Behavioral Ecology and Sociobiology (2017) 71:178 https://doi.org/10.1007/s00265-017-2407-0

ORIGINAL ARTICLE

Inbreeding, but not seed availability, affects dispersal and reproductive success in a seed-inhabiting social beetle Gal Sitkov-Sharon 1,2 & Martin Tremmel 2 & Amos Bouskila 1 & Yael Lubin 3 & Ally R. Harari 2,3 Received: 3 September 2017 / Revised: 8 November 2017 / Accepted: 15 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017

Abstract Societies based on family groups have flexible social structures, where genetic relatedness and ecological or demographic factors exert pronounced influence on the social organization. Individual decisions that play an important role in determining the degree of cooperation and resulting social structure include whether to disperse from the natal patch, to breed within the group, or to help rear the young. We investigated individual decisions regarding dispersal and possible cooperation in offspring rearing using the date stone beetle, Coccotrypes dactyliperda (Scolytidae). In this species, cooperative breeding is suggested by the observation that genetically similar beetles share galleries in a single seed (stone) over several generations. We used beetles from two populations to create offspring of higher and lower relatedness, and provided them with high and low availability of date seeds as food and alternative breeding sites for dispersing individuals. We found that care of the eggs by adult females is obligate for offspring development. Seed availability did not affect individual behaviors, but dispersal of inbred beetles was lower compared to outbred individuals. Although the number of eggs did not differ between breeding treatments, more offspring were found in colonies of inbred females, indicating either a greater level of cooperation or reduced conflict in the inbred beetles. Taken together, these results emphasize the importance of relatedness among individuals in determining the social strategy and the potential reproductive output of a colony. Significance statement The evolution of cooperative group living is often ascribed to two main factors: high relatedness, promoting indirect fitness benefits, and environmental constraints on dispersal. We investigated the influence of relatedness between individuals and food and breeding site availability in the habitat on individual decisions relating to dispersal and reproductive success in the date stone beetle, Coccotrypes dactyliperda. The beetles share galleries in the seed (stone), and several generations may occupy a single seed. We found more independent young and fewer dispersers in inbred colonies than in outbred colonies, but expected food availability (i.e., additional dates in the surroundings) had no influence on dispersal or reproductive success. We suggest that colony social organization in this species is strongly influenced by the level of relatedness among colony members. Keywords Coccotrypes dactyliperda . Dispersal . Inbreeding . Maternal care . Reproductive success . Seed availability Communicated by J. Heinze Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00265-017-2407-0) contains supplementary material, which is available to authorized users. * Martin Tremmel [email protected] 1

Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel

2

Department of Entomology, Agricultural Research Organization, Beit Dagan, Israel

3

Mitrani Department of Desert Ecology, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Beer-Sheva, Israel

Introduction Social groups often consist of family members that share a common domicile in which cooperation can occur in activities such as foraging, offspring care, and defense. Explanations of how kin-based societies evolved and are maintained have focused largely on the levels of relatedness among group members, using Hamilton’s theory (Hamilton 1964) of kin selection (Crozier and Pamilo 1996; Gardner et al. 2011; Bourke 2011; Rubenstein and Abbot 2017) to predict the degree of cooperation within groups. Other studies emphasize the demographic and ecological factors that promote or constrain

