Kin selection theory predicts that individuals should behave less aggressively (enhanced social tolerance) or more amicably toward closer kin than less related individuals ([@zoy082-B22], [@zoy082-B23]). In species that live in family groups, individuals may simultaneously gain indirect and direct benefits from helping kin. For example, meerkats *Suricata suricatta* are more likely to perform sentinel behavior when pups join foraging groups ([@zoy082-B57]) and the sentinels have a lower predation risk when they are guarding ([@zoy082-B15]). Moreover, helpers may gain kin selection benefits from raising related offspring, and these offspring might also boost the direct fitness of helpers in the future ([@zoy082-B22], [@zoy082-B23]; [@zoy082-B32]; [@zoy082-B55]). Nevertheless, agonistic behaviors commonly occur between relatives due to competition for resources ([@zoy082-B24]; [@zoy082-B61]). In many cases, animals exhibit their peak aggression during the breeding season (BS) (e.g., eastern broad-toothed field mouse, *Apodemus mystacinus*: [@zoy082-B63]), and they behave less amicably toward kin during the BS compared with other times of the year (e.g., black-tailed prairie dog, *Cynomys ludovicianus*: [@zoy082-B25]). Thus, how individuals treat conspecifics depends on their genetic relatedness, but also on the ecological context, which influences the benefits and costs of cooperative and competitive interactions.

Evidence increasingly indicates that unrelated conspecifics that engage in frequent interactions due to their close spatial proximity may develop "kin-like" behavior ([@zoy082-B5]; [@zoy082-B42]; [@zoy082-B56]). Unrelated individuals may gain immediate shared benefits from mutualism or reciprocity ([@zoy082-B11], [@zoy082-B12]; [@zoy082-B40]). For instance, as the home range overlap increases, there is a higher probability of nest-sharing among unrelated female dusky-footed woodrats *Neotoma fuscipes* ([@zoy082-B28]), thereby suggesting that proximity can lead to social tolerance in this species. In addition, many territorial species are less aggressive toward known neighbors than they are toward strangers ([@zoy082-B39]; [@zoy082-B43]; [@zoy082-B62]), and individuals may save time and energy by averting a costly and unnecessary fight with familiar neighbors ([@zoy082-B20]; [@zoy082-B48]).

Mongolian gerbils *Meriones unguiculatus* are geographically widespread in the typical steppe, desert steppe, or desert areas of northern China, Mongolia, and the Trans-Baikal region of Russia ([@zoy082-B69]). Mongolian gerbils live in social groups comprising 2*--*18 individuals throughout the year ([@zoy082-B37]), where each group occupies an exclusive territory and all group members share the burrow system ([@zoy082-B3]). The reproduction and recruitment of Mongolian gerbils occur mainly from March to August ([@zoy082-B35], [@zoy082-B37]), and they start to store food from September to October ([@zoy082-B3], [@zoy082-B4]). Thus, there are 2 distinct annual life-history stages in Mongolian gerbils: the BS (March to August) and food-hoarding season (FHS) (September to October). Previous studies have shown that the home-range size of a social group increases with the number of male gerbils in the group during the BS, whereas it is positively correlated with the number of female group members during the FHS ([@zoy082-B66]). In addition, field observations and genetic data have demonstrated that social groups are basically family groups ([@zoy082-B3], [@zoy082-B4]; [@zoy082-B67]), and that inter-group genetic distances and geographic distances are positively related ([@zoy082-B65]). Trespassing by neighbors and chases involving individuals from adjacent groups are frequently observed during the BS ([@zoy082-B3], 1989b), and females commonly copulate with neighboring males ([@zoy082-B1], [@zoy082-B2]; [@zoy082-B3]). Consequently, the gerbils that live in different social groups may be kin. These characteristics make the Mongolian gerbil a suitable model species for investigating the influence of kinship and spatial proximity on social behavior, which have not been thoroughly examined in this species.

