Size and dimorphism

Size and dimorphism - Journal of Fish Biology (2001) 59,...

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Unformatted text preview: Journal of Fish Biology (2001) 59, 1614–1621 doi:10.1006/jfbi.2001.1790, available online at http://www.idealibrary.com on Sexual size dimorphism and male contest in wild Siamese fighting fish M. J* K. J Institute of Science, Walailak University, 222 Thasala District, Nakhon Si Thammarat, 80160, Thailand (Received 10 May 2001, Accepted 11 September 2001) Larger males of the wild Siamese fighting fish Betta splendens were more successful in male contests. There were no differences in fighting duration among treatments. Comparing agonistic behaviour between large and small males in 1 and 2 .. treatments, larger males attacked, chased and performed total agonistic behaviour more than smaller males. There were no differences between larger and smaller males concerning other agonistic behaviour during fighting. Females presented with two potential mates of different sizes did not prefer larger  2001 The Fisheries Society of the British Isles males. Key words: male body size; female preference; male contest; Siamese fighting fish; Betta splendens. INTRODUCTION Sexual size dimorphism results from the relative importance of natural selection and sexual selection on both sexes (Selander, 1965; Shine, 1989). If natural selection for female fecundity is less intense than sexual selection, males should be larger than females in mating systems in which large size provides an advantage in male-male competition, female choice, or both (Pyron, 1996). Male body size may affect reproductive success by influencing the number of mates obtained and the quality of care provided to the broods. Larger males tend to win in male contests, hold higher quality territories and gain greater access to females in several fish species (Hanson & Smith, 1967; Kodric-Brown, 1977; Constantz, 1979; Downhower & Brown, 1980; Fricke, 1980; Rubenstein, 1981; Barlow, 1983; Downhower et al., 1983; Thresher & Moyer, 1983; Rowland, 1989; Howard et al., 1998; Chellappa et al., 1999). Female reproductive success is limited by the number of eggs they can produce, therefore, they are expected to maximize their genetic contribution to future generations by practising selective mating (Trivers, 1972). Females in some species may gain information about a potential mate’s ability to provide good territorial defence and parental care for developing eggs and young by using body size to assess a male’s quality. The effect of size in female preference for large and competitively superior males has been described in several fishes (Schmale, 1981; Downhower et al., 1983; Noonan, 1983; Berglund et al., 1986; Bisazza & Marconato, 1988; Hastings, 1988; Andersson, 1994; Howard et al., 1998). *Author to whom correspondence should be addressed. Tel.: +66 75 672005; fax: +66 75 672004; email: jmullica@wu.ac.th 1614 0022–1112/01/121614+08 $35.00/0  2001 The Fisheries Society of the British Isles 1615 During the breeding season, male Siamese fighting fish Betta splendens Regan establish and defend territories in the water column near the surface which are centred on a bubble nest built by the males (Forselius, 1957). They construct their bubble nests in rice paddy fields, entice females to spawn in them and then care for the developing eggs and newly hatched fry (Gordon & Axelrod, 1968; Jaroensutasinee & Jaroensutasinee, 2001). The ability to compete for mates is therefore critical to the reproductive success of male Siamese fighting fish through male-male competition and possible direct benefits of female mate choice. In the present study the relationship between male body size, male contest and female preference in the wild Siamese fighting fish was investigated. The question of whether the sexually dimorphic trait (i.e. male standard length, LS) improves a male’s fighting ability and relates to female preference is addressed. This study predicts that: (1) if male contest plays an important role in mating success in this species, then larger males should win fights; (2) fighting duration should decrease as the size difference between males increases; (3) if females prefer larger males as mates, then they should spend more time with larger males than smaller males. MATERIALS AND METHODS FISH BIOLOGY The Siamese fighting fish is an anabantid native to Thailand, Malaysia, Cambodia and Myanmar (Smith, 1945). Typical fighting fish habitats in Thailand are quiet freshwater ponds with muddy bottoms or flooded rice paddy fields (Gordon & Axelrod, 1968). Unlike domesticated fighting fish, wild fighting fish are small, inconspicuous, and dull brown or green in colour (Jaroensutasinee & Jaroensutasinee, 2001). They hide beneath water plants, presumably to minimize predation from turtles, snakes, fish-eating egrets, herons and kingfishers. Male Siamese fighting fish have very aggressive social displays including gill cover erection, biting, tail beating, attacking and chasing (Clayton & Hinde, 1968; Simpson, 1968). Fighting usually involves physical damage and can result in death. Females are duller in colour and usually smaller than males (Jaroensutasinee & Jaroensutasinee, 2001). After the females