Costs of loading associated with mate-carrying in the

Costs of loading associated with mate-carrying in the waterstrider, Aquarius remigis

Department of Biology, Concordia University, 1455 de Maisonneuve Boulevard West, Montreal, Quebec H3G 1M8, Canada

R

Received 8 November 1991 Revised 22 June 1992 Accepted 10 July 1992 1045-2249/93/J5.00 © 1993 International Society for Behavioral Ecology

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eproduction generally entails increased weightbearing for at least one sex. When one sex carries the other during some phase of courtship and mating, loading associated with mate-carrying may be both prolonged and extreme. For example, male isopod and amphipod crustaceans carry females during a precopulatory phase that may last several days (Adams and Greenwood, 1987; Adams et al., 1985; Elwood et al., 1987). Many insects perform nuptial flights in which one partner (usually the male) supports the other in flight during some phase of the reproductive cycle (Alcock and Gwynne, 1987; Alcock et al., 1976, 1977; Marden, 1989; O'Neill and Evans, 1983; Thornhill, 1980; Thornhill and Alcock, 1983). Some dipteran and mecopteran males offer nuptial gifts of captured prey to females and must not only take off and fly carrying their gift, but in many cases they must also carry the female plus prey once the gift has been accepted (Marden, 1989; Thornhill and Alcock, 1983). For motile organisms, reproductive loading must entail an energetic cost for the load-bearing individual. In addition, loaded individuals may be more susceptible to predation because of reduced mobility or increased visibility to predators (Arnqvist, 1989a; Hairston et al., 1983; Koufopanou and Bell, 1984; Lima and Dill, 1990; Shine, 1980; Winfield and Townsend, 1983; but see Gwynne, 1989, for exceptions). If reproductive loading is prolonged, reduced mobility may also affect the foraging efficiency of loaded individuals (Arnqvist, 1989a; Rowe, 1992; Wheeler and Greenwood, 1983; Wil-

Behavioral Ecology Vol. 4 No. 3

cox, 1984). Because of these costs, loading associated with carrying reproductive material, mates, or nuptial gifts is generally presumed to select for large size in the carrying sex and has been suggested as a selective force affecting sexual size dimorphism in a number of taxa (Adams and Greenwood, 1987; Adams et al., 1985; Naylor and Adams, 1987; O'Neill, 1985; O'Neill and Evans, 1983; Vepsalainen and Nummelin, 1985; Wheeler and Greenwood, 1983). Where one sex carries the other, reproductive loading may also influence the duration and frequency of mating (Rowe, 1992; Sih et al., 1990; Strong, 1973) and patterns of mate choice (Adams et al., 1985; Crespi, 1989). Prolonged mating during which females carry males is characteristic of temperate waterstriders (Hemiptera, Gerridae; Andersen, 1982; Fairbairn, 1990). These are semi-aquatic insects, and both mating and foraging occur on the water surface. Mating includes precopulatory and postcopulatory phases, during which the male remains mounted on the female's back (Andersen, 1982; Arnqvist, 1988, 1989b; Rowe, 1992; Rubenstein, 1989; Wilcox, 1984). In some species, this physical pairing may extend from several hours to several weeks, and females continue to forage while in tandem (Clark, 1988; Fairbairn, 1990; Rubenstein, 1984, 1989; Sih et al., 1990; Vepsalainen and Nummelin, 1985; Wilcox, 1984). Females are larger than males in most species, and loading during prolonged mating has been proposed as a possible selective factor favoring this female-biased size dimorphism (Andersen, 1982; Fairbairn, 1990; Vepsalainen and

