Introduction

In eusocial Hymenoptera, foraging strategies generally balance the energy expenditure of individual foragers with net energetic gains to the colony (Seeley et al. 1991). For example, honeybee (Apis mellifera) foragers trade off food value of nectar with the caloric costs of food transport, colony recruitment, and competition (Dyer 2002). The value of carbohydrates in influencing nectar foraging behavior and communication is well known (Pankiw and Page 2000; Kay 2002; Nieh et al. 2003; Scheiner et al. 2004; Nieh and Sanchez 2005). These studies discuss sophisticated behavioral mechanisms that play key roles in colony food gathering. By comparison, the role of protein quality in hymenopteran foraging has received less attention, although protein is essential to colony growth. The few existing studies on this topic all address pollen collection in bees (Pernal and Currie 2001; Pankiw and Rubink 2002; Bertram et al. 2003).

During resource collection, foraging insects incur a metabolic cost in part because flight muscles must achieve a minimum threshold temperature (Coelho and Ross 1996). Variation in ambient air temperature (T a) can greatly affect the energy expended by foragers to reach this flight threshold. Thus, some wasps and bees alter their metabolic or activity rates to respond to changes in T a (Heinrich 1993). For example, honeybees exhibit a positive correlation between thoracic temperatures (T th) and T a over certain ranges of T a (Schmaranzer and Stabentheiner 1988; Woods et al. 2005). The ability to maintain a positive relationship between body temperature and T a is a form of thermoregulation (Coelho and Ross 1996).

Foragers can minimize thermoregulatory costs incurred while foraging by varying T th in response to the carbohydrate content of food (Heinrich 1984; Dyer and Seeley 1987; Schmaranzer and Stabentheiner 1988; Waddington 1990; Stabentheiner and Hagmüller 1991; Underwood 1991; Kovac and Stabentheiner 1999; Stabentheiner 2001; Nieh and Sanchez 2005; Nieh et al. 2006). Honeybee foragers thermoregulate in response to sugar concentration, maintaining a higher T th in response to more concentrated sucrose solutions compared to less concentrated solutions over the same range of T a (Stabentheiner 2001). Using a similar approach, Kovac and Stabentheiner (1999) demonstrated that Vespula vulgaris foragers adjust T th in response to sucrose concentration. The ability to maintain T th above T a may reduce the time needed to attain the minimum threshold temperature for flight even at low environmental temperatures (Heinrich 1984; Coelho and Ross 1996). In this way, thermoregulation may facilitate resource exploitation (Kovac and Stabentheiner 1999). In Vespine wasps, T th closely corresponds with activity level; T th decreases during inactivity and increases during high activity (Kovac and Stabentheiner 1999). Although such thermoregulation depends on the behavioural context of the forager (e.g., leaving the nest, entering the nest, or interacting with nestmates; Coelho and Ross 1996), no study to date has examined whether thermoregulatory behaviors also change with the type of resource (such as carbohydrates or proteins).

Protein foraging has been examined primarily with respect to pollen foraging by honeybees. Pernal and Currie (2001) demonstrated that A. mellifera colonies respond to deficiencies in their pollen reserves by increasing the proportion of foragers collecting pollen rather than by accumulating pollen of higher quality. In this paper, we define protein quality or concentration as the relative proportion of chicken bait that consists of protein. Individual honeybee foragers can evidently assess protein resource quality (Waddington et al. 1998). However, forager sensitivity to pollen quality may not be highly refined because pollen is scraped off the abdomen and packed directly into the corbiculae without first being sampled (Hodges 1984; Pernal and Currie 2001).

In contrast to honeybees, wasp foragers macerate prey, providing them with an extended opportunity to assess protein quality. To examine the role of thermoregulation in protein foraging by predatory wasps, we measured the thermal responses of yellowjacket foragers to varying qualities of protein resources. Because most studies of social wasp foraging address prey preferences, our study is novel in its focus on the behavioral responses of foragers to protein quality. Our results indicate that foraging yellowjackets elevate T th in response to bait concentration. We suggest that yellowjackets, and perhaps other social wasps, may increase their foraging efficiency by modulating T th relative to protein quality.