finish laying eggs, the males chase the mated females out of the bubble nest areas and solely provide parental care for developing eggs and fry. DATA COLLECTION The test subjects were wild caught fish collected in April–December 2000 from Nakhon Si Thammarat, Thailand. The fish were maintained in the laboratory with natural light and fed daily with mosquito larvae. Males and females were housed in separated 1 l bottles to prevent fighting. Gravid females were used as the test subjects. The test aquarium for the female preference tests was a 60 l aquarium, measuring 0·60 0·36 0·30 m high, and divided into three compartments with a removable clear Plexiglas partition and an opaque Plexiglas partition between the two males. The central compartment comprised 67% of the total test arena and contained the focal female. The two similar adjacent compartments were of equal size and each contained a male. The preference zone was demarcated in the centre compartment by drawing a line 10 cm away from the clear Plexiglas partition on the outside of the aquarium. The region next to the partition of each male was the preference zone for that male and the rest of the central compartment was considered to be a no-preference zone. Trials were conducted from 0900 to 1700 hours. All test animals were fed prior to testing. 1616 . . Each trial consisted of two 10 min observation periods. At the beginning of each trial, a focal female was placed in the centre of the no preference zone. A 5 min acclimation period preceded each set of observations in the trial. Then the first observation period was initiated. During this period, the behaviour of the three test subjects was noted and the number of seconds the females spent in each of the three centre sections was recorded. At the completion of this observation period, the location of the two males was reversed to preclude any side bias by females. The female was again placed in the centre section of the no preference zone for a 5 min acclimation period. This was followed by a second observation period. The time of the two observation periods was summed for statistical analysis. For the female preference test, successful trials were those that the test female approached each male at least once to ensure that both males were viewed by the female and all three participants actively courted. Three treatments were conducted using males in which there was a LS difference of 0, 1 and 2 .. of male LS (i.e. 0, 0·48 and 0·96 cm). Male LS was 2·91 0·24 cm (mean .., n =70) and female LS 2·43 0·52 cm (n =70) (Jaroensutasinee & Jaroensutasinee, 2001). LS was measured as the distance from the anterior-most point of the upper jaw to the end of the caudal peduncle. Each trial was conducted with new females and the new pairs of matched males. Forty, 30 and 27 trials were conducted in 0, 1 and 2 .. treatments respectively. In the 2 .. treatment, there were only 27 trials conducted due to some difficulties in finding fish with the 2 .. difference. Female preference for a male was expressed as the difference between the total nearness time with the larger male and the total nearness time with the smaller male divided by the total nearness time with both males. For the male contest test, the same pairs of test males from the female mate choice tests were placed in a 1 l bottle. The number of the five agonistic behaviours (i.e. gill cover erection, biting, tail beating, attacking and chasing) was observed for both males until one fish retreated. Gill cover erection (G) was regarded as males erecting the gill covers while oriented towards or parallel to intruders. Lowering of the gill cover or swimming away from intruders ended gill cover erection. Biting (B) was recorded when males used their mouthpart to bite or tear at intruders. Tail beating (T) was defined as each separate beat of tail towards intruders. Attacking (A) was recorded when the focal male swam rapidly towards its intruder. Chasing (C) was defined as rapid and continuous following. The contest duration was recorded as the period from the first agonistic behaviour until one fish retreated. DATA ANALYSIS Parametric statistics were used when underlying assumptions were met, otherwise non-parametric tests were used. A 2 test was used to test for the number of fights won by the larger males. Paired t-tests were used to test for the amount of agonistic behaviour differences between large and small males. One-way ANOVAs were used to test for fighting duration and female preference to among the three size category treatments. All significant tests were two-tailed. RESULTS FEMALE PREFERENCE TEST There was no female preference for males with greater LS (one-way ANOVA, F2,94 =1·37, NS; Fig. 1). MALE CONTESTS Not all-male pairs fought. Only 28 pairs out of 40 pairs, 25 pairs out of 30 pairs and 22 pairs out of 27 pairs fought in the 0, 1 and 2 .. treatments 1 and 2 .. respectively. Larger males won more fights ( 2 tests for treatments: 17·64, P< 0·001 and 22·00, P< 0·001, respectively). Larger males won 23 pairs out of 25 fighting pairs in the 1 .. treatment and all 22 fighting 1617 1 0.75 Female preference 0.5 0.25 0 –0.25 –0.5 –0.75 –1 0 S.D. 1 of male Ls difference 2 F. 1. Female preference and male size differences of 0, 