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When one sex carries the other during some phase of courtship or mating, the associated loading may entail a significant cost to the carrier. This paper presents a series of laboratory experiments designed to identify the costs of mate-carrying in Aquarius remigis. Female A. remigis mate repeatedly and carry each mate for several hours. Dead males and lead weights were used to simulate normal mating and loading associated with mate-carrying, respectively. Females carrying weights equivalent to the weight of an average male showed no detectable reductions in survival, lipid reserves, or foraging success, and maintained themselves on the water surface for more than 10 days without access to resting sites. Weights equivalent to two males were supported for 6.1 ± 4.9 days. Thus, female A. remigis appear to be very well adapted to carrying their mates and are unlikely to be near their load limits when carrying a single mate. However, females carrying males or equivalent weights suffered a significant reduction in maximum mobility (stride length and speed), and an increased risk of predation by frogs (Rana clamitans). Females carrying weights were more susceptible to predation than unburdened females but were less susceptible than females carrying males, suggesting that loading contributes significantly to, but does not fully explain, the increased predation risk. This risk probably results from both reduced mobility due to loading and greater visibility (size). Possible influences of the costs of loading on mating behavior and sexual size dimorphism are discussed. Key words: Aquarius remigis, Gerridae, mating, cost of reproduction, predation risk, sexual size dimorphism. [Behav Ecol 4:224-231 (1993)]

D. J. Fairbairn

Table 1 Description of treatments used to detect maximum sustainable load Treatment* Mean female weight (g) Mean weight of lead (g) Mean proportion of female weight

1

1

2

3

4

5

6

0.047 (0.007) 0.040 (0.002) 0.86 (0.15)

0.046 (0.009) 0.081 (0.002) 1.84 (0.43)

0.050 (0.006) 0.119 (0.004) 2.41 (0.30)

0.047 (0.007) 0.160 (0.003) 3.46 (0.62)

0.050 (0.009) 0.201 (0.002) 4.15 (0.74)

0.047 (0.007) 0.242 (0.003) 5.25 (1.04)

Standard deviations are in parentheses. Lead weights equivalent to one to six times the average weight of a male.

Study animal Aquarius remigis is a large (11-16 mm in length) waterstrider found in lotic habitats throughout North America (Andersen, 1990; Calabrese, 1977; Polhemus and Chapman, 1979). Adults and nymphs feed on arthropods trapped in the surface film, and foraging occurs primarily in areas of calmer water (surface current < 15 cm/s) below riffles (Cooper, 1984; Fairbairn and Brassard, 1988; Rubenstein, 1984). Females are larger than males, and sexual size dimorphism is greater than expected for gerrids of this size (Fairbairn, 1990). Mating duration (an average of 3-7 h under laboratory conditions; Fairbairn, 1988b; Sih et al., 1990) is determined primarily by a prolonged period of copulatory guarding after sperm transfer (Rubenstein, 1989). Reproductive females spend much of their time carrying males and continue to forage while mating (Fairbairn DJ, personal observation; Wilcox, 1984). METHODS

I examined the cost of mate-carrying in A. remigis through a series of five experiments using artificially loaded females. Artificial loading was accomplished by gluing (Instant Krazy Glue) small, flattened lead weights ("leads") to the posterior end of the pronotum. Preliminary experiments indicated no effects of the glue or lead on fertility, fecundity, or life span (Fairbairn DJ, unpublished data). The leads were irregular in shape, tending to be roughly circular, and those approximating the weight of a male A. remigis were 2.5 ± 0.6 mm in longest dimension and 0.06 ± 0.001 mm thick. Applying the leads took only a few seconds, and they did not appear to interfere with normal female movement. Short-term assessments of mobility, foraging success, and risk of predation also included a "tandem" treatment designed to assess the cost of not only the weight of the carried male, but also his bulk or volume. I created the tandems by gluing a freshly killed male to the back of each female. Males were killed by freezing overnight and

were attached to their partners in natural mating position. To control for handling effects, I handled females not receiving leads or mates in a manner that mimicked the application of these loads. I collected adult A. remigis from Brochet Creek, approximately 125 km northwest of Montreal, Canada, and held them in the laboratory at room temperature and 16 h light: 8 h dark until used in an experiment (see Fairbairn, 1988a, for culture methods). Collections were made on 29 July and 12 August 1990. I began all experiments within 6 days of capturing the test animals and conducted experiments at room temperature (22.6°C ± 1.8°C23.4°C ± 1.2°C) under natural photoperiod (experiment 3) or 16 h light: 8 h dark (experiments 1, 2, 4, and 5). I tested all data for normality using Kolmogorov-Smirnov goodness-of-fit tests and for homoscedasticity using Bartlett's test. Data were transformed where necessary before analyses using parametric statistical methods. Means are given ± SD.