In this study, we conducted an experiment to test our hypothesis that the social behavioral traits of individuals living in different groups are affected by genetic relatedness and spatial distance (distance between burrow systems) in a natural population of Mongolian gerbils *Meriones unguiculatus.* We predicted that gerbils may exhibit higher rates of amicable interactions and allocate less time toward exploring genetically and spatially close individuals. We also tested whether these effects on social behavior differed between sexes and seasons. The social associations and home-range overlap frequently occur among individuals from adjacent social groups during the BS ([@zoy082-B4]; [@zoy082-B66]), and fewer intergroup social connections are established during the FHS than the BS ([@zoy082-B17]). Thus, we predicted that the effects of kinship and spatial proximity on social behavior might be stronger during the BS than the FHS. In addition, multiple breeding females can be found at the same time in natural populations of wild gerbils and reproductively active male gerbils mainly defend the territories ([@zoy082-B4]). Therefore, we predicted that these effects may be stronger in males but not in female gerbils given that females usually encounter relatively lower reproductive competition than males ([@zoy082-B14]).

Materials and Methods
=====================

Study site, trapping, and animal identification
-----------------------------------------------

Our studies were conducted at Houhatai (42°23.613′N, 116°06.524′E, altitude 1300 m), which is located about 25 km north of Shangdu, Zhenglan Qi, Inner Mongolia, China. The study site is a typical steppe area. The average monthly temperature ranged from --13.9°C to 21°C and the annual total precipitation was 324.6 mm during 2014.

Our trapping plot comprised a 2-ha (100 m × 200 m) grassland dominated by *Leymus chinensis*, *Artemisia sieversiana*, *Thalictrum petaloideum*, *Stellera chamaejasme*, *Klasea centauroides*, and *Aster altaicus*, which provided food or cover for gerbils. The sympatric small mammals comprised the Daurian ground squirrel *Spermophilus dauricus*, Daurian pika *Ochotona dauurica*, and striped dwarf hamster *Cricetulus barabensis.* The potential predators of Mongolian gerbils were the steppe polecat *Mustela eversmanii*, corsac fox *Vulpes corsac*, and some raptors such as the common kestrel *Falco tinnunculus*and upland buzzard*Buteo hemilasius.* No livestock grazed on the study site during our study period.

Mark-recapture experiments were conducted from April 29 to October 24 at 2-week intervals in 2014, where each trapping session lasted for 3 consecutive days. We did not trap during the winter to avoid mortality. Mongolian gerbils were live-trapped using wire mesh live traps (28 cm × 13 cm × 10 cm) baited with fresh peanuts. To enhance the likelihood of successful trapping, we used a concentric circle trapping method ([@zoy082-B35]). The trap station was arranged in 3 or 4 concentric circles within a burrow system to cover most of the range. In total, around 380 traps were set each time to cover all of the trap stations. The traps were set at 05:00*--*06:00 h from May to August, and checked every 1*--*2 h until about 1100 h. The traps were then closed between 11:00 and 15:00 h to avoid trap mortality due to the heat, and trapping was resumed at 16:00 h and continued until 19:00 h. Trapping started between 06:30 and 07:30 h, and continued until 17:30 h during September and October. We also checked the traps every 1--2 h during this period. Gerbils were active during the trapping periods employed in this study ([@zoy082-B35]).

All of the captured gerbils were toe clipped when initially captured to allow permanent identification. The clipped toes were preserved in 95% ethanol for subsequent genetic analyses. All of the burrow systems were marked and their coordinates were recorded with a tape. In the field, a typical Mongolian gerbil burrow system comprises entrance holes, feeding chambers, nest chambers, and tunnels ([@zoy082-B71]). The core area of the burrow system usually has about 10--20 entrances holes in the ground, which form a burrow entrance cluster ([@zoy082-B3]). There are areas with few or no entrances between the burrow systems, so it is easy to determine the distribution of the burrow systems based on the burrow entrance clusters ([@zoy082-B3], [@zoy082-B66]). Therefore, coordinates were used to calculate the distance between each 2 burrow systems according to the Pythagorean Theorem, where the distance from the center point of the core area of the burrow system was the average distance between all of the burrows in one social group and those of another social group. The distance between burrow systems was used as an index of the spatial distance between Mongolian gerbils living in different social groups.

