Idotea baltica, Asellus aquaticus (Isopoda), and. Gammarus zaddachi (Amphipoda). Specifically, we manipulated, by osmotic stress or a neuromuscular blocking ...
Behav Ecol Sociobiol (1995) 36 : 43M8
© Springer-Verlag 1995
V. Jormalainen • S. Merilaita
Female resistance and duration of mate-guarding in three aquatic peracarids (Crustacea)
Received: l l March 1994/Accepted after revision: 17 September 1994
Abstract Both theoretical and empirical studies have treated mate-guarding in aquatic Crustacea purely as a male decision problem. However, male and female interests are rarely identical, as implied by observations of female resistance against guarding attempts. We tested experimentally the occurrence of sexual conflict over guarding duration in three crustacean species: Idotea baltica, Asellus aquaticus (Isopoda), and Gammarus zaddachi (Amphipoda). Specifically, we manipulated, by osmotic stress or a neuromuscular blocking agent, the female's ability to resist guarding attempts. Female manipulation, by both methods, roughly doubled precopula duration in I. baltica (Figs. 1 and 2) showing that female resistance effectively diminishes guarding duration. However, in A. aquaticus and G. zaddachi female manipulation had no effect on guarding duration, which also was longer than in I. baltica (Fig. 2). This implies either that male and female interests are equal or that the conflict is resolved according to the male interest in these species. The lack of female resistance in such species allows long precopulatory guarding. In I. baltica we also manipulated, by osmotic stress and by clipping nails, male ability to hold the female. These treatments had no effect on guarding duration (Figs. 1 and 2). Male size tended to correlate positively with guarding duration in control groups, but not in female manipulation groups (Fig. 3). Thus, conflict is mainly resolved according to the female interest i n / . baltica. Results in this species also suggest that female resistance selects
V. Jormalainen([]) 1 • S. Merilaita2 Satakunta Environmental Research Centre, University of Turku, FIN-28900 Pori, Finland Present addresses:
1Department of Biology, Laboratory of Ecological Zoology, University of Turku, FIN-20500 Turku, Finland 2Department of Zoology, Uppsala University, Villavfigen 9, S-75236 Uppsala, Sweden
for large male size. Consequently, mechanisms of sexual selection may differ considerably between species with otherwise comparable mating patterns. Key words Precopula • Sexual selection • Female resistance • Isopoda • Amphipoda
Introduction Recent studies of mating behaviour have increasingly emphasized the role of female behaviour in affecting males' mating objectives (Ahnesj6 et al. 1993; Birkhead and Moller 1993; Rowe et al. 1994). Although female behaviour patterns are not well known (Rosenqvist and Berglund 1992) there are a number of ways whereby females may affect male mating aspirations, even in species in which the male takes the active part in mate searching (e.g. Parker 1974; Janetos 1980; Arnqvist 1992). The payoffs of a given mating pattern may differ between the sexes, which leads to intersexual conflict. The resolution of the conflict may be a compromise for both sexes, or it may favour either male or female interests (Parker 1979; Ahnesj6 et al. 1993). The mating pattern of many aquatic isopods and amphipods can be described as serial monogamy (cf. Ahnesj6 et al. 1993), in which a male guards a female (amplexus) for some time before actual copulation. The opportunity for fertilization is time-limited as it is possible only during the parturial moult of a female. The time between successive matings is relatively long in females due to incubation of broods in a marsupium. In males this time varies according to the availability of receptive females, which, in turn, is largely determined by the amount of synchronization of the female moulting cycle. Males are active in searching for mates (Parker 1974; Ridley and Thompson 1985) and, thus, regulate the number of possible encounters with females. However, during encounters, male and female
44
interests concerning the start of guarding may not be identical, and, consequently, a conflict may arise over whether to start or delay precopulatory guarding (Jormalainen and Merilaita 1993). From the male's point of view, guarding is a beneficial competitive strategy by which he monopolises a female while waiting for the onset of her receptivity (Parker 1974). Precopulatory mate-guarding may evolve when female receptivity is time-limited (Parker 1974; Grafen and Ridley 1983) or even when her receptive period is long but guarding costs are sufficiently low as compared to searching costs (Yamamura 1987). Optimal guarding duration is sensitive to factors such as encounter probability, sex-ratio, occurrence of takeovers, synchronization of female moulting cycle, guarding and searching costs, and duration of receptive period (Parker 1974; Grafen and Ridley 1983; Yamamura 