Introduction

The nutritional composition of foods rarely matches the nutritional requirements of an organism. Moreover, nutrients in plants that are important for animal growth, such as protein, vary both temporally and spatially, as do characteristics such as digestibility and secondary metabolites [1,2]. The ratio of protein to digestible carbohydrate in plants is thus likely to determine feeding patterns in herbivores, influencing foraging rates and strategies and other aspects of performance [3]. Generalist herbivores tend to be both mobile and selective [4], moving widely to encounter suitable foods and avoid nutrient limitation. They frequently switch between foods, but are likely to eat large amounts of imbalanced foods [5,6]. Nonetheless, many organisms have a demonstrated ability to select combinations of complementary foods that individually are nutritionally imbalanced, but together allow an animal to reach its multidimensional intake target [2,7,8].

Natural selection should favour individuals that can regulate nutrient intake using resource selection strategies such as carnivory or predation (including occasional cannibalism), or differential absorption and utilisation of nutrients 1], [6], [because failure to balance nutrient intake can result in large fitness costs [4,9,10]. This might occur, for example, if intake of a key limiting nutrient is prioritized, and results in a situation where other nutrients are over-consumed. The extent to which an individual overeats one nutrient while under-eating another thus represents a compromise [11]. Experiments which limit key nutrients during short periods of pre-feeding have shown that individuals in a range of species are capable of generating nutrient specific compensatory responses (e.g. cockroaches [12], beetles [13], and spiders [14]).

It has been argued, however, that nutritionally explicit models should consider functional consequences and mechanistic constraints of nutritional decisions [3,15]. For example, where males and females differ in their reproductive strategies, such as the timing and costs of reproductive effort [16,17,18,19], sexually dimorphic optima in nutrient requirements and the allocation of resources might also occur. Nutrient requirements could also differ between the sexes because of different costs associated with somatic maintenance, intra-specific competition, or immune function [20,21,22,23]. Here, we consider nutrient requirements associated with one particular organismal trait, sexually dimorphic weaponry. If the sex-specific resource optima hypothesis is correct, then males and females might demonstrate different nutrient intake targets in late instar stages when sexual weaponry is developing. Alternatively, the sexes could differ in the type and amount of nutrients that are most tightly regulated or prioritised during food selection and hence differ in patterns of compensatory nutrient intake. Characterization of sex-specific nutritional optima thus has clear utility for probing behavioural and physiological mechanisms governing how each sex forages.

We investigated nutrient intake target and consumption in a sexually dimorphic cricket the Wellington tree weta (Hemideina crassidens), using the geometric framework for nutrition [24,25]. Tree weta (Hemideina spp.) are large bodied Orthopterans that are ecologically important in New Zealand forests, and preferred prey for both native birds and invasive mammals such as rats [26,27,28]. These cryptic foragers are nocturnal and have an adult weight range of approximately 3–6 g). However, whereas females mature at the 10^th instar only, males can mature asynchronously at either 8^th, 9^th or 10^th instar [29,30]. Instar at maturity reflects male investment in heavy, mandibular weaponry [30], such that weight is a useful proxy for male investment in these sexually selected weapons. Adult weta can survive for at least a year [31] and MMR and PMW (personal observation).

Most Anostostomatidae crickets tend towards carnivory, but the Wellington tree weta Hemideina crassidens is likely a generalist herbivore, similar to other species in this genus [32,33]. Nonetheless, informal reports also indicate occasional carnivory as a foraging strategy [32,34]. In H. crassidens, large headed males with elongated mandibles engage in combat to defend cavities with female harems in a polygynandrous mating system [35,36]. The heads of the two sexes are completely indistinguishable prior to the 5^th instar, and most dimorphic mandibular development occurs in late instar males immediately prior to sexual maturity, consistent with other Orthopterans [37,38]. It is thus possible that late sub-adult and adult males could have specific nutrient needs based either on the cost of developing or maintaining large mandibles, or costs associated with male-male combat.