1, and 2 .. in male standard body length. Female preference for a male was defined as the difference between the total nearness time with the larger male and the total nearness time with the smaller male divided by the total nearness time with both males. pairs in 2 .. treatment. The average fighting duration in all treatment was 584·97 425·09 s (mean ..), n =75. There were no differences in fighting duration among treatments (one-way ANOVA, F2,72 =2·96, NS). BEHAVIOURAL DIFFERENCES BETWEEN LARGE AND SMALL MALES From 1 and 2 .. treatments, smaller males started all the fights in 47 fighting pairs. Comparing agonistic behaviour between large and small males in these two treatments, larger males fish performed more chasing and attacking, and displayed more total agonistic behaviour than smaller males (paired t-tests for chasing: t =6·17, d.f.=46, P< 0·001; attacking: t =3·45, d.f.=46, P< 0·001, total agonistic behaviour: t =2·47, d.f.=46, P< 0·05, Fig. 2). There were no differences between larger and smaller males concerning other agonistic behaviours during fighting (paired t-tests for gill cover erection: t =0·86, d.f.=46, NS; tail beating: t =1·57, d.f.=46, NS, biting: t =0·09, d.f.=46, NS, Fig. 2). DISCUSSION It is well established that individuals differ in their ability to win fights, and that winners often have priority of access to mates and critical resources (Huck et al., 1986; Huntingford & Turner, 1987; Archer, 1988; Dewbury, 1990; Schuett, 1997). In species where males compete for mates, males are usually larger than females (Jarman, 1983; Kirkpatrick, 1987; Hedrick & Temeles, 1989; Poole, 1989). Large male fish are usually at an advantage in competing for access either to females or to resources preferred by females (Hanson & Smith, 1967; Constantz, 1975; Cole, 1982; Hughes, 1985; Bisazza & Marconato, 1988). In fish species in which males defend territories, large size has been shown to confer success in competition among males (Constantz, 1975; Warner et al., . . 1618 140 * Level of aggression 120 100 80 60 40 20 ** ** G T B A Male agonistic behaviour C Total F. 2. Mean ( ..) of agonistic behaviour between large ( ), and small () males in 1 and 2 .. treatments, n =47 fighting pairs. Male agonistic behaviour includes gill cover erection (G), tail beating (T), biting (B), attacking (A), chasing (C) and total agonistic behaviour (Total). *P< 0·05, **P< 0·001. 1975; Schmale, 1981; Chellappa et al., 1999). The present study is the first demonstration in fighting fish that larger males tend to win contests. A number of studies have shown that winners and losers may differ in their behaviour during contests (Reichert, 1978; Enquist et al., 1985, 1990; Harvey & Corbet, 1986; Turner & Huntingford, 1986; Popp et al., 1990; Turner, 1994) but few cases have tested the effect of size asymmetries (Turner & Huntingford, 1986; Turner, 1994). Turner (1994) has shown that larger males swim around in circles more often than smaller males, suggesting that this activity may carry a higher cost for smaller males than larger males. In the present study, larger male fighting fish performed more chasing, and attacking, and generally displayed more total agonistic behaviour than smaller males but no difference in the number of gill cover erections, tail beatings, attacking and biting was noted. Chasing behaviour usually occurred near the end of male contests. This indicates that chasing by larger males occurred when smaller males were fatigued and tried to escape. Female preference has been reported for many teleost fishes (Hay & McPhail, 1975; Kodric-Brown, 1977; Hughes, 1985). In species with paternal or biparental care, females prefer larger males as mates (Perrone, 1978; Downhower & Brown, 1980; Noonan, 1983; Berglund et al., 1986; Bisazza & Marconato, 1988). In some of these species, male size is known to be correlated with the quality of parental care (Perrone, 1978; Downhower & Brown, 1980). However, in this study, female fighting fish showed no preference for larger males. This result was not because females were sexually unreceptive. Gravid females were used that were sexually active and ready to mate within 3 h of time spent with males who had their bubble nest present. Since larger males are more likely to win contests, it is very probable that female mate choice falls solely upon the males with the female passively accepting the winning male. Similar behaviour have been shown in many other species such as bees, harvester ants, scorpionflies, 1619 damselfish, bullfrogs, toads and iguanid lizards (Davies & Halliday, 1977, 1979; Ruby, 1981; Thornhill, 1981; Davidson, 1982; Thresher & Moyer, 1983; Alcock, 1984; Howard, 1988; Anderson, 1994). In conclusion, sexual size dimorphism is assumed to be adaptive and is expected to evolve in response to a difference in the net selection pressures on both sexes (Blanckenhorn, 2000). The wild Siamese fighting fish are sexually dimorphic that is males are usually bigger than females. Sexual size dimorphism in this species may derive from male contests, not female preference. The results showed that large males often win fights in male contests, and there is no female preference for larger males. 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