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group living and cooperation (Hatchwell and Komdeur 2000; Pen and Weissing 2000). Clearly genetic and ecological influences interact, and the two approaches are complementary (West-Eberhard 1975; Kokko et al. 2001; Costa 2006; Korb and Heinze 2016). Non-eusocial societies based on kin groups may show considerable flexibility in social structure under different conditions, with individual decisions playing an important role in determining the degree of cooperation within groups (Helms Cahan et al. 2002; Rubenstein and Abbot 2017). Within its lifetime, each group member may decide whether to disperse from the group, whether to breed in the group, and whether to help in rearing the young of other group members. How these three major decisions are integrated within the organisms’ life history will influence group composition, dynamics, and social organization. Given this flexibility, the effects of genetic relatedness and ecological or demographic factors on these decisions, and ultimately on the social organization, may be best investigated in societies based on kin groups. Dispersal from the natal site constitutes an important factor in population dynamics and social structure (Kirkendall et al. 1997; Clobert et al. 2001), and when delayed, it enables extended and more complex interactions among family members that share the habitat and breeding place (Lehman and Rousset 2010). Delayed natal dispersal may increase the potential for kin competition on the one hand, but also the value of helping to rear kin on the other (Peer and Taborsky 2007; Hatchwell et al. 2013). Several studies have demonstrated that the decision whether or not to disperse from the natal group is influenced by ecological constraints such as availability of food (Berger-Tal et al. 2016; Ventura et al. 2017), suitable breeding sites (Koenig et al. 1992; Balshine-Earn et al. 1998), and predation pressure (Arnold and Owens 1999; Heg et al. 2004). When dispersal is delayed or avoided altogether, individuals may breed within the group. Breeding can be shared by all or most adults, or be restricted to a few or even only a single individual. The loss of breeding potential may be compensated by inclusive fitness benefits owing to the high relatedness among all group members (Crozier and Pamilo 1996; Kirkendall et al. 1997; Korb and Heinze 2016). Individuals that remain in the natal group may decide to help to rear the young of other individuals in an attempt to gain future breeding options (Emlen and Wrege 1994; Balshine-Earn et al. 1998) or to increase their inclusive fitness (Stacey and Ligon 1991; Koenig et al. 1992; Hatchwell et al. 2004). Helpers often delay dispersal and their own reproduction in order to promote the survival of related offspring and by that increase their own reproductive success (Komdeur 1992; Emlen 1994). The mechanisms by which delayed dispersal or philopatry and helping behavior evolved are likely diverse and influenced by life history and environmental factors as well as phylogeny. We investigated the effects of genetic relatedness, on the one hand, and availability of food, on the other, on decisions

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to disperse out of the group and breed independently in the beetle Coccotrypes dactyliperda Fabricius (Coleoptera: Curculionidae, Scolytinae). We asked also whether remaining in the group increased reproductive success, possibly as a consequence of helping behavior. This highly inbred, haplodiploid beetle establishes colonies in stone seeds of date palm trees (Phoenix dactylifera L., Arecaceae), and despite the limited space imposed by the volume of the seed, up to about 50 adults can be found in a single date palm seed (Bar-Shalom 2002; Gottlieb et al. 2009). Coccotrypes dactyliperda exhibits life history traits that potentially support cooperative breeding. Usually, the seed is penetrated by a single female that lays eggs into a single gallery where the life cycle of the beetle is completed. Females are long lived (~ 90 days) with a generation time of ~ 25 days (Gottlieb et al. 2011). Individuals inhabiting a seed are closely related, as virgin mothers colonizing a seed often mate with their sons, and sib mating among offspring within the seed occurs frequently (Gottlieb et al. 2009). Continuous oviposition results in temporal overlap between offspring produced early and late in the life of the colony, and more than three generations may occupy one seed before it is completely consumed. These traits provide opportunities for extended social interactions among the related colony members (Kirkendall et al. 1997; Costa 2006; Peer and Taborsky 2007; Kirkendall et al. 2015). Cooperative breeding in the form of alloparental brood care of siblings has been found in scolytine ambrosia beetles (Xyleborini), but has not been investigated in seed-breeding Scolytinae (reviewed in Kirkendall et al. 2015). We first investigated whether maternal care is obligatory at the egg stage, namely, if the eggs will hatch in the absence of the mother. Second, we investigated whether inbreeding level and resource availability influence the offspring’s decision to disperse or remain in the colony. We manipulated the level of inbreeding among females in a gallery (colony) and their opportunities for independent breeding away from the colony (i.e., availability of seeds). In the laboratory, we mated females to obtain inbred (sib-mated) and outbred females (females mated with males from a different population). If remaining in the seed results in indirect fitness benefits, we predicted that offspring of sib-mated (inbred) females should delay dispersal compared to those of outbred females. Helping to rear close kin may be more profitable in terms of inclusive fitness and assured fitness returns than dispersing and rearing one’s offspring alone, risking the loss of the entire brood (Gadagkar 1990). We predicted that breeding opportunities, namely, seed availability, should influence the dispersal behavior: when seeds are scarce, offspring should be more inclined to remain in the colony regardless of their level of inbreeding. When more seeds are available, however, offspring may seek independent breeding opportunities in order to reduce potential competition for their own offspring, and this may be expressed more strongly in outbred beetles.