1987). Thus, under certain conditions, a male's chances of obtaining a mate are better the earlier he can start guarding. Prolonged guarding may incur costs for females. In a precopula pair the male is usually responsible for motion (e.g. Adams et al. 1985; Naylor and Adams 1987), and hence he selects microhabitats for the pair. This may contradict female interests concerning feeding or predation avoidance (Jormalainen and Merilaita, 1993). A model of optimal guarding duration (Jormalainen et al., in press) revealed two conditions favouring shorter optimum guarding duration for females than for males: (1) higher mortality of males during mate searching than during precopula, and no mortality differences or higher mortality during than before precopula in females; and (2) a male-biased operational sex ratio, which is usually true in isopods and amphipods because of the sexual difference in time investment per brood. Thus, behavioural means to shorten guarding duration may have evolved in females. These may be, for example, avoiding and hiding from males, or struggling against males' mating attempts, and body manoeuvres fostering escape. Female behaviour, especially resistance, is important, as it may act as a selective agent for male characteristics. In the present study, for the first time, we test the effects of female behaviour on precopula duration in mate-guarding Crustacea. This kind of experimental design, distinguishing between female manipulation of males and male exploitation of females, is suggested to be important in the study of sexual selection (Birkhead and Moller 1993). Specifically, we experimentally manipulated the female's ability to perform behaviour patterns that may affect the formation of a precopula pair in two isopod (Idotea baltica, Asellus aquaticus) and one amphipod (Gammarus zaddachO species by means of osmotic stress or a neuromuscular blocking agent. I n / . baltica we further manipulated the male's ability to hold the female in order to evaluate whether the conflict was
resolved according to male or female interests. The species were chosen because of the evident controversy in the literature over the role of female behaviour both within and between species of mate-guarding Crustacea (see e.g. Ridley and Thompson 1979; Thompson and Manning 1981; Birkhead and Clarkson 1980; Ward 1984; Dick and Elwood 1989; Jormalainen and Merilaita 1993; Jormalainen et al. 1994).
Materials and methods A male and female were allowed to form a precopulatory pair in 1-1 (I. baltica, G. zaddachi) or 0.15-1 (A. aquaticus) jars under laboratory conditions (aeration, 15°C, natural light rhythm). A piece of bladderwrack (Fucus vesiculosus) with epiphytic algae and a stone were provided as habitat and as food for/. baltica and G. zaddachi, and a decaying birch leaf for A. aquaticus. "Lapis" fish food (containing 45% raw protein) was offered as extra nourishment to G. zaddachi and A. aquaticus. The pairs were checked three times a day, between 5 and 9 a.m., 11 and 3 p.m., and 7 and 10 p.m. Precopulatory guarding, moults of both the male and female, and oviposition were observed. Experiments ended at parturial moult and oviposition. The aim of the female manipulation was to decrease their ability to resist male mating attempts. Females often resist by kicking and by body manoeuvres (references in Jormalainen and Merilaita 1993), in which muscular strength is needed. Females were exposed to a mixture of neuromuscular blocking agent (alloferin, 5 mg/ml, Roche) and water. This neuromuscular blocking agent affects receptor activation by blocking competitively the transmitter action of ACh, and is used as an adjuvant in surgical anesthesia to obtain relaxation of skeletal muscles (Taylor 1980). The treatment was repeated daily until the pair had started precopulatory guarding. After treatment the animal was rinsed in brackish (/. baltica, G. zaddachi) or fresh (A. aquaticus) water before returning to the original container. Females in control groups were also removed from the experimental containers for the duration of the treatment. In I. baltica, manipulation of male ability to hold a female was included, and an additional experiment with osmotic stress treatment to both males and females was done. A randomly selected male was given to each female and the pairs were randomly allocated to different treatments. Details of sampling and experimental procedure in each species are given below. I. baltica We sampled animals from the bladderwrack in Gulf of Bothnia, west coast of Finland (61 ° 21'N, 21 ° 26'E, and 61 ° 29'N, 21 ° 21'E) on 5, 14 and 25 May, and reared them in groups before the start of experiments. At the start we measured animals to the nearest 0.5 mm. Females were in the preparturial stage, their next moult being the one during which copulation takes place. The cases in which the male moulted during the experiment were excluded from the analysis in order to decrease variation in guarding duration due to male moult.
