition to the single analysis for each treatment. We compared data for the natural conditions and the first experiment to analyze whether the perception of predation risk affected the outcome of fights, and we compared the data for the first and second experiments to analyze whether the acquisition of a mate motivated the resident crabs and affected the outcome of fights.

Results In fights under natural conditions with low predation risk and without a female mate in the burrow of the resident male, the larger male was significantly more likely to win the contest and the status of a male as intruder or resident had no effect on outcome (Table 1 and Fig. 1a). In the first experiment in which the intruder had experienced a predation threat, although the difference in body size largely determined the outcome of contests, intruders won significantly more often than expected (Table 1 and Fig. 1b). In the second experiment, we either put a female directly into the burrow of the male when he was in his burrow (n=26) or a female was released near the resident male when he was on the surface, and he captured her and dropped her into his burrow (n=26). In both cases, the males soon appeared on the surface to plug the entrance, as happens in natural burrow

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Table 1 Estimated values of the parameters and the standard errors in the the logistic intruder analyses was of motivated contests by for aa perceived burrow under increase natural in conditions, predation risk for the logistic experiment analyses in which of contests only theforintruder a burrow wasunder motivated naturalbyconditions, a perceived increase and the inresident predation by risk, the acquisition and for the of experiment a female,inseparately; which the not intruder any was the for motivated experiment by a perceived in which increase only theinintruder predation was riskmotivated and the resident by a bycomparisons the acquisition between of a female, these treatments separately;(see notthe anytext) comparisons between these treatments perceived increase (see in the predation text) risk, and for the experiment in which Categories

Natural conditions Intercept Difference in body size (carapace width of the resident minus that of the intruder, mm) First experiment Intercept Difference in body size Second experiment Intercept Difference in body size

Estimated values of the parameters

Standard error

Chi-square

P value

0.015 3.666

0.476 1.181

0 9.64

0.975 0.0019

−0.926 2.659

0.386 0.919

5.74 8.37

0.0166 0.0038

0.790 2.450

0.400 0.765

3.90 10.25

0.0483 0.0014

The intercept represents the difference in carapace width (mm, resident minus intruder) at which each contestant has an equal probability of winning. The differences in body size listed are those at which the resident would win all contests (see also Fig. 1)

mating. The way in which females were provided did not result in a difference in the fighting success of residents (14 resident victories when females placed in the burrow and 13 resident victories when females released at the surface; P=0.556, Fisher’s exact test), so we pooled the data. Fight outcomes were largely determined by relative body sizes, but residents were more likely to win even when they were smaller (Table 1 and Fig. 1c), indicating that a resident that had a 1.0 0.8

4 2 2 3 23 3

A

0.6

Fight outcomes for resident (win = 1, loss = 0)

0.4

and logistic regression curve by the outcomes

Fig. 1 Relationships between outcomes of contests for burrows and the difference in body size between the resident and the intruder. The graphs show data for each pair and the logistic regression curve under natural conditions (a), for the experiment in which only the intruder was motivated by a perceived increase in predation risk (b), and for the experiment in which the intruder was motivated by a perceived increase in predation risk and the resident by the acquisition of a female (c). The numbers above some circles represent the number of overlapped data points

female in his burrow fought harder than did the intruder that had been subjected to a predation threat. However, when we pooled all data from the natural condition and the two experiments (n=145), there was no significant effect of perception of predation risk on outcomes of the contests (Table 2), different from the result obtained by the simple analysis (Table 1). In contrast, the existence of a female in the burrow significantly contributed to the

0.2

2

0.0 -3

-2

-1

0

1.0 0.8

2

3

B

1

2

3

1

2

3

1

2

3

3

0.6 0.4 0.2

2 2

0.0 -3

-2

6 3 43 24

-1

1.0 0.8

6

0

2 222

C

4 23

22

0.6 0.4 0.2 0.0 -3

-2

22

5 222 3

-1

0

Difference in carapace width (resident minus intruder; mm)

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Table 2 Estimated values errors in the under the experiment onlyofintruders posed predation threat and valuesof ofthe theparameters parametersand andthe thestandard standard errors in the logistic regression analysis contests were for a burrow on the entire pooled logistic of contests forwith a burrow on thelow entire pooled risk,residents were intact, andonly under anotherwere experiment intruders were posed dataset, regression i.e., underanalysis the natural condition relatively predation under the experiment intruders posed predation threat and dataset, under the natural condition with relatively low predation a female mate residentsi.e., were intact, and under another experiment intruders wererisk, posed predation predation threat threatand andresidents residentswere weregiven given a female mate Categories

The intercept Differences of body size (carapace width of the resident minus that of the intruder, mm) Predation threat to the intruder Defense of female mate by the resident

victories of the resident over the intruder (Table 2), the same as the result by the simple analysis (Table 1). These results indicated that the acquisition of a female mate motivated the resident more than the perception of predation risk motivated the intruder, causing asymmetric increases of RV between these two treatments. The differences of carapace width necessary for a resident to win with a probability of 50% were −0.0041 mm (body sizes of both contestants were nearly equal) under natural conditions (Fig. 1a), 0.348 mm (the resident needed to be large) in the first experiment (Fig. 1b), and −0.322 mm (the intruder needed to be large) in the second experiment (Fig. 1c), as calculated from values of the estimated parameters in Table 1. The fight seldom escalated to a pushing contest when only the intruder was motivated to win the burrow (first experiment), whereas the fight significantly more often escalated to pushing when both the intruder and the resident were motivated to win the burrow (second experiment; P=0.007, Fisher’s exact test, Table 3).