The following data were recorded for each captured gerbil: location, sex, body mass, and reproductive condition (male: testes scrotal or abdominal, ventral scent glands invisible, clear contour or large visible pores surrounded by secreted substance; female: vulva closed or open, pregnant, lactating) ([@zoy082-B35]). Age was estimated based on body mass and the developmental stage of the ventral sebaceous gland (juvenile: \<30 g and with no sign of ventral gland; subadult: 30--50 g, unless they had a ventral active gland wider than 4.2 mm; adult: \>50 g) ([@zoy082-B3]; [@zoy082-B35]). We treated gerbils captured in the same burrow system in 2 consecutive trapping sessions as members of the same social group ([@zoy082-B3]). After recording the data, all of the gerbils were taken to a tent beside the plot and only adults from different groups were selected for behavioral tests. The trapping and handling procedures for the Mongolian gerbils were approved by the Institutional Animal Use and Care Committee of the Institute of Zoology, Chinese Academy of Sciences (Ethical Inspection License No: IOZ13047).

Procedures in behavioral tests
------------------------------

To test the social behavior in pairwise encounters, we used a neutral arena ([@zoy082-B49]), which has been employed widely in rodent behavioral studies ([@zoy082-B31]; [@zoy082-B59]; [@zoy082-B70]). Adult gerbils (\>50 g) of the same sex captured from different burrow systems were selected for pairwise encounters in behavioral tests and they were matched randomly. Social behavior was tested by staging paired encounters in a rectangular neutral arena (42.5 cm × 31 cm × 19 cm), where each dyad was tested only once during a specific season in our study. The arena was divided into 2 equal compartments using a removable opaque partition. Two individuals were placed on either side of the partition and allowed to acclimatize to the novel environment for 5 min. The barrier was then removed and interactions were observed for a 10-min period. A digital voice recorder was used to record any behavior, which was observed continuously by a specific observer. We terminated the tests immediately if one of the following occurred: 1) continuous fighting physically for almost 1 min at a time; or 2) an actual injury occurred.

We measured the durations of the following 4 categories of behavior observed during each 10-min test in the arena: 1) investigative behavior, including sniffing the nose, body, and anal zone of the other gerbil; 2) neutral behavior where the gerbils remained more than 5 cm apart without exhibiting any agonistic behavior and they ignored each other; 3) amicable behavior defined as the pair of gerbils located less than 5 cm apart and they exhibited affiliative behaviors such as remaining side by side, one over the other, or grooming; and 4) agonistic behavior such as upright boxing, defense, and wrestling. All of these behaviors have been observed in natural and captive populations of Mongolian gerbils ([@zoy082-B27]). The arena was cleaned thoroughly with 75% ethanol between tests to remove any odors from the previously tested individuals. At the end of each trial, all of the gerbils were released back into the burrow systems from which they were captured. Gerbils were housed individually in plastic cages (30 cm × 15 cm × 20 cm) during the behavioral tests and adequate food was provided. Animals were kept in the tent for less than 3 h.

Pairwise relatedness based on microsatellite markers
----------------------------------------------------

DNA was extracted from tissues using a TIANamp Genomic DNA Kit (TianGen Biotech Company Ltd, Beijing, China). Nine microsatellite loci (*Mung*µ1, *Mung*µ2, *Mung*µ3, *Mung*µ4, *Mung*µ5, *Mung*µ6, *Mung*µ7, *Mung*µ8, and *Mung*µ9) developed for Mongolian gerbils were used to estimate relatedness ([@zoy082-B46]). Polymerase chain reaction (PCR) was conducted in a 10-µL reaction mixture containing 0.5 ng genomic DNA, 5 µL Premix Taq (TianGen Biotech Company Ltd), and 0.6 µM of the forward (fluorescently labeled with 5^\#^-TAMARA, HEX, or FAM) and reverse primers. PCR was run under the following conditions: initial denaturation at 95°C for 5 min, followed by 30 cycles of denaturation at 94°C for 40 s, annealing at a specific temperature (*T*~a~) for 45 s, and extension at 72°C for 1 min, with a final extension at 72°C for 5 min. The specific annealing temperature (*T*~a~) for each locus was specified by [@zoy082-B46].