Osmotic stress experiment This experiment was started in successive subsets between 17 and 22 May. The design was planned for a two-way ANOVA. We had (1) control, (2) male stress, (3) female stress, and (4) both male and female stress groups. Osmotic stress was produced by placing the animal(s) in stagnant tap water for 1 h twice a day at about 12-h intervals until precopulatory guarding started. This treatment
45 caused slow movement or immobility for a few hours. Female stress treatment did not affect timing of parturial ecdysis as the time between start of experiment and completion of parturial ecdysis did not differ between control (ff = 208 h, SD = 43, n = 40) and stress (2 = 228 h, SD = 65, n = 46; ANOVA: FI,,4 = 2.76, P = 0.10) groups.
Neuromuscular blocking agent
nail clipping experiment
We started the experiment on 28 and 31 May. We had three different groups: (1) control, (2) manipulated female with control male, and (3) manipulated male with control female. The female manipulation was done by placing females once a day into a mixture of brackish water (18 ml) and alloferin (3 ml) for 1 h. In a preliminary survey this concentration (about 14%), had a clear reductive effect on female activity. Female treatment did not affect timing of ecdysis (time between start of experiment and completion of ecdysis: control group: x = 104 h, SD = 49, n = 30; stress group: x = 93 h, SD = 31, n = 18; ANOVA: F~, 46= 0.79, P = 0.38). Males in group 3 were manipulated by cutting the pincer-like "nails" from the third and fourth pair of pereiopods a few days before the start of the experiment. This nail clipping had no noticeable effects on male behaviour, but we assume that their ability to hold a resisting female was decreased as compared to control males because males use nails of all of their legs to hold the female (Jormalainen and Merilaita 1993). A. aquaticus
marsupia. In the latter case a female was allowed to copulate and oviposit, after which the experiment was started with a new male. As with A. aquaticus, we had a control and a female manipulation group. The manipulation was done by exposing the female to a 25% mixture of alloferin and brackish water solution for 1 h each day. The acute effect of this treatment was variable. Some females lay on the bottom ,on their sides for hours, but were often able to move quickly when disturbed. Some individuals were able to swim relatively normally. Furthermore, the alloferin treatment tended to increase female mortality (61%) as compared to control group (39%, G = 3.54, df= 1, P = 0.06).
Results Precopula duration in I. baltica was about one third of that in A. aquaticus and G. zaddachi (Figs. 1 and 2). Guarding duration was also shortest in I. baltica in relation to intermoult interval. We did not measure the duration of this interval directly, but the m i n i m u m estimate for it from the osmotic stress experiment was 242 h (n = 38, SD = 59), giving a m a x i m u m estimate for relative guarding duration of 14% of the intermoult cycle. A. aquaticus males guarded for on average 38% of the intermoult duration (x = 273 h, SD = 78, n -- 47).
We sampled animals weekly from leaf litter of a large freshwater spring (Kuninkaanl~ihde 61 ° 05'N, 22 ° 24'E). We started the experiment with newly sampled individuals in successive subsets between 7 and 25 July. The experiment was started by offering a male to a female in the preparturial stage. A t this stage the animals were measured to the nearest 0.5 mm. Some of the females were in preparturial stage when sampled while a part moulted and became to this stage in the laboratory. We had a control and female manipulation group. The manipulated females were exposed to a 15 % mixture of alloferin and fresh water for 0.5 h each day. This caused temporary immobility or slow motion in most individuals. Cases in which the male underwent ecdysis before half of the duration of the female moulting cycle were deleted from the analysis.
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Fig. 2 Precopula duration (h; ~, SE, n) in neuromuscular blocking agent experiment i n / .
baltica, AseIlus aquaticus, and Gammarus zaddachi
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