Materials and methods

Study site and colonies

We performed all experiments in the Hilina Pali region of Volcanoes National Park (HAVO) on the Big Island of Hawaii (19°20.6 ′N, 155°16.4′ W, elevation 994 m). HAVO supports locally abundant, non-native populations of V. pensylvanica as well as diverse assemblages of native arthropod prey such as Araneae, Coleoptera, Diptera, and Lepidoptera (Gambino 1992). At these sites, V. pensylvanica also actively scavenges vertebrate carrion. We collected data in September 2005 on 134 foragers from four different colonies (N = 31, N = 33, N = 29, N = 41 wasps, respectively) located in an area of approximately 0.18 km2. We tested each colony once per day. Therefore, trials of different protein qualities occurred on separate days but at the same time of day (1000 or 1400) to control for diurnal patterns of activity. Trials were conducted in diffuse sunlight.

Bait composition and experimental setup

We measured T th of V. pensylvanica foragers collecting baits with either higher protein content (100% chicken by volume) or lower protein content (50% chicken by volume). Invertebrate species preyed upon by wasps generally consist of 40–70% protein by dry weight (Allen 1989). Lean chicken (Hormel® 98% fat-free chicken breast; 0.3 g fat, 2.7 g protein, 0.0 g carbohydrate) was used as the protein source. Canned chicken is currently used in baiting V. pensylvanica at HAVO (Gruner and Foote 2000) and was preferred over ground beef and tuna in our pilot studies. Each 100% chicken bait consisted of 15 cm3 of canned chicken (mean ± SD, 28.6 ± 3.4 g). A concentration of 50% chicken by volume was obtained by mixing 7.5 cm3 of chicken with an equal volume of packed alpha-cellulose powder (Sigma, EC 232-674-9), an inert indigestible compound used to vary the protein content of resources for foraging honeybees (Pernal and Currie 2001) and caterpillars (Lee et al. 2004). Alpha cellulose has a low density (bulk density, 110–145 g/l), and thus, we diluted the protein content of the bait by volume instead of by mass. The difference in the consistency of the two bait types is unlikely to have affected foraging responses because grinding meat and altering its texture have no effect on Vespula visitation or acceptance of baits (Reid and Macdonald 1986). Although other factors such as weight or odor intensity may also be affected by this dilution process (Waddington et al. 1998), natural prey vary in macronutrient concentration, weight, density, and odor even within the same prey species. We therefore expect foragers to have the ability to assess protein quality in addition to other physical characteristics of their food.

Individual wasps foraged at protein bait stations, each of which consisted of a single bait on a blue circular plastic dish (14-cm diameter) placed on an ice pack (12.1 × 19.1 × 1.9 cm). The ice pack ensured that bait temperature remained relatively constant throughout the trial and provided a sufficiently low background temperature against which to measure the T th wasps. The mean temperature of the bait achieved by this process was 21.2 ± 1.8°C, while T a ranged from 21.1°C to 29.0°C. Because of air flow and wasp movements during foraging, the ice pack had a negligible effect on wasp body temperature.

We measured surface T th to the nearest 0.1°C with an infrared thermometer (Radio Shack, cat #220-0325, spot size of 5 mm). Temperature measurements were accurate to 2.0°C, but they were all precise and repeatable to within 0.2°C as compared with a reference mercury thermometer. Thermal measurements were taken with the infrared thermometer placed within 5 mm of the thorax. Ambient temperature data were measured to the nearest 0.1°C with a digital temperature probe (Pyrex model 7738). Processing T th were measured while foragers processed the chicken into a ball. To collect control T th (non-processing temperatures), we placed a domed wire screen (11.1-cm diameter × 3.7-cm height; 0.1-cm mesh spacing) over the bait to prevent wasps from contacting chicken. Trials lasted between 45 and 60 min depending on wasp activity. Trials ended after data were collected for at least 15 individual foragers per colony per day.

Thoracic temperature measurements

Before each trial, we stimulated colony foraging activity by placing 30 cm3 of 100% chicken on the ground 20 m away from a nest entrance. After 15–20 min, we removed this initial bait to ensure that it did not affect the body temperatures of foragers during the trial. We waited 20–30 min after removal of the initial bait before placing experimental bait stations at the same locations as the initial baits.

We measured ambient temperature and chicken temperature at approximately 15-min intervals throughout the trial. Each wasp was uniquely marked on her abdomen with acrylic paint (Duncan Craft, RM# 0010788) for individual identification (Gambino 1990). In each trial, temperatures were taken from 15–20 different foragers that were provided with either 100% or 50% chicken by volume.

Pilot studies showed that wasps were unable to imbibe detectable quantities of liquids from baits during short time periods (10 s). We observed that 93.2% of foragers (68 of 73) produced no regurgitate after processing 100% chicken bait for 10 s; this observation indicates that 10 s is too short of a time interval for liquids from the bait to be consumed and metabolized. Thus, in the current study, we measured body temperatures within 3 s of initiation of bait processing to ensure that any observed changes in temperature were not caused by metabolic activity. Repeat temperature measurements (mean ± SD, 2.82 ± 1.84) of individual wasps were taken on successive visits to the bait. We used the mean T th per individual in all statistical analyses.