Successful PCR amplification was verified by examining samples on agarose gels. Amplified fragments were electrophoresed on an ABI 3730 XL capillary sequencer (Applied Biosystems, Foster City, CA) and the allele size was determined with GENEMAPPER 4.1 software (Applied Biosystems).

Hardy--Weinberg equilibrium tests were conducted with GenePop 4.3 ([@zoy082-B54]). Micro-checker 2.0 ([@zoy082-B64]) was used to test for the probability of null alleles. We calculated genetic diversity metrics comprising the observed heterozygosity (*H*~o~), expected heterozygosity (*H*~e~), number of effective alleles (*N*~a~), and inbreeding coefficient (*F~IS~*) using GenAlEx 6.501 ([@zoy082-B51]). The pairwise genetic relatedness was estimated between test pairs using GenAlEx 6.501 ([@zoy082-B51]) with Lynch and Ritland's estimator ([@zoy082-B38]). The polymorphic information content (PIC) and discriminatory power (DP) of loci were estimated using Cervus 3.0.7 ([@zoy082-B30]).

Statistical analysis
--------------------

The behavioral data were examined using Shapiro--Wilk test to determine their normality and the results did not indicate normal distributions (*P *\<* *0.001). Thus, we transformed the data using the Box-Cox method ([@zoy082-B21]) in the MASS package ([@zoy082-B53]). We used linear mixed effects (LME) models to determine whether the durations of 4 social behaviors were affected by the pairwise relatedness, spatial distance, sex, and season. Relatedness, distance, sex, and season were treated as fixed effects, and pairwise encounter and month as random effects. The results were expressed as the mean ± standard error (*SE*). *P *\<* *0.05 was considered to indicate a statistically significant difference. LME models were analyzed using the lme4 package ([@zoy082-B8]). All statistical analyses were performed with R software ([@zoy082-B52]).

Results
=======

In total, 253 gerbils (137 females and 116 males) from 24 burrow systems were identified during the study period. We captured gerbils 1371 times during the BS and 609 times during the FHS. The distance between 2 burrow systems ranged from 11.6 m to 147.0 m with an average of 66.1 ± 1.6 m. We conducted 249 pairwise behavior tests and about 77.1% (192/249) pairwise encounters were observed aggression. Initially, the gerbils in the pairwise encounters generally sniffed each other (i.e., investigative behavior) or kept away and ignored each other (i.e., neutral behavior). After several investigative and neutral behaviors, the encounters became amicable or agonistic. Amicable and agonistic behaviors did not occur within 10 min in some tests ([Table 1](#zoy082-T1){ref-type="table"} , [Figure 1](#zoy082-F1){ref-type="fig"} ). The relatedness between pairs of individuals was relatively low ([Table 1](#zoy082-T1){ref-type="table"}).

###### 

Pairwise relatedness (Lynch and Ritland's genetic relatedness coefficients) and the duration (seconds) of 4 behaviors in encounters during the breeding season (BS) and food-hoarding season (FHS)

  Sex      Season   Relatedness      Investigative behavior   Neutral behavior   Amicable behavior   Agonistic behavior   Sample size (No. of paired encounters)
  -------- -------- ---------------- ------------------------ ------------------ ------------------- -------------------- ----------------------------------------
  Female   BS       0.019 ± 0.011    33.75 ± 2.89             496.66 ± 11.46     23.76 ± 10.35       34.20 ± 4.97         92
  Female   FHS      0.028 ± 0.018    62.51 ± 12.46            396.68 ± 26.09     17.12 ± 11.99       114.51 ± 21.81       41
  Male     BS       −0.005 ± 0.010   36.35 ± 4.27             462.17 ± 12.96     6.81 ± 6.10         81.42 ± 11.07        78
  Male     FHS      0.017 ± 0.009    44.18 ± 6.88             432.95 ± 22.86     0.00 ± 0.00         110.29 ± 22.02       38