Nummelin, 1985). This paper examines the costs of loading associated with mate-carrying in the waterstrider, Aquarius remigis Say [formerly Gems remigis (Andersen, 1990)], as a first step in assessing the influence of such loading on the evolution of sexual size dimorphism and mating behavior in the Gerridae.

Experiment 1: maximum sustainable load Methods

The first experiment was designed to determine the maximum load that could be supported by female A. remigis on the water surface. Loading associated with mate-carrying was simulated with lead weights, as described above. I randomly assigned 72 females to six treatments (Table 1), simulating one to six times the weight of a single male. I allowed females to rest on land for 10 min after the leads were attached. One female from each treatment was then placed on the water surface in each of 12 plastic cages (48 x 38 * 20 cm), containing water to a depth of 10 cm. The cages contained no resting sites, and females were not able to climb the sides. I recorded time to sinking for each female. Females were fed before the experiment, and five frozen Drosophila melanogaster per female per day were

provided throughout the experiment. I ended the experiment after 10 days. Results

Mean times to sinking are shown in Figure 1. All females in treatment 1 and 7 of 12 females in treatment 2 were still on the water surface at the end of the experiment. These animals were assigned a score of 10 days, and the means for these treatments therefore underestimate the true means. Females were clearly able to support loads equivalent Fairbairn • Cost of mate-carrying in Aquarius remigis

225

for capture at all times. The time to first capture of a fly, total number of flies captured after 5 and 10 min, and position in the stream after 5 and 10 min were recorded for each female. To ensure that all females were active on the water surface, I returned females that attempted to rest on the walls during the first 5 min of the trial to the water within 0.1 m of their resting site. Only 2 of 20 females (both tandems) attempted to rest during the first 5 min, and 3 (all tandems) rested during the second 5 min of the trial.

Figure 1 Mean times (± 1 SD) to sinking for females carrying one to six times the normal reproductive loads. Note that the ordinate is a log scale. Dotted area indicates mean duration of pairing reported in the literature (Fairbairn, 1988, 1990; Sih et al., 1990). Hatched area indicates the duration of copulation necessary for completion of sperm transfer (Rubenstein, 1989).

Results

0 08

0 12

016

0 20

0 24

Added weight (g)

Experiment 3: long-term foraging success to twice the weight of a male for much longer than necessary for normal copulation and guarding. Even females carrying the equivalent of the weight of three males were able, on average, to remain on the surface long enough to allow complete sperm transfer. If maximum sustainable load is defined as the load that can be carried for long enough to permit normal copulation and postcopulatory guarding, we may conclude, by extrapolation from Figure 1, that the maximum sustainable load under the conditions of testing was approximately 0.11 g. This is equivalent to 2.7 times the weight of an average male and 223% of the weight of the carrying female. These results indicate that the loads associated with carrying single males are well below the maximum capacity of female A. remigis. Experiment 2: short-term foraging success Methods

To determine the influence of loading on shortterm foraging success, I observed females foraging in a linear portion (110 x 20 cm) of an artificial stream (see Fairbairn and Brassard, 1988, for a complete description). The surface current varied from 7.3 to 13.9 cm/s, with a mean of 11.3 ± 2.7 cm/s, which approaches the upper limit for natural foraging in this species (Fairbairn and Brassard, 1988). The experiment consisted of three treatments: control, weighted, and tandem females. The males used weighed 0.033 ± 0.004 g, and the leads weighed 0.034 ± 0.002 g. Females were deprived of food for 24—29 h and allowed a minimum of 2 h to recover from handling before being used in the experiment. I ran the experiment as a series of 20 trials, each consisting of one female from each treatment. Within each trial, I matched females with respect to time since manipulation and time since last feeding. The three females were placed on the water surface at the head of the stream section and allowed 1 min to acclimate. During the subsequent 10 min, I sprinkled frozen Drosophila melanogaster on the water surface at the upstream end of the stream section, so that at least one fly was available