We asked whether Hemideina crassidens individuals regulate their nutritional intake to meet the demand for specific nutrient combinations [39]. Using artificial foods, we investigated nutrient intake in these individuals to determine their nutritional targets. We predicted that males would show increased protein demand relative to females if the development and maintenance of male weaponry has high nutrient costs. We then conducted trials to determine whether a period of enforced imbalanced nutrient intake would result in subsequent compensatory feeding, using both artificial foods and foods available to them in their natural environment, over two time scales. We compare our results with data for other insects, to examine the possibility that feeding differences might be related to key differences in life history, and discuss the implications of the data for the understanding of weta foraging patterns and their ecological niche in New Zealand forests [40].

Methods

Results

Nutrient intake trial

Twenty seven Hemideina crassidens individuals completed the feeding trial (n = 17 females and n = 10 males). The ratio of carbohydrate:protein in the selected diet was significantly greater than 1 (Wilcoxon One-sample Signed-rank Test, p=0.009; Figure 1). There was no evidence, however, that this differed between the sexes (Independent-samples Mann-Whitney U test, p=0.12; Figure 2). The data thus indicate non-random selection of the test foods by both male and female weta, to a protein:carbohydrate intake target of 0.84.

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Figure 1. Mean amount of artificial food eaten by Hemideina crassidens individuals over a seven night experimental trial period (x ± SE).

Weta selected foods non-randomly so that carbohydrate consumption was greater than protein on average. Dotted line indicates a 1:1 ratio.

https://doi.org/10.1371/journal.pone.0084641.g001

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Figure 2. Carbohydrate and protein intake in individual male and female Hemideina crassidens, shown by sex, over a seven night experimental trial period after weta were captured from the wild.

https://doi.org/10.1371/journal.pone.0084641.g002

Influence of Nutritional State on Subsequent Diet Choice

In the first phase of this experiment (eight days), tree weta in the control group (i.e. the CP group, which was able to self-select the macronutrient composition of their diet) selected a protein:carbohydrate ratio that was neither significantly different from the target ratio established in the initial experiment with wild weta, nor from the 1:1 ratio expected if weta were eating equal amounts of the two foods (One-sample Wilcoxon Signed Rank Test, p = 0.445 and p = 0.647, respectively; Figure 3).

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Figure 3. Feeding in the three groups in the eight-day phase 1 of the experiment.

Middle line indicates the model if weta were eating the same amount of P-rich and C-rich diet in the mixing treatment (CP). CC indicates the group offered carbohydrate rich food only, CP the group offered both carbohydrate rich and protein rich food, and PP the group offered protein rich food only.

https://doi.org/10.1371/journal.pone.0084641.g003

In the second phase of the experiment, the weta that were maintained on the CP treatment selected a diet with a carbohydrate:protein ratio that was significantly greater than 1 (One-sample Wilcoxon Signed-rank Test, p = 0.043, Figure 4). There was no significant effect of diet in phase 1 on the macronutrient ratio selected in phase 2, either in a comparison of the groups that were confined to the PP and CC treatments in phase 1 (Independent Samples Median Test, p > 0.99) or in a comparison of all three pre-treatment groups (p = 0.855). This indicates that the weta did not compensate for a prior period of confinement to a diet of either excessive protein or excessive carbohydrate. The self-selected diet of all three pre-treatment groups combined was significantly higher in carbohydrate than protein in phase 2 (median protein:carbohydrate ratio = 0.76, p = 0.004, One-sample Wilcoxon Signed Rank Test).

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Figure 4. Proportions of carbohydrate and protein ingested by weta during the 48 hour period in phase two.