Data represent the mean ± *SE.*

![Relationships between the duration (seconds) of social behavior and pairwise relatedness (Lynch and Ritland's genetic relatedness coefficients) and spatial distance (distance between burrow systems) in Mongolian gerbils (*N* = 209): **(A, B)** investigative behavior, **(C, D)** neutral behavior, **(E, F)** amicable behavior, and **(G, H)** agonistic behavior.](zoy082f1){#zoy082-F1}

Genetic analysis
----------------

The *Mung*µ8 allele was not amplified by PCR and it exhibited relatively low polymorphism, so we excluded it from subsequent analyses. *Mung*µ3 deviated from Hardy-Weinberg equilibrium after sequential Bonferroni corrections for multiple comparisons (*P *\<* *0.01). Thus, we removed this locus from any further estimations of relatedness. The number of alleles per locus ranged from 8 to 11, whereas the mean *H*~e~ = 0.783 ± 0.020 and mean *H*~o~ = 0.789 ± 0.030. The mean PIC was 0.756 ± 0.022, the mean DP for 7 loci was 0.927 ± 0.014, and mean *F~IS~* was --0.211 ± 0.020 ([Table 2](#zoy082-T2){ref-type="table"} ). We detected no evidence of null alleles.

###### 

Characteristics of microsatellite loci used to estimate relatedness in Mongolian gerbils

  Locus      Allele size (base pairs)   *N* ~a~   *H* ~o~   *H* ~e~   PIC     DP      *F~IS~*
  ---------- -------------------------- --------- --------- --------- ------- ------- ---------
  *Mung*µ1   212--232                   10        0.720     0.764     0.734   0.917   −0.223
  *Mung*µ2   107--129                   10        0.829     0.848     0.822   0.959   −0.194
  *Mung*µ3   125--149                   9         0.683     0.688     0.683   --      −0.199
  *Mung*µ4   204--220                   8         0.646     0.659     0.646   0.847   −0.128
  *Mung*µ5   161--185                   11        0.805     0.808     0.780   0.936   −0.261
  *Mung*µ6   113--131                   10        0.829     0.802     0.770   0.933   −0.230
  *Mung*µ7   162--184                   11        0.927     0.856     0.833   0.962   −0.145
  *Mung*µ9   119--205                   9         0.793     0.812     0.780   0.941   −0.311

Na, number of alleles; Ho, observed heterozygosity; He, expected heterozygosity; PIC, polymorphic information content; DP, discriminatory power; *F~IS~*, inbreeding coefficient.

Effects of relatedness and spatial distance on behavioral traits
----------------------------------------------------------------

The pairwise relatedness had significant effects on investigative, neutral, and amicable behavior. In particular, the durations of investigative behavior (*t *=* *2.869, *P *=* *0.0046) and amicable behavior (*t *=* *5.432, *P *\<* *0.0001) increased significantly, whereas the duration of neutral behavior (*t*= --2.734, *P *=* *0.0071) decreased significantly with increasing relatedness ([Figure 1](#zoy082-F1){ref-type="fig"}, [Table 3](#zoy082-T3){ref-type="table"} ). We also found a significant negative relationship between the durations of amicable behavior and spatial distance (*t*= --2.562, *P *=* *0.0111, [Table 3](#zoy082-T3){ref-type="table"}), and amicable behavior only occurred between individuals with a spatial distance of less than 40 m ([Figure 1](#zoy082-F1){ref-type="fig"}f). In addition, the interaction between relatedness and distance had significant negative effects on investigative behavior (*t*= --2.022, *P *=* *0.0475) and amicable behavior (*t*= --7.505, *P *\<* *0.0001), but positive effects on neutral behavior (*t *=* *4.764, *P *\<* *0.0001, [Table 3](#zoy082-T3){ref-type="table"}). However, pairwise relatedness (*P = *0.1248) or spatial distance (*P = *0.5687) had no significant effects on the intensity of agonistic behavior ([Figure 1g-h](#zoy082-F1){ref-type="fig"}, [Table 3](#zoy082-T3){ref-type="table"}).