226


Methods

I divided 80 females equally and at random between two treatments: weighted and control. The leads weighed 0.031 ± 0.002 g and mimicked the weights of males in the sampled population (0.034 ± 0.005 g). Ten females from each treatment were randomly assigned to each of four plastic cages (48 x 38 x 20 cm). I placed an airstone in one corner of each cage and placed styrofoam floaters in each cage for resting. Frozen Drosophila melanogaster were added twice each day, at the rate of three flies per waterstrider per day. Preliminary experiments indicated that wild-caught females fed two D. melanogaster per day show declining lipid reserves when maintained under conditions similar to those used in this experiment (Fairbairn DJ, unpublished data). A ration of three flies per day was therefore considered an adequate but not excessive maintenance ration. I always sprinkled flies on the water surface at the site of the airstone. The vigorous current caused by the airstone carried the food away from the delivery site, mimicking the delivery of food by downstream drift in the natural habitat of the waterstriders. Aquarius remigis establish feeding territories preferentially in areas of high food delivery (Blanckenhorn, 1991; Rubenstein, 1984), and these territories, essentially foraging positions, may be maintained for several months. Thus, some stability in the distribution of foraging positions was expected within each cage. I ran the experiment for 31 days. Beginning on day 11, the positions of all animals were recorded on a scale drawing of the cage interior, 1-2 h after the morning feeding, 5 days per week. From these maps I determined the distance of each female from the airstone (food source) and noted whether each female was active on the water surface or resting (touching one of the styrofoam floaters or the sides of the cage). In A. remigis, lipids (mainly triglycerides) stored in the fat body are the primary energy reserves for diapause, migration, and egg production (Lee et al., 1975). I therefore used lipid content as an assay of the ability to accumulate energy and thus of longterm foraging efficiency. On day 31, I assayed all


0 04

Control, weighted, and tandem females did not differ significantly with respect to time to first capture of a fly, number of flies captured, or foraging position (Table 2). The data do not indicate any tendency for control females to forage more efficiently than weighted or tandem females. Thus, there is no indication that short-term foraging success is inhibited by mate-carrying in female A. remigis.

Table 2 Comparisons of short-term foraging success of control, weighted and tandem females

Variable Time to first capture (5) Number of flies captured in 5 min Number of flies captured in 10 min Position after 5 min* (cm) Position after 10 min" (cm)

Treatment Control 114.6 (76.9)

Comparison of treatments Weighted

Tandem

X?

P

107.4 (53.9)

147.4 (157.9)

0.225

>.75

0.250

>.75

3.528

>.10

2.60

>.25

4.831

>.O5

2.2

2.2

2.2

(1-3)

(15)

(1.4)

4.4

4.2

3.7

(2.2)

(2.4)

(2.1)

91 (5) 90

83

92

(14)

(16)

81

93

(13)

(14)

(13)

surviving females for lipid content following the methods of Lee et al. (1975). I killed females by freezing them overnight and then placed them in preweighed vials containing 1.5 nil of a 2:1 v/v solution of chloroform and methanol. I removed the bodies from the vials after 42 days, and both bodies and vials (containing lipids) were dried at 55°C for 24 h and then weighed. Results