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Finally, inbreeding increases the degree of relatedness within the group in comparison with the population at large, and this could promote increased cooperation and reduced competition. Thus, we expect that sib-mated colonies will produce more independent offspring than colonies with outbred individuals, due either to increased cooperation in care of the offspring or to reduced conflict over breeding among colony members.

Methods Study organism and lab rearing The adult C. dactyliperda beetles bore into date seeds that have fallen to the ground, after the flesh of the fruit has dried or fallen off (Kirkendall et al. 2015). It is an obligate seeddwelling (spermatophagous) species, and as such presents a challenge for behavioral observations, as no artificial diet has yet been developed for this species and information on behavior can be obtained only when destructively opening the seed. Nevertheless, considerable information on life history and breeding biology has been acquired from raising cultures in the laboratory and opening seeds at intervals after initial invasion (Herfs 1950; Gottlieb et al. 2011). Mature seeds of the date palm (P. dactylifera) were collected from beneath date palms in Kibbutz Sa’ad (31° 28′ 13.44″ N, 34° 32′ 5.64″ E), in the western Negev and Kibbutz Sha’ar HaGolan (32° 41′ 11.4″ N, 35° 36′ 11.87″ E), north of the Sea of Galilee, and examined for the presence of C. dactyliperda. Former studies revealed that the two beetle populations differ in their inbreeding status (relatively inbred, Sa’ad: FIS = 0.66, allele diversity = 2.3; relatively outbred, Sha’ar HaGolan: FIS = 0.24, allele diversity = 2.6), and the two populations are ~ 200 km apart such that migration between the two locations is unlikely (Gottlieb et al. 2009). The highly femalebiased sex ratio (100 to 84%, Gottlieb et al. 2009) in both populations further suggests ongoing inbreeding under local mate competition (Hamilton 1967) in both populations. Laboratory stock populations were established with a few hundred individuals from each location, and new individuals were added routinely. For the experiments, beetles from both populations were reared in the laboratory (28 °C, 12:12 L:D) in glass jars (500 cm3, 100 P. dactylifera seeds) for about four generations, promoting within-population inbreeding in each population. Every other week, frass and debris were removed from the jar. Consumed seeds were replaced by intact seeds that were collected from the Arava valley, southern Israel, where the beetles do not occur. Seeds were kept frozen and were soaked in water to soften before use. The life history and development of C. dactyliperda are described in Gottlieb et al. (2011) and Gottlieb et al. (2014). To minimize observer bias, blinded methods were use when data were recorded and analyzed.

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Testing obligate maternal care To verify whether tending the eggs is essential for their survival to hatching, we introduced two mated females from Kibbutz Sa’ad’s colony into a Petri dish in the presence of one seed (n = 120). Beetles usually excavate galleries in one end of the date seed. When two beetles encounter a seed at the same time, each may go to a different end or both may excavate a hole at the same end (personal observation ARH, GSS). Only seeds with two holes at different ends of the seed were included in this experiment (n = 77 seeds). Six days after providing the beetles with a seed, one beetle in each seed was forced out of the gallery using an insect pin, while the other beetle was not interrupted and served as control for the survival of the brood of the female removed from the same seed. We were able to remove the female from her guarding position at the gallery entrance without harming the eggs that are placed deeper in the gallery (Sitkov-Sharon 2010). All seeds were cut open 5 days after the removal of females in the treatment seeds, and we searched for eggs and larvae inside all excavated galleries.