Statistical analyses

Only individual wasps for which we had both control and processing temperature data were included in the analyses (N = 134 wasps). We report mean ± SD. We performed three analyses of covariance (ANCOVAs). In the first ANCOVA, we assessed whether colony (2, 3, 4, and 5), protein quality (100% and 50% chicken), and T a (covariate) affected T th. In the remaining ANCOVAs, we analyzed how these same independent variables affected the following dependent variables: (1) non-processing T th (control temperatures) and (2) the mean differences between the control and processing temperatures of the thorax. JMP In v. 5.1 (SAS Institute) statistical software was used for all analyses.

Results

Foraging wasps visiting the baits for the first time circled bait stations aerially before landing. Yellowjackets orient towards odor plumes emanating from food resources (Hendrichs et al. 1994), and we observed that low flying foragers on their first approach flew in a zigzag pattern into the wind, apparently following the scent plume created by the bait. After initial location and exploitation of the bait, all returning foragers flew directly to and from the bait station. As more wasps visited the bait, foragers had a greater chance of interacting. Although aggressive behaviors such as buzzing, pouncing, and grappling may increase body temperatures, we observed minimal aggression among foragers irrespective of bait type (100% or 50% chicken), colony, and T a.

Our results indicate that V. pensylvanica thermoregulate in response to processing protein. In the first ANCOVA, there were significant effects of protein quality and colony, but no effect of T a (Table 1). Wasps collecting 100% chicken exhibited higher mean T th (31.2 ± 1.4°C) compared to wasps collecting 50% chicken (30.0 ± 1.1°C; Fig. 1). Although colonies differed in the extent to which foragers adjusted T th to bait type, foragers from all nests consistently exhibited higher T th when collecting from a 100% chicken bait as opposed to a 50% chicken bait (colony 2: N = 31, F 1,29 = 36.11, p < 0.0001; colony 3: N = 33, F 1,31 = 20.89, p < 0.0001; colony 4: N = 29, F 1,27 = 10.88, p = 0.0027; colony 5: N = 41, F 1,39 = 5.18, p = 0.028).

Fig. 1
figure 1

Change in thoracic temperatures (T th) with protein concentration as a function of ambient temperature (T a; N = 31, 33, 29, 41 wasps from four colonies, respectively). Although T th increase with T a, T th are significantly greater when wasps forage on 100% chicken compared to when they forage on 50% chicken across all T a sampled (Table 1)

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Table 1 Results of the three ANCOVAs that examine the effect of ambient temperature (T a), colony and protein quality on thoracic temperatures (T th)
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Before encountering the resource, T th of foragers in 100% chicken trials did not differ from those of workers in the 50% chicken trials. The second ANCOVA demonstrated no statistical effect of protein quality on control T th (Table 1). However, we observed significant effects of colony and the interaction between colony and protein quality that were driven by one trial of one colony that exhibited low control T th.

Thoracic temperature increases were influenced both by forager behavior (processing or control) and by bait type (100% or 50% chicken). When foragers processed meat, their T th increased relative to control temperatures at both protein qualities (Fig. 2). The third ANCOVA showed no effect of colony or T a on the mean differences between processing and control T th (Table 1). Compared to control foragers, wasps foraging on 100% chicken increased their T th by 1.7 ± 0.2°C, while those foraging on 50% chicken increased their T th 1.1 ± 0.2°C (Fig. 2).

Fig. 2
figure 2

The difference between processing and control T th as a function of T a for two different protein concentrations (N = 134 wasps pooled from all colonies). Across all T a, differences in T th are greater when foragers collect 100% chicken compared to when they collect 50% chicken (Table 1)

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Discussion

Our study demonstrates that foraging V. pensylvanica can adjust T th depending on protein concentration despite the fact that chicken baits are resources from which foragers do not derive any immediate caloric benefit. This finding is, to our knowledge, the first demonstration that foraging Hymenoptera modulate body temperature in response to protein quality. Thoracic temperatures of wasps processing 100% chicken were significantly greater compared to those of wasps processing 50% chicken (Fig. 1). Agonistic interactions among wasps at the bait likely had a negligible effect on T th because aggressive behaviors were not commonly observed at baits. Therefore, we suggest that the observed increase in T th is an active behavioral response to protein quality that may assist in foraging efficiency and contribute to colony fitness.