Data are presented according to the previous treatment group of weta, who had been fed for eight days on specific foods in phase 1. CC= carbohydrate rich food only, CP = carbohydrate and protein rich food, PP = protein rich food only.

https://doi.org/10.1371/journal.pone.0084641.g004

All tree weta fed leaves and dead conspecifics (rather than the artificial foods) for 48 hours in phase two ate leaf material with the exception of one individual previously in the carbohydrate rich group in phase one. There was no relationship between the amount of flesh eaten (estimated using the weight change of the dead conspecific weta) and previous diet (Independent Samples Median Test, p > 0.93; Figure 5a). Individual weta in the carbohydrate rich group appeared to compensate by eating greater amounts of flesh than leaf material compared to weta in the protein rich group (Figure 6), but a number of weta in the trial ate very little. Feeding in the control group appeared more similar to the carbohydrate rich group than to the protein rich group. Individuals previously on the protein rich diet ate more leaf material on average, but again there was considerable individual variation in the amount of leaf material eaten within each group (Figure 5b). Differences between groups in the amount of leaf material eaten were not significant (Independent Samples Median Test, p > 0.67).

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Figure 5. Estimates for the amount of (a) dead conspecific cannibalised and (b) leaf material (dry weight, mg) eaten by tree weta during phase two of the nutritional state trial.

Weta had previously been assigned to one of three diets for 8 days. Group CC had carbohydrate rich food only (n=6), CP had a balanced diet (protein and carbohydrate rich foods both available, n=5) and PP, protein rich food only (n=6). Means ± SE presented.

https://doi.org/10.1371/journal.pone.0084641.g005

Discussion

Many species of animals have been demonstrated to selectively forage based on the macronutrient content of available foods [25]. Resource acquisition can thus be understood by comparing the balance of nutrients available in food to that required by an organism’s phenotype [39,44,45]. Protein and carbohydrate are among the nutrient groups most strongly regulated by herbivorous insects, and are thus expected to play a dominant role in ingestive behaviour [46,47]. We identified a nutrient target for the Wellington tree weta, Hemideina crassidens, where individuals select more carbohydrate relative to protein. However, we did not detect a difference between male and female nutrient optima despite strong sexual dimorphism in adults. Although we detected a positive relationship between weta weight and quantity of food consumed, their nutrient target did not change with weight (and, by inference, instar and investment in mandibular weaponry).

Male tree weta guard tree cavities inhabited by harems of females during the mating season, and expend energy fighting other males for access to these females, using their mandibular weaponry [48]. Male combat can result in significant injury to combatants; male reproductive strategies in highly polygynous species often appear to involve somatic damage and increased viability costs [22,49]. We had therefore hypothesised that substantial dietary costs could be associated with either performance or somatic maintenance in mature males, or that late instar males might require higher protein diets than females during the period when their mandibles develop. However, our data did not show any difference between the sexes in nutrient ratios. The lack of sex-specific differences in nutrient intake probably reflects an equal net reproductive cost for the two sexes, as observed in other species [49].

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Figure 6. Feeding in the three groups in phase two of the long-term cannibalism experiment.

Group CC had previously eaten carbohydrate rich food only for six weeks during phase one of the experiment (n=6), CP had a balanced diet (protein and carbohydrate rich foods both available, n=5) and PP, protein rich food only (n=6).

https://doi.org/10.1371/journal.pone.0084641.g006

It has been argued that natural selection should favour herbivores that regulate nutrient intake, using pre and post ingestive mechanisms [6]. In the first experiment, after a three day period of starvation weta ate towards a nutrient target that favoured carbohydrate ingestion. In the first phase of the second experiment the group of weta given a combination of complementary foods selected an intake that was not significantly different from the carbohydrate-biased target selected in experiment 1, but was neither different from the 1:1 expected under random feeding. However, in the second phase of that experiment the intake point selected by the same treatment group of weta was significantly higher in carbohydrate than protein, again demonstrating non-random feeding. Together these results show that Hemideina crassidens do select their food to reach a carbohydrate:protein intake target that is significantly, if marginally, greater than 1:1.