Experimental setup To obtain different levels of inbreeding among offspring, we collected pupae of the highly inbred Sa’ad population maintained in the lab (Gottlieb et al. 2009) and used newly emerged virgin females. Females were placed individually in a Petri dish (diameter 8.5 cm) with a slice of date seed as food and either with a male sibling, to obtain highly related daughters (n = 29), or with a male from Sha’ar HaGolan population, to obtain less related daughters resulting from outbreeding (n = 26). After mating, each female was given an intact seed to establish a new gallery. To obtain high and low seed availability treatments, the populated seeds were further divided into two groups. In the high seed availability treatment (n = 23), we placed each populated seed in a glass jar (500 cm3) with additional 50 intact fresh seeds, which exceeds the potential number of adult offspring (F1) in a developed gallery. In the low seed availability treatment (n = 32), we placed one populated seed in a jar with only one additional seed. The fresh seeds were added in order to provide new breeding sites to potential dispersing adults. We assumed that 50 fresh seeds will provide a strong olfactory cue to beetles inside the natal seed, which can be detected via the entrance hole that opens into the gallery. In both treatments, the additional seeds were added 18 days after providing the mated female with a seed, at a stage when the first clutch of offspring is likely to have reached pupation (Gottlieb et al. 2009). The combined breeding level and seed availability treatments yielded the following four groups: outbred, high seed availability, n = 12; outbred, low seed availability, n = 14; inbred, high seed availability, n = 11; inbred, low seed availability, n = 18.

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Dispersal

Reproductive success

In order to determine the composition of individuals in the natal seeds after dispersal, we opened all seeds (n = 52) 21 days after the natal seeds were occupied by the experimentally mated females. At this stage, mature, first-generation offspring begin to appear in the seed, but have not yet begun to lay eggs (based on Gottlieb et al. 2014). Thus, counts of adults include the founding mother plus her first-generation adult offspring. At the same time, we also opened the newly invaded seeds (i.e., 1 to 2 days after the dispersers had invaded, n = 30). We counted the dispersed beetles and recorded their sex.

The effect of breeding and seed availability treatments on reproductive success was analyzed using non-parametric Kruskal-Wallis tests, followed by post hoc tests (package: agricolae, using fdr). We compared the following variables per seed between the treatments: (i) the number of adult females (i.e., the foundress and her adult female offspring, including dispersers), (ii) the total number of individuals (i.e., the foundress and her offspring including adults, pupae, larvae, and eggs), (iii) the number of immature offspring (i.e., larvae and pupae), and (iv) the number of eggs.

Breeding treatment and reproductive consequences We characterized the gallery composition in the natal seed including adults, pupae, larvae, and eggs. We used the data to calculate the total reproductive output per seed, including the beetles that dispersed. To determine mating status, dispersing females and all adult females (including the foundress and her offspring) in a seed were dissected, the spermatheca of each female was isolated into a drop of distilled water, and the presence of sperm was noted under microscope (magnification of ×40 and ×100).

Statistical analysis Statistical analyses were conducted with the program R 3.0.1 (R Development Core Team 2014). We used Bartlett’s test and Shapiro-Wilk tests to test for variance homogeneity and normal distribution of data, respectively. Data were transformed whenever necessary to meet parametric assumptions. Maternal care To determine whether maternal care is essential for hatching of larvae from the eggs, we compared the number of living offspring between galleries with and without the foundress female with a binomial test. Dispersal from the natal seed To compare the proportion of dispersing female beetles, we used ANOVA (type II) after arcsine square root transformation of the data, with breeding treatment (inbred, outbred) and seed availability (high, low) as independent variables, using the Akaike information criterion (AIC) to find the minimal adequate model. The absolute numbers of dispersers and remainers were analyzed with Kruskal-Wallis tests followed by post hoc tests (package: agricolae, using fdr).

Availability of data and material The datasets generated during and/or analyzed during the current study are not publicly available but are available from the corresponding author on reasonable request.

Results Maternal care Out of the 120 Petri dishes with two beetles provided with a single seed, 77 resulted in beetles penetrating the seeds at two ends. In all 77 cases, removal of the female resulted in hatching failure of the entire clutch. In the galleries from which the females were removed, the eggs were covered by mold and no larvae, dead or alive, were found, while offspring in various immature stages were present in all but two of the galleries from which females were not removed (binomial test, p < 0.001).