Ninety-five percent of control females and 90% of weighted females were still alive on day 31. These proportions did not differ significantly (Fisher's Exact test, p = .31). The proportion of females active on the water surface declined with time, from 0.91 on day 11 to 0.58 on day 31 (F = 6.051, df = 12,72, p < .001) but did not differ significantly among cages (F = 2.521, df = 3,4, p = .20) or between treatments [F = 0.006, df = 1,6, p = .94, repeated measures ANOVA, with arcsinev^proportion active) as the dependent variable]. The interaction between time and treatment was also not significant (F = 0.400, df = 12,72, p = .96). Thus, weighted females were as likely as control females to be active on the water surface. Distances of individual females from the airstone (food source) were corrected by subtracting weekly cage means to remove differences in average distances among cages and dates. (Two-way ANOVAs by week revealed significant differences among cages in 3 weeks, but no significant interactions between treatment and cage. Thus, the effects of the treatments did not differ among cages.) Repeatedmeasures ANOVA of the corrected distances revealed no significant difference between treatments (F= 4.529, df = 1,6, p = .08). Overall, the controls were farther from the airstone on eight sampling dates, whereas the weighted females were farther from the airstone on five sampling dates, and the mean corrected distances were 0.27 cm and —0.30 cm for nonweighted and weighted females, respectively. Thus, there is no tendency for weighted females to forage farther from the food source than control females. Both absolute lipid dry weight and lipid as a proportion of total dry weight were analyzed using twoway ANOVA with treatment and cage as indepen-


Comparisons were made using Friedman's chi-square for randomized blocks (df = 2). Data are means with SDs in parentheses. ' Distance from upstream boundary of test area.

dent variables. Neither cage effects nor interactions were significant for either dependent variable (p > .20). The treatments also did not differ significantly for absolute lipid weight (F = 0.318, df = 1,62, p = .575) or for proportion lipid (F = 1.659, df = 1,62, p = .203). Although mean absolute lipid dry weight was similar for control (0.0205 ± 0.0016 g) and weighted females (0.0207 ± 0.001 g), proportion lipid tended to be slightly higher in control females (0.557 ± 0.025, and 0.553 ± 0.024, for control and weighted females, respectively). A paired t test, using cages as blocks, effectively removed the variance within cages and yielded a onetailed probability of less than 0.10 for proportion lipid (t = 1.902, df = 3). Thus, there was a slight but not significant tendency for weighted females to accumulate less lipid than control females over the 31 days of the experiment. Experiment 4: maximum mobility Methods

1 randomly assigned 39 female A. remigis to three treatments as in experiment 2. Males and leads weighed 0.033 ± 0.003 g and 0.033 ± 0.002 g, respectively. For each trial, I selected one female from each treatment matched with respect to recovery time (minimum of 1 h) and time since feeding. Trials were conducted in still water in a short section of artificial stream (50 x 20 cm). The test arena was marked off with string suspended above the water surface so that the females could freely exit and enter along the water surface. A trial began when one female, selected at random from the triad, was placed on the water surface several centimeters outside the test arena. The observer then chased the female through the arena by waving his or her fingers in the air just behind the female, encouraging flight at maximum speed. This chase was repeated until the test animal had made a minimum of three complete transits of the long axis of the test arena, providing a record of >1.5mof rapid skating. The trial ended when all three test females had been run. I recorded each trial on videotape for analysis of speed, stride length, and stride rate. Path lengths, times, and number of strides (completed rowing strokes of the midlegs)

Fairbairn • Cost of mate-carrying in Aquarius remigis

227

11

x:

10

ide Leng

I

8

35

6

15.8 mm). I placed each frog in a plastic cage (48 x 38 x 20 cm) containing water to a depth of 10 cm and a large, emergent rock. The frogs were given 24 h to acclimate, during which time they were not fed. A trial began when one female A. remigis from each treatment was introduced into the cage. I observed each cage continuously for the first hour and then once every 10-15 min between 0830 and 1800 h. Trials were run for 48 h or until two of the females had been captured. In 21 trials, I observed first captures 19 times and second captures 13 times. On several occasions, the frogs consumed two females overnight, and order of capture could not be determined. In several other trials, the frogs consumed only one female, and so second captures could not be recorded.

9 7

cm-si

50 40

Q. 20 -

Figure 2 Mean (± SD) stride length, speed, and stride rate for control, weighted, and tandem female A. remigis in continuous, rapid skating.