Discussion Although the larger male sand bubbler crab usually won the contest for a burrow, the experimental increases in RV also affected fight outcomes. Under natural conditions, the resident did not have an advantage over the intruder with regard to level of motivation. Thus, a male’s resource holding power as determined by his size relative to his rival most strongly affected the fight outcome. However, in the experimental treatments, the intruders who were escaping a greater perceived threat of predation won the contests with the residents. With additional treatment, the residents who were defending potential mates clearly won the contests with the intruders, indicating that the acquisition of a female gave higher RV to the burrow than the perception of predation risk. In addition, fights more often escalated when both males had increased motivation than when the motivation of only the intruding male was elevated. When perceived information of RV is symmetric in both contestants and level of motivation for fighting is even,

Estimated values of the parameters

Standard error

Chi-square

P value

0.482 2.831

0.281 0.518

2.94 29.90

0.086 <0.0001

0.495 −0.921

0.291 0.273

2.88 11.35

0.090 0.0008

resource holding power may depend primarily on body size or other determinants of fighting ability (Humphries et al. 2006). In the present study, under natural conditions with low predation risk, the resident did not have an advantage; the larger male won. In Ilyoplax pusilla, a small intertidal crab that is closely related to S. globosa, residents also have no advantage in contests for burrows, and body size strongly affects contest outcomes (Wada 1993). However, in a previous study of S. globosa (Takahashi et al. 2001), smaller residents sometime adopted a motionless stance at a distance from their own burrows and were able to avoid fights and maintain burrow ownership when larger wanderers approached. But when a smaller crab retreated to the entrance of his burrow, fights occurred and the large wanderer won, as noted in the present study. In the burrow mating of S. globosa under natural conditions, neighboring resident males often disturb mating males and fights occur. The larger male usually wins the fight and obtains both the female and the burrow regardless of the previous ownership of the burrow (Koga and Murai 1997), probably because both contestants recognize nearly equally value of the female and burrow in contrast to the condition of the second experiment in this study. That is, the level of RV is symmetric to both contestants, usually resulting in victory for the larger one. The reason why there is no residency advantage in the fighting for burrows of S. globosa should be clarified in the future. Recent studies of the effects of an asymmetrical increase in RV on fight outcomes have experimentally varied either predation risk or mating opportunities. When hermit crabs fight for shells and perceived risk is equal between contestants, Table 3 Relationship between motivation and contest escalation Motivated contestant(s) Only the intruder Both the intruder and resident

Escalated

Did not escalate

18 33

32 19

The intruder was motivated by the perception of predation risk and the resident by the defense of a female P=0.007 (Fisher’s exact test)

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the larger crab wins (Briffa et al. 1998). However, in the hermit crabs Pagurus filholi and P. longicarpus, when one of the contestants occupies a shell that is too small to provide adequate protection, this crab will initiate more fights and fight harder to win a shell that provides better protection (Gherardi 2006; Yoshino and Goshima 2002); the level of motivation, which is governed by perceived risk, affects the outcome. Thus, when a win increases the probability of survival to a greater degree for one contestant, that individual is likely to win. Our results are consistent with these studies. The imposition of a predation threat on the intruding S. globosa male who had lost his own burrow increased his aggressiveness and chance of winning. An increase in the opportunity to mate also has been shown to affect fight outcome. Male crickets that have not had recent sexual access to females fight more intensely and more often win than those that have mated recently (Brown et al. 2007). Female house finches (Carpodacus mexicanus) were more motivated to initiate fights and win more contests against intruding females when their mates were near the nest (Jonart et al. 2007). In the present study, after the resident male had a potential mate in his burrow, he fought better than did the intruder motivated by risk of predation. Thus, in the second experiment, a residency effect appeared when the RV of the burrow was increased for the resident, as predicted by Enquist and Leimar (1987). Although escape from predation is generally prioritized ahead of other activities, including mating (Endler 1986; Helfman 1989; Lima and Dill 1990; Magnhagen 1991), in the present study, the resident male defending a female was more likely to win than the intruder exposed to a predation threat. Because we caused the resident to retreat into his burrow before placing the intruder there, the resident likely perceived predation risk to some degree even though it was less than the intruder, which had been captured and manipulated. Thus, in the first experiment without a female in the resident’s burrow, the intruder was more motivated than the resident and was more likely to win, but the results were significant only in one of the two analyses (P=0.0166 in a single analysis for the first experiment, see Table 1; P=0.090 in the analysis using all data, see Table 2). In the second experiment, the resident was motivated to retain his burrow due to a potential predation risk from us in addition to the acquired mate. This might cause asymmetric increases of RV between the two experimental treatments. The present study showed that an increase in absolute RV, either for escaping predation or defending a mate, increased an individual’s motivation to fight. In addition, we provided the first evidence that a simultaneous increase in both these RV components increased the motivation of both contestants, which jointly affected the outcome of a single contest. Multifunctional territories occur in many taxa (Wilson 1975). Studies that change or manipulate the

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multiple factors that affect the value of a territory to either or both rivals will help to clarify the evolution of fighting behavior. Acknowledgments We thank Patricia Backwell, Michael Jennions, and John Christy for assistance with editing and correction of the English. Kenji Yoshino and Yoiti Yusa helped with some of the statistics. Kenji Yoshino also gave suggestions for literature references and composed the figure. Many valuable comments by the associate editor and two reviewers greatly improved the final version of this paper.

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