Dispersal from the natal seed All dispersing beetles were females. Opening the natal and newly invaded seeds and dissecting the spermathecae revealed that all adult females, dispersers and residents, were mated. The proportion of dispersing females out of the total number of females was influenced significantly by the breeding treatment (ANOVA, F1,53 = 31.93, p < 0.001; for the complete model see Table 1), with an almost fourfold higher proportion of dispersers in the outbreeding than in the inbreeding treatment (outbred 0.43 ± 0.22 [mean ± s.d.]; inbred 0.11 ± 0.19; Table 1 The effect of breeding and seed availability treatments on the arcsine transformed proportion of dispersing females (out of the total number of adult females that dispersed and remained in the natal seed)

Relatedness Seed availability Relatedness × seed availability Residuals

Sum sq.

df

F

p

3.235 0.033 0.082 5.255

1 1 1 51

31.401 0.322 0.796

≪ 0.001 0.573 0.377

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Fig. 1). Seed availability and the interaction between breeding treatment and seed availability did not influence the proportion of dispersing beetles, and they were removed from the minimal adequate model (Table 1). There was a significant difference in the absolute number of dispersing females between the inbred (median, range 1, 0–4) and outbred beetles (0, 0–2) (Kruskal-Wallis, χ2 = 20.55, df = 3, p < 0.001, post hoc, p < 0.05), but seed availability had no effect on the numbers of dispersers (Kruskal-Wallis, post hoc, p > 0.05). The number of non-dispersing females that remained in the natal seeds did not differ among the treatments (Kruskal-Wallis, χ2 = 3.32, df = 3, p = 0.345).

Reproductive success

Proportion of dispersers

Seed availability had no effect on any measures of reproductive success (Fig. 2; for comparison of breeding treatment pooled data, see supplementary table). The number of adult females (remainers and dispersers combined) did not differ between the four treatments (Kruskal-Wallis, χ2 = 4.129, df= 3, p = 0.248; Fig. 2a). Only 38% of the seeds had adult male offspring (range 0–3 males), and the number of males did not differ among treatments (Kruskal-Wallis, χ2 = 3.624, df = 3, p = 0.305). However, the total number of individuals (adults and immature offspring) per seed was greater in the inbred treatment than in the outbred treatment (Kruskal-Wallis, χ2 = 12.450, df = 3, p = 0.006, followed by post hoc comparisons, p < 0.05; Fig. 2b). More immature offspring (excluding eggs) per seed were found in the inbred treatment compared to the outbred treatment (Kruskal-Wallis, χ2 = 26.601, df = 3, p < 0.001, followed by post hoc comparisons, p < 0.05; Fig. 2c), although the number of eggs counted in the seed did not differ between treatments (Kruskal-Wallis, χ2 = 0.522, df = 3, p = 0.914; Fig. 2d).

High seed availability Low seed availability

1.0 a

b

0.8 0.6 0.4 0.2 0

Outbred Inbred Breeding treatment

Fig. 1 The mean proportion of dispersers (± SD) in relation to relatedness and seed availability. We used ANOVA, with the proportion of dispersers as response variable, and relatedness and seed availability as explanatory variables. Least significant difference tests were used as post hoc tests. Different letters indicate significant differences (p < 0.05)

Discussion Obligate maternal care No larvae developed from galleries from which the mother was removed after egg-laying. Similar total clutch failure was observed in other populations when the mother died or was removed from the seed. Observations of cut seeds containing females and eggs showed that mothers frequently touched and rolled eggs with their mouthparts. Eggs that were left without their mother inside the seed developed mold and desiccated. We concluded therefore that eggs require care, in the form of grooming, egg rolling and probably antibiotic salivary secretions (D. Ezra, The Volcani Center, Israel, preliminary results), without which they do not survive. Similar egg-care behaviors by adult beetles were observed in another seed-boring scolytine beetle, the coffee berry borer, Hypothenemus hampei (Vega et al. 2017). In ambrosia beetles (Curculionidae, Xyloborinae), these tasks are performed by larvae and adults (Biedermann and Taborsky 2011). Eggs laid in organic substances often require care to prevent fungal or bacterial growth (Kudo et al. 2011; Boos et al. 2014), and this may be critical where individuals live in close proximity or in high densities. In C. dactyliperda, eggs are laid communally in chambers and may be tended by multiple females. Pathogenic microbes and parasites may spread easily due to frequent interactions among colony members.