The importance of nutrient balance for fitness would lead us to expect compensatory intake responses after a period of restriction to an imbalanced diet. However, Hemideina crassidens did not show a tightly regulated protein response in its patterns of food selection, in contrast to more tightly regulated responses in other insects that have been studied, e.g. [12,50]. As noted above, those that had previously been on a balanced diet ate towards a nutrient target that was weighted towards carbohydrate, supporting the results of the initial experiment. Most individuals were slow to compensate for imbalance through selective nutrient intake. First, tree weta on the protein rich artificial diet did not increase their subsequent (phase two) feeding rate on the carbohydrate rich food available to them. Second, tree weta that had previously been maintained on a carbohydrate rich diet continued to eat a greater amount of carbohydrate rich food than expected in the second phase of the experiment. Third, there was large individual variation in the amount of flesh scavenged by weta in phase two of the eight day intake experiment and no significant differences identified. However, when tested in the long-term trial, a significantly higher number of weta from the protein deprived group cannibalised their dead conspecifics compared to the protein rich group.

In the wild, individual weta do not forage nightly, and instead favour intermittent feeding based on weather and other environmental conditions (P. Wehi and M. Morgan-Richards, unpub. data). The variability in carbohydrate intake over the seven day period of the initial trial is consistent with a general tolerance to nutrient imbalance. This strategy contrasts with that seen in lab experiments using synthetic foods where tight, target-like, homeostatic regulation of the balance and amounts of nutrients eaten appears to predominate e.g. [12,25]. Nonetheless, non-nutrient components of food, such as indigestible bulk, can also influence food selection [1,47] and nutrient utilisation [51]. It is also possible that if individuals rely on endosymbionts to aid digestion, as has been hypothesised for the Auckland tree weta Hemideina thoracica [32], the presence and activity of these endosymbionts could influence both food selection and consumption rate.

Our results contrast with an extensive literature demonstrating strong short-term compensatory intake responses in a wide range of species [12,13,52,53]. The long term cannibalism trial provides some evidence that weta respond to protein deficit, but this response was weak compared to other Orthopterans such as Mormon crickets, which strongly cannibalise conspecifics when on migratory marches, and when protein deprived in captivity [54]. However, compensatory responses may occur over longer periods than those currently documented with other species, given the wide individual variation in feeding observed in the current experiments. We therefore infer from our data that weta are able to sustain nutrient imbalance for longer periods than other invertebrate species previously used in captive trials. That is, they might have developed adaptive tolerance to imbalance based on intermittent access to high protein sources, e.g. [55,9,56,57], and use this as a strategy to increase fitness in a fluctuating environment.

Overall, these results contrast with other insects that have shown strong and immediate compensatory feeding responses [12,13]. Nonetheless, one difference between our study and most others is that we tested individuals caught directly from the wild. Populations of laboratory organisms are usually inbred (so exhibit little genotypic variation), and through either explicit or implicit selective breeding can show adaptation to laboratory environments including diets. For example, diamondback moth larvae reared in captivity for 350 generations developed fastest on a diet with the same protein/carbohydrate ratio as their lab diet compared with other ratios, and self-selected a diet with the same ratio as their lab diet [58]. However, this same moth species showed adaptation in only 8 generations when fed a high carbohydrate diet [59]. Wild caught individuals, in contrast, can face large variation in the nutrient composition of their naturally occurring diet. In addition, the composition of the population sample is less likely to demonstrate the genotypic and phenotypic uniformity found in lab cultured insects. It is unlikely, however, that our results can be explained entirely by unaccounted variation among field-caught weta.

Finally, our knowledge of both current and ancestral nutrient environments in the wild remains somewhat limited for tree weta, despite their ecological importance in forest food webs. Future studies that determine heterogeneity in the naturally occurring feeding environment, and diet breadth in the naturally occurring diet of tree weta, will shed further light on the likely strength of selection for nutrient regulation.

Melissa Griffin and Martin Hanson performed preliminary experiments, and Tracy Harris and Melissa Griffin made up artificial diets. William Wehi and Niwa Wehi helped collect and maintain wild weta. Robyn Dewhurst weighed foods, maintained and fed weta in the second nutrient experiment, and measured leaf area. Daniel Laughlin, University of Waikato, kindly provided data on SLA and LDMCs for mahoe. We thank two anonymous referees for constructive comments on an earlier version of this manuscript. Palmerston North City Council supported the research at Turitea Reserve.