Most studies on thermoregulation in social insects examine foraging at carbohydrate rewards, which can benefit not only workers and brood at the nest but also the forager directly (Heinrich 1984; Schmaranzer and Stabentheiner 1988; Stabentheiner and Hagmüller 1991; Kovac and Stabentheiner 1999; Stabentheiner 2001; Nieh and Sanchez 2005). Our study demonstrates that individual wasps collecting protein, a resource from which foragers do not derive any immediate caloric benefit, still adjust the energetics of foraging. Metabolic costs incurred during thermoregulation may be offset by increased colony fitness or by compensation at the nest. The absence of any immediate energetic benefit from the collected protein also suggests that increases in T th do not correspond to higher hemolymph sugar titers in foragers.

Protein can be eventually converted into metabolic energy (Hunt et al. 2003), and wasps can imbibe some liquid from killed prey (Michener and Michener 1951). However, we observed that foragers primarily tore at the bait with their mandibles and shaped it into a ball rather than directly feeding on it while manipulating the chicken meat. Based on our preliminary studies, wasps did not consume measurable amounts of liquid from chicken bait before our measurement of their T th. Therefore, it seems unlikely that the observed T th changes were due to metabolizing the bait they collected. Although no immediate carbohydrate reward was provided at the meat bait, it is possible that such payment was received upon returning to the nest. In response to being fed protein, vespid larvae produce a nutrient-rich saliva that is consumed by adults; the nutritive value of this secretion is comparable to that of floral nectar, which is the primary source of nutrition for adult wasps (Hunt et al. 1982). Further investigation may reveal whether larvae adjust the volume or concentration of carbohydrate rewards according to the quality of the protein that they receive.

Although protein resources provide no immediate metabolic compensation for the energetic output necessary to raise T th, wasps may adjust their foraging efficiency (i.e., reduce the amount of time required for flight muscle warm-up) to protein quality. Previous studies of both Vespidae and Apidae suggest that elevated T th enhance the power output of flight muscles and allow for faster foraging and a higher rate of resource exploitation (Esch 1976; Schmaranzer and Stabentheiner 1988; Coelho 1991; Stabentheiner and Hagmüller 1991; Kovac and Stabentheiner 1999). If workers expend energy in proportion to resource quality (Kovac and Stabentheiner 1999), then a greater increase in T th may be expected in response to higher protein concentration. Similarly, lower T th may help the forager conserve energy while collecting a relatively poor resource. Such flexibility could allow a colony to adjust energy consumption during foraging to the quality of the available protein. This hypothesis is further supported by the result that T th in both meat concentrations increased relative to control T th. Wasps raise their body temperatures when processing any type of protein compared to temperatures during unsuccessful foraging (control temperatures), but the magnitude of this increase in T th is dependent on resource quality.

Vespula wasps also thermoregulate in response to carbohydrate quality. Kovac and Stabentheiner (1999) reported that when sucrose concentrations were doubled, Tth of V. vulgaris foragers increased between 1.4°C and 1.7°C. In our study, V. pensylvanica experienced a comparable 1.2°C increase in Tth when protein content was doubled. Although individual foragers increase Tth with a twofold increase in resource quality, the degree of thermoregulation (as measured by thoracic temperature excess = Tth − Ta (Herrera 1997) may be specific for species or resource. At high Ta (~28°C), V. vulgaris exhibited a thoracic temperature excess of 6–9°C, whereas at low Ta (~19°C), thoracic temperature excess was ~16°C (Kovac and Stabentheiner 1999). In our study, we observed a thoracic temperature excess of 5–6°C in V. pensylvanica at moderate Ta (~24°C); this value falls below the range reported for V. vulgaris and may indicate that wasps may invest less energy in thermoregulation when foraging for protein than when foraging for carbohydrates. Because the present study and that of Kovac and Stabentheiner (1999) occurred at similar times of the year, the disparities observed in thoracic temperature excess are unlikely to be the result of differences in colony cycle. However, the roles of food type, colony need, or interspecific differences in influencing how Tth are adjusted remain to be tested. Although we detected a colony effect on Tth that was independent of Ta, estimated colony size (Malham et al. 1991), and nest location, it is important to note that foragers from all colonies increased Tth in response to increased protein concentration. Our study measured temperatures of foragers only at the bait and not in the nest. To fully contextualize the thermal behavior observed at the resource, future studies should also examine the motivation and behavior of protein foragers once they have returned to the colony.