Dispersing from the natal seed In our experiments, only female offspring left the seed, although male dispersal is known to occur in this species (Gottlieb et al. 2014). All females, both remainers and dispersers, had mated in the natal gallery. This result, together with the low genetic variation observed in all C. dactyliperda populations (Gottlieb et al. 2009), corroborates previous studies showing that inbreeding is a common breeding system in the species (Holzman et al. 2009). The decision whether to leave or stay in the natal seed was significantly affected by the breeding treatment: A greater proportion of the outbred female offspring left the natal seed than of inbred sisters. Various reasons may cause females to delay or forego dispersal from the natal patch, such as avoiding predation risk (Arnold 1988; Bilde et al. 2007), a lack of alternative breeding sites (Du Plessis 1992), assurance of finding a mate in the natal patch (Komdeur 1992), or gaining fitness benefits through assisting in care of the young (Kirkendall et al. 1997; Peer and Taborsky 2007). Our experiments shed light on some of these potential factors. Predation risk was negligible in the experimental populations, but could influence dispersal in nature (Benton and Bowler 2012). A lack of breeding opportunities is not a likely explanation for the observed differential dispersal. A single date seed may

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High seed availability Low seed availability

b

a

Individuals per seed [n]

n.s.

Females per seed [n]

8 6 4 2

c 30

a

a

b

b

40 30 20 10 0

0 a

a

b

b

d

n.s.

8

25

Eggs per seed [n]

Fig. 2 The influence of relatedness and seed availability on the number per seed of a adult females (i.e., the foundress and her adult female offspring), b total individuals (i.e., the foundress and her offspring including adults, pupae, larvae, and eggs), c immature offspring (i.e., pupae and larvae), and d eggs. Dependent variables were analyzed with Kruskal-Wallis tests followed by Kruskal-Wallis post hoc tests (R package: agricolae). Boxplots show the median and quartiles, maximum and minimum values are depicted as dots. Different letters indicate significant differences (p < 0.05)

Immatures per seed [n]

178

20 15 10 5

6 4 2 0

0 Outbred

Inbred

Breeding treatment

supply resources for three overlapping generations (more than 50 individuals), and can easily support the first generation of offspring. The lack of response to available seeds further indicates that at least for the first brood, lack of food was not a major factor in the decision to disperse or remain. The offspring of outbreeding females should have greater genetic variability overall and lower relatedness among siblings than those in the sib-mating treatment. Thus, we predicted that the more highly inbred, and thus more closely related, female offspring would show reduced dispersal in comparison with less closely related offspring, and that indirect benefits of helping to raise additional broods should be greater when relatedness is higher (Emlen 1994). Furthermore, if inbred individuals have less within-seed conflict over resources such as food and breeding than outbred ones, we would expect them to disperse less. Both helping and lower levels of conflict over resources would require some sort of group or kin recognition. While this has not been investigated directly, Gottlieb et al. (2009) found that females distinguish between males from the same population and from a different population. An alternative explanation for the different levels of dispersal in relation to the breeding experiments could be that the behavior is not facultative, but rather under local selection. Thus, beetles in the source population for females (Sa’ad) are more inbred and therefore may have undergone local selection to be more cooperative and less dispersive than those in the genetically more variable and less inbred population at Sha’ar HaGolan. Environmental differences between the sites

Outbred

Inbred

Breeding treatment

could result in adaptive differences in social behavior (Hendry et al. 2001) including the tendency to disperse or remain.

Breeding treatments and possible indications of cooperative breeding Although the number of adult females per seed (including dispersers) did not differ between relatedness levels, inbred females had higher total number of offspring (including eggs), and more immatures (excluding eggs) (Fig. 2). Thus, the mean reproductive success of females was higher for inbred beetles compared to outbred beetles, and was independent of seed availability. Since there was no difference in the number of eggs between the two breeding treatments, these results indicate that more eggs of the inbred beetles survived to the stages of larvae and pupae. Higher survival rate of eggs may be explained by better care of eggs in the presence of inbred sibs, by reduced egg cannibalism in the more highly related sibs, or by a generally greater viability of eggs from within-population matings compared to between-population matings. No egg cannibalism was ever observed in many hours of observations when opening seeds (GSS, MT, and ARH). Nevertheless, egg cannibalism by females was reported recently in a different scolytine beetle, the coffee berry borer, Hypothenemus hampei (Vega et al. 2017). Intrinsic differences between populations in egg viability or hatching success are less likely. We have not observed consistent differences in egg viability among different populations raised in the lab (ARH and D. Gottlieb), and any initial differences should not be strongly expressed after