###### 

LME models for testing the effects of pairwise relatedness (Lynch and Ritland's genetic relatedness coefficients) and spatial distance (distance between burrow systems), sex, seasonality, and their interaction on the duration of social behaviors in Mongolian gerbils

  Behavioral category        Estimate    SE         *t*      *P*
  -------------------------- ----------- ---------- -------- -------------------
  Investigative behavior                                     
   Sex ¦ Male                −0.9020     5.7218     −0.158   0.8750
   Seasonality ¦ FHS         18.7815     9.2196     2.037    0.0718
   Distance                  −0.1565     0.1363     −1.148   0.2523
   Relatedness               83.0134     28.9383    2.869    **0.0046** ^†^
   Sex×seasonality           −22.1472    11.9224    −1.858   0.0654
   Sex×distance              −0.0271     0.2682     −0.101   0.9197
   Sex×relatedness           −18.4826    60.3405    −0.306   0.7595
   Seasonality×distance      −0.5388     0.2845     −1.894   0.0603
   Seasonality×relatedness   201.9876    59.4921    3.395    **0.0007** ^†^
   Distance×relatedness      −2.8057     1.3875     −2.022   **0.0475** ^†^
  Neutral behavior                                           
   Sex ¦ Male                −12.6218    16.9520    −0.745   0.4577
   Seasonality ¦ FHS         −69.3533    28.1718    −2.462   **0.0385** ^†^
   Distance                  0.3481      0.4028     0.864    0.3962
   Relatedness               −232.7818   85.1354    −2.734   **0.0071** ^†^
   Sex×seasonality           83.9565     34.8727    −2.408   **0.0188** ^†^
   Sex×distance              0.1029      0.7908     0.130    0.8967
   Sex×relatedness           224.6669    176.3941   1.274    0.2035
   Seasonality×distance      −0.4918     0.8408     −0.585   0.5717
   Seasonality×relatedness   −358.4613   175.9891   −2.037   **0.0425** ^†^
   Distance×relatedness      18.5908     3.9025     4.764    **\< 0.0001** ^†^
  Amicable behavior                                          
   Sex ¦ Male                −8.8841     10.0254    −0.886   0.3760
   Seasonality ¦ FHS         −2.7259     6.9205     −0.394   0.6945
   Distance                  −0.5665     0.2211     −2.562   **0.0111** ^†^
   Relatedness               251.6024    46.3225    5.432    **\< 0.0001** ^†^
   Sex×seasonality           −5.7055     13.8146    −0.413   0.6797
   Sex×distance              0.5058      0.4339     1.166    0.2444
   Sex×relatedness           −209.8939   90.9768    −2.307   **0.0235** ^†^
   Seasonality×distance      0.0862      0.3388     0.255    0.7992
   Seasonality×relatedness   7.5315      71.0561    0.106    0.9158
   Distance×relatedness      −14.3321    1.9096     −7.505   **\< 0.0001** ^†^
  Agonistic behavior                                         
   Sex ¦ Male                26.8136     12.7975    2.097    **0.0379** ^†^
   Seasonality ¦ FHS         59.4638     18.1132    3.283    **0.0123** ^†^
   Distance                  0.1795      0.3046     0.589    0.5687
   Relatedness               −100.1879   64.8515    −1.545   0.1248
   Sex×seasonality           −56.0029    26.8553    −2.085   **0.0410** ^†^
   Sex×distance              −0.1014     0.6008     −0.169   0.8664
   Sex×relatedness           −129.0281   135.2281   −0.954   0.3405
   Seasonality×distance      1.1558      0.6306     1.833    0.0734
   Seasonality×relatedness   142.8407    136.6413   1.045    0.2973
   Distance×relatedness      1.3495      3.1414     0.430    0.6706

^†^Significant *P*-values are indicated in bold. FHS, food-hoarding season.