£CO rr

6 5 4 3

\

2

'&

CO

1

Control

Weighted Tandem

were noted for two complete transits of the arena for each animal. Results

The data were analyzed as randomized blocks using parametric ANOVA followed by SNK pairwise comparisons. Speed (cm/s) was transformed to natural logs before analysis to correct for heteroscedasticity. Stride length during continuous, rapid skating differed significantly among treatments (F = 4.571, df = 2,24, p = .021), as did average speed (F = 7.727, df = 2,24,p = .003; Figure 2). In both cases, the control females scored higher than the weighted and tandem females (p < .05 for stride length; p < .005 for speed), but the latter two treatments did not differ significantly (p > .50). Stride rate did not differ significantly among treatments (F = 1.239, df = 2,24, p = .194; Figure 2), indicating that the di.-Terence in speed was primarily due to the greater length of each stride taken by the control females rather than an increased number of strides. On average, stride length was reduced by 1296 for weighted females and 15% for females in tandem, and overall speed was reduced by 7.5% in both groups. Experiment 5: predation risk Methods

I assigned 63 females at random to three treatments as in experiments 2 and 4. I captured six green frogs (Rana clamilans) from two streams within 35 km of Montreal. The frogs were captured in A. remigis habitat and are known to prey upon adult A. remigis in the wild (Fairbairn DJ, personal observation). The test frogs ranged in size (snoutvent length) from 38 to 80 mm (x = 49.7, SD =

228


An overall contingency test indicated highly significant differences among treatments in order of capture by the frogs (G = 23.21, df = 4, p < .001). The females in tandem tended to be taken first and the weighted females second, while control females dominated the not-captured group (Figure 3). The overall risk of predation in the experiment was similar for tandem and weighted females (65% and 67%, respectively) but was significantly lower (35%) for control females (G = 5.444, df = \,p = .02). Tandem females were 4.3 times more likely to be eaten first than weighted or control females. Once the tandems had been eaten, however, weighted females were seven times more likely to be eaten than controls. Thus, loading associated with matecarrying has a significant effect on vulnerability to predation in female A. remigis. DISCUSSION Female A. remigis appear to be well adapted to sustain the load associated with prolonged matecarrying. On still water, they are able to support up to 223% of their body weight, equivalent to 2.7 times the mean male weight, for a period equivalent to the average duration of mating, including copulatory guarding. The times to sinking observed in the laboratory may not accurately reflect load limits in the wild, where surface currents vary and resting sites are available. However, the magnitudes of the weights carried by females in the laboratory experiment do indicate that females are unlikely to be near their load limits when carrying single males, even under natural conditions. Previous laboratory and field studies indicated that female A. remigis forage successfully while carrying males (Clark, 1988; Rubenstein, 1984; Sih et al., 1990; Wilcox, 1984). My results confirm this; females carrying weights or males showed no reduction in short-term foraging success. Long-term foraging success also did not differ significantly between weighted and control females, although the former may have accumulated slightly less lipid. Carrying mates did reduce the maximum mobility of female A. remigis. Comparisons of tandem and weighted A. remigis indicate that this reduction is a function of the weight carried, rather than the bulk or volume of the male, and thus represents a true cost of loading. Amqvist (1989a) observed a similar reduction in mobility of mating female Gerris odoniogasler. It is perhaps surprising that the


as)

b

Results

Fairbairn * Cost of mate-carrying in Aquarius remigis

FigureS Proportions of control (solid bars), weighted (hatched bars), and tandem (open bars) females captured by green frogs (Rana clamitans).

Separate histograms are shown for first and second captures and for females not captured.


reduction in mobility did not lead to significant reductions in foraging success in either experimental assay. In the long-term foraging experiment, subtle differences among treatments may have been obscured by benign foraging conditions: lipid levels at the end of the experiment were very high, indicating a superabundance of food (Lee et al., 1975). The short-term assay was designed to mimic foraging conditions in a relatively fast current where prey capture would be expected to be more difficult, but again no difference was found among First capture Second capture Not captured treatments. Aquarius remigis are essentially sit-andwait predators, holding position in the stream and capturing passing prey in the surface drift (Blanckencounter predators (Gwynne, 1989; Walker, 1980). enhorn, 1991; Jamieson and Scudder, 1979; RuHowever, physical pairing is more commonly prebenstein, 1984; Wilcox, 1984). Natural prey tend sumed to increase risk of predadon because mating to be small (