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many generations of common laboratory rearing conditions. Another possibility is that lower reproductive success of the outbred offspring was due to outbreeding depression (perhaps caused by bacterial reproductive manipulators found in this species; Zchori-Fein et al. 2006; Peer and Taborsky 2005). This cannot be assessed with these data; however, Gottlieb et al. (2009) found that when given a choice of mates, females do not avoid outbreeding. Whether or not there is greater cooperation in brood care, our results indicate that sharing a resource patch with related females has reproductive benefits. The observations of care of eggs and gallery plugging (ARH) also suggest that gallery maintenance activities and parental care duties are shared by sisters. Parental care is a common phenomenon in the Scolytinae, and is mostly carried out by females. Several species exhibit cooperation in parental care as well as in gallery maintenance duties (Kirkendall et al. 1997; Costa 2006). In some species, males help in the maintenance of the galleries as well (Kirkendall et al. 1997; Costa 2006; Peer and Taborsky 2007), though there is no such indication in C. dactyliperda, where males are scarce in the population. In some cooperatively breeding species, reproduction is skewed and helper females hold back oviposition (Strassmann et al. 2002; Hart and Ratnieks 2005; Smith et al. 2009; Herrera 2013). In C. dactyliperda, the analysis of spermathecae showed that all adult female offspring within the gallery were sib-mated and therefore capable of reproducing. However, we did not dissect the ovaries, and therefore we do not know whether all females realized their reproductive potential in the gallery and laid eggs, or if only one or a few females reproduced inside the gallery. In the present study, we investigated the behavior of beetles in early state of colony development. As more offspring join the gallery and the seed is continuously consumed, the complexity of colony composition increases. This may lead to additional factors that will influence the decision whether to remain in the natal seed (Peer and Taborsky 2007). Dispersal is often associated with an increase in competition for food and shelter among siblings, and this may affect their future direct reproductive value (Emlen 1994). According to the Bdelayed dispersal model^ (Koenig et al. 1992), there is a threshold group size above which an individual would benefit from leaving the group due to increased kin competition. The greater reproductive success of inbred females in early stages of colony development enables the colony to grow rapidly. This would be particularly important in unstable environments where high-quality breeding sites are available for only a short season, as is typical of C. dactyliperda habitats (Bar-Shalom 2002). Thus, highly related females may exploit more seeds in the season and have a higher rate of population growth. Extreme inbreeding in this species, including both mother-son and sib-

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mating (Gottlieb et al. 2009; Holtzman et al. 2009), may be seen as an adaptation to a short-lived resource, while high relatedness among females within a seed favors reduced or delayed dispersal and perhaps cooperative brood care. We suggest that both factors increase reproductive success within the gallery. Similar results were found by Peer and Taborsky (2007) in the scolytid beetle Xyleborinus saxeseni. In their study, they demonstrated that adult individuals tend to delay dispersal and benefit from a higher reproductive success, possibly through the care of offspring kin. In summary, we suggest that three factors associated with sociality may play a role in colonies of C. dactyliperda: (1) Overlapping of generations (Wilson 1975), enabling beetles to exploit a date seed for shelter and food for up to three overlapping generations (Gottlieb et al. 2011) with all instars cohabiting inside the seed. (2) Obligatory care of eggs, without which they cannot survive. Eggs left unattended in the date seed gallery are rapidly attacked by pathogens and die (this study; see also social spiders, Solomon and Lubin 2007). Current experiments are focusing on determining whether there is also allomaternal care of young at different developmental stages. (3) High genetic similarity among the seed inhabitants (Hamilton 1964; Gadagkar 1990; Gottlieb et al. 2009; Holzman et al. 2009). Sib mating and occasional mother-son mating is the norm in the beetle’s life history. Thus, the inclusive fitness benefit of helping kin may exceed that of dispersing and establishing a new colony alone (Koenig et al. 1992) at least during the early stages of colony growth. Acknowledgements We thank Daphna Gottlieb, Tamar Keasar, and Na’ama Morag for fruitful discussions and two anonymous reviewers for helpful comments on an earlier draft of the manuscript. This is publication number 950 of the Mitrani Department of Desert Ecology. Conflict of interest The authors declare that they have no conflict of interest. Funding information The study was funded by the Israel Science Foundation (grant no. 1321/12 to AH and YL).

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