The effects of relatedness and spatial distance on social behavior exhibited no consistent patterns in different sexes or seasons. The interaction between relatedness and sex had significant negative effects on amicable behavior (*t*= --2.307, *P *=* *0.0235, [Table 3](#zoy082-T3){ref-type="table"}). The interaction between relatedness and seasonality had significant positive effects on investigative behavior (*t *=* *3.395, *P *=* *0.0007), but negative effects on neutral behavior (*t*= --2.037, *P *=* *0.0425, [Table 3](#zoy082-T3){ref-type="table"}). In addition, there were significant negative interaction effects of sex and seasonality on neutral (*t* = --2.408, *P *=* *0.0188) and agonistic behavior (*t* = --2.085, *P *=* *0.0410, [Table 3](#zoy082-T3){ref-type="table"}, [Figure 2](#zoy082-F2){ref-type="fig"} ).

![Interaction effects between sex and seasonality on the duration (seconds) of **(A)** neutral behavior and **(B)** agonistic behavior.](zoy082f2){#zoy082-F2}

Discussion
==========

As expected, the social behavior in staged encounters between Mongolian gerbils was influenced by relatedness, spatial distance, and their interactions. Gerbils spent significantly more time sniffing related individuals and behaved more amicably with increasing pairwise genetic relatedness. Thus, gerbils were more indifferent to each other as the pairwise relatedness decreased. However, the significant interaction effects between relatedness and spatial distance on investigative, neutral, and amicable behavior indicate that the effects of kinship on social behavior were constrained by space. In addition, we found that amicable behavior occurred only between individuals with a spatial distance of less than 40 m, which is approximately the active range of Mongolian gerbils ([@zoy082-B3]). Furthermore, amicable behavior was exhibited by both close relatives and distantly related gerbils. These results imply that familiarity may play a key role in social associations of Mongolian gerbils. It should be noted that a higher level of familiarity does not necessarily indicate closer spatial distance, but it implies a closer social distance based on frequent interactions. For example, female banner-tailed kangaroo rats *Dipodomys spectabilis* avoid inbreeding via the development of familiarity based on prior associations rather than by using spatial cues, even if males live in familiar spatial locations ([@zoy082-B68]). Thus, the relatively low inbreeding coefficient in gerbils may be related to a similar behavioral strategy for inbreeding avoidance, although females commonly copulate with neighboring males ([@zoy082-B1], [@zoy082-B2]).

However, there were no significant effects of relatedness or spatial distance on agonistic behavior. We observed that 192 out of 249 pairs of encounters resulted in aggression, thereby confirming previous direct observations that most intergroup interactions between gerbils are aggressive ([@zoy082-B3]). Many studies have demonstrated that high relatedness increases the incidence of amicable behavior among relatives such as supportive behavior ([@zoy082-B60]) and cooperation ([@zoy082-B34]), and it can reduce the intensity of aggression ([@zoy082-B18]). However, we found no evidence that agonistic intergroup interactions between gerbils changed with variations in the genetic or spatial distance. [@zoy082-B58] also reported that the intensities of inter- and intra-family aggression in Mongolian gerbils were generally similar. This suggests that the effects of relatedness and familiarity on aggression may be limited in this species, which indicates a high level of competition for food or mate resources between intergroup individuals.

We also found that the effects of relatedness and spatial distance on social behavior exhibited sexual or seasonal patterns. Analyses of the LME models showed that there were interaction effects between relatedness and seasonality on investigative and neutral behavior, thereby suggesting that the effects of kinship on investigative behavior are stronger during the FHS but stronger on neutral behavior during the BS. The negative interaction effects between sex and relatedness on amicable behavior suggest that male gerbils are less intimate with individuals of the same sex than females when the pairwise relatedness is low, but the intensity of the increase in intimacy was higher for males as their relatedness increased. These findings support the idea that individuals may alter their behavioral strategies to adapt to variations in the ecological context ([@zoy082-B44]; [@zoy082-B50]).

Our results showed that both male and female gerbils were highly aggressive during the FHS. The food-hoarding period is known to be a crucial period for non-hibernating northern small mammals ([@zoy082-B33]; [@zoy082-B45]). Our previous field study showed that fewer intergroup social connections were established during the FHS than the BS ([@zoy082-B17]). Other previous studies of Mongolian gerbils have demonstrated that the home range size is positively related to group size ([@zoy082-B3]; [@zoy082-B66]), and the home ranges overlap less during the FHS than the BS ([@zoy082-B66]), thereby demonstrating that food defense is an important function of territorial behavior ([@zoy082-B3]; [@zoy082-B19]). Furthermore, food availability is a determinant of winter survival and of the social groups or population sizes for Mongolian gerbils in the following spring ([@zoy082-B36]). Similarly, highly aggressive behavior during the defense of territories is also found in American red squirrels *Tamiasciurus hudsonicus*, which are highly aggressive during both the breeding and non-BS while defending food stored in their individual territories ([@zoy082-B9]). Therefore, we consider that the high levels of aggression between intergroup gerbils during the FHS associated with an increase in territoriality were caused by food resource competition.

Nevertheless, we found significant negative interaction effects between sex and seasonality on agonistic behavior, which indicates that although Mongolian gerbils exhibited enhanced aggression during the FHS, the intensities of the changes differed significantly between the sexes. Female gerbils exhibited significantly lower aggression during the BS than the FHS, whereas the intensity of aggression in males did not differ significantly between the 2 seasons. Moreover, female gerbils exhibited significantly lower aggression than males during the BS. In most groups of animals, the intensity of aggression is lower in females than males, and the secondary sexual characteristics are generally less developed ([@zoy082-B16]), which suggests that females usually experience relatively lower reproductive competition than males ([@zoy082-B14]). Females primarily fight to defend access to resources such as food and shelter according to Bateman's principle ([@zoy082-B6]; [@zoy082-B7]). In Mongolian gerbils, males have opportunities to copulate with visiting or resident females by defending an undisturbed area, but they are probably unable to prevent their female partners from leaving their territories to solicit copulations in adjacent territories ([@zoy082-B3]). In our study, males were more aggressive against out-group individuals of the same sex than females during the BS, which probably reflects the paternity benefits of repelling intruding male gerbils. This finding supports the suggestion that intergroup encounters may have very different fitness impacts on males and females, and that responses to intruders may reflect differences in the benefit and cost trade-offs between the sexes ([@zoy082-B41]; [@zoy082-B47]).

A second reason why agonistic interactions are less frequent and intense in females than males during the BS is that the risks associated with escalated conflict are usually higher for females than males ([@zoy082-B10]; [@zoy082-B13]). For example, a fatal injury to a female may also lead to increased mortality for any dependent offspring (e.g., ring-tailed lemurs, *Lemur catta*: [@zoy082-B29]). In addition, even when the risk of injury is relatively low, there may be an energy trade-off between chasing and pup feeding ([@zoy082-B41]). Thus, lactating females may rarely engage in aggressive interactions (e.g., chacma baboons, *Papio ursinus*) ([@zoy082-B26]).

In conclusion, our results demonstrate that the behavioral traits of Mongolian gerbils that live in different social groups are affected by kinship, spatial proximity, and their interaction. Furthermore, we showed that the effects of kinship and spatial proximity on social behavior exhibited sexual or seasonal patterns, thereby indicating the occurrence of context-dependent responses to out-group individuals in Mongolian gerbils. Further studies are required to investigate the social association patterns and to quantify their costs and benefits in order to advance our understanding of behavioral interactions in social rodents.

We are grateful to all the members of the Animal Physiological Ecology Group for helpful discussions. We also thank Mr Bin Wu, Plant Protection Station of Taipusiqi, for help with the field work. We are grateful to Dr Michael Cant for his help revising the manuscript. This study was financially support from the National Natural Science Foundation of China (No. 31372211) for W.L. and from the Chinese Academy of Sciences (KSCX2-EW-N-005) for D-H.W.
