DOI: 10.5061/dryad.s7h44j1dm
title: Male reproductive tactics in house mice: consistent individual differences, intrinsic factors and density effects

description: Alternative reproductive tactics (ARTs) describe fixed or flexible
 alternative strategies to secure fertilization within species. For
 example, while some males defend territories to attract females, others
 invade them to attempt sneaky matings. Often, male ARTs are considered to
 be status-dependent, explained by mass or competitive
 differences. Here, we used 244 male mice, Mus musculus
 domesticus, from semi-natural populations to address those caveats and
 describe ARTs in Mus musculus for the first time. We followed males
 throughout their life and categorized them as territorials or roamers over
 multiple monthly intervals, after validating our method of assigning a
 tactic with detailed spatial data. We explored if tactic choice
 is consistent, whether multiple social and/or intrinsic factors predict
 tactic choice, and tested for fitness consequences and physiological
 differences between ARTs. Tactic choice was consistent and
 associated with mass, age, the operational sex ratio, and population size.
 We also found that territorials had a higher probability of reproduction
 but a lower gonadosomatic index. Our results reveal a personality
 component of ARTs, confirm equal fitness among tactics, and show ARTs as
 multifaceted traits that probably are under various selective pressures.
description: Mice (<em>Mus musculus domesticus</em>) descended
 from wild populations sampled in the Cologne/Bonn region of Germany (N =
 18 original breeding pairs) (50°45′N–51°N, 6°45′E–7°E). Founding mice (N =
 160, 80 males, 80 females) for our semi-natural populations were
 distributed to four replicate enclosures (19.6 m2 each), with 20 males and
 20 females founding one population.     For house mice,
 the natural environment is a barn or a human shelter, and, to mimic such
 an environment as closely as possible, each semi-natural enclosure was
 equipped with various nesting materials and 12 nest-boxes/houses
 (hereafter we use the word territory to describe that a male occupies one
 nest-box/house—the territory holder—; however, in many species “territory
 holder” refers to a male monopolizing multiple houses—which is not the
 case here). Food and water were provided ad libitum and distributed
 uniformly across the room at nine feeding stations. Indeed, such
 conditions are similar to those found in natural European populations of
 <em>M. m. domesticus</em>. After releasing the founders, we
 monitored population developments every 4–5 weeks. Population development
 and densities in semi-natural enclosures mimic mice populations that are
 under long-term observation with the possibility for dispersal. During
 monitoring, we caught all mice within an enclosure, measured body mass,
 checked for fresh bite marks, and took a tissue sample of new individuals
 (weighing &gt;10 g) for parentage assignment. New animals received an
 RFID pit tag (Planet ID, 1.4 × 9 mm) for permanent recognition. Whenever a
 given population reached at least 80 chipped offspring, we removed the
 older generation to keep the density below the carrying capacity. Some of
 the removed animals were euthanized in order to obtain physiological
 characteristics, e.g., conduct sperm counts.  We
 classified reproductive tactics according to males’ location during our
 monitoring. If a male was located inside a nest-box/house it was
 identified as the territory holder and if roaming outside nest-boxes as
 roamer. We excluded all observations from months when more than one
 potentially mature (&gt; 4 weeks) male was found at a particular
 house, as it was unclear which male was the territory holder. 
 We used a stepwise approach to determine our final dataset: from
 an initial of 1699 observations from 336 males, we excluded (1) the first
 two observations of monitoring for each male since they were still
 immature (males only reproduce vary rarely within the first two months;
 only 1 male did from Nmales = 336). Moreover, we rejected (2) those males
 that were not assigned as, or were not possible to categorise them as,
 territorials or roamers (explained below). In total, we ended up with 727
 observations from 244 males. We used seventeen
 microsatellite markers to determine parentage and assign fitness using the
 procedure adapted from Linnenbrink et al. (2013). In summary, DNA was
 extracted from ear clips, amplified using a Multiplex PCR kit (QIAGEN),
 and the samples were run on an ABI 3730 Sequencer (Applied Biosystems). We
 used GeneMarker (V2.6.4) to identify alleles, and Colony [©COLONY |
 Zoological Society of London (ZSL)] to assign the parentages based on the
 maximum likelihood of each potential parental pair. We conditioned our
 parentage assignment on the following assumptions: sexual reproduction,
 polygamous mating system, possible inbreeding, and all animals being
 present the month before sampling (i.e., when juveniles were born) being
 possible parents. Overall, our fitness estimates quantify the number of
 offspring that survived to at least 10g.  We used
 fitness to explore differences between ARTs in reproductive success
 (Nmales = 104), we quantified fitness in four different ways: (1) We first
 used a binary variable describing if a male reproduced (1) or not (0)
 during its lifetime; (2) we also added the total number of offspring each
 male produced as a measure of absolute fitness. We then used two different
 measures of relative fitness, (3) the number of offspring produced by each
 male during its life divided by the total yield of offspring of all males
 that belonged to the same generation, and (4) the total number of
 offspring of each male divided by its lifespan. Measure 3 allowed us to
 identify the relative fitness of each individual at the population level
 while the latter (i.e., 4) “stratifies” individuals based on both their
 offspring yield and their survival rate. We used individuals that
 reproduced successfully, along with those that did not, as omitting the
 latter would inevitably increase average estimates of fitness for each
 tactic. <strong>Physiological
 traits</strong>: Sperm quantity was assessed using a modified
 version of the procedure described in Wang Y. (2002). Epididymal sperm
 count. Curr. Protoc. Toxicol. 14 (1), 16–16. doi:
 10.1002/0471140856.tx1606s14. The epididymis was sliced in a Phosphate
 buffer solution (PBS). The sperm suspension was incubated in a thermomixer
 for 10 min at 40°C to kill the sperms. In a Burker chamber, 10 μl of 1:20
 or 1:40 diluted sperm suspension was used accordingly to get a sperm count
 of around 3–10 per square. Sperms were counted twice under a microscope at
 40× with a PH2 filter in 25 squares.<br>Randomly chosen adult males
 out of all males present in the enclosure were dissected to assess testes’
 mass and sperm concentration. Males were killed using carbon dioxide,
 followed by cervical dislocation. The whole body and testes were weighed,
 and the testes/body mass percentage was calculated.
 <strong>Instrument- or software-specific information needed
 to interpret the data</strong>: All statistical analyses were
 performed using R (version 4.2.1 2022-06-23). Repeatability (R) was
 calculated using the rpt function (rptR package). For the modeling part,
 we used the packages stats, lme4, and pscl.
description: # Male reproductive tactics in house mice: consistent individual
 differences, intrinsic factors and density effects Dataset DOI:
 [10.5061/dryad.s7h44j1dm](10.5061/dryad.s7h44j1dm) ## Description of the
 data and file structure This readme file was generated on 16/03/2025
 **Geographic location of data collection:** MPI for Evolutionary Biology,
 August-Thienemann-Straße 2, 24306 Plön **File List:**
 Darmis_et_al_Male_ARTs_in_house_mice.xlsx ### DATA-SPECIFIC INFORMATION
 FOR: Darmis et al.\_Behavioural data **Number of variables:** 17 **Number
 of cases/rows:** 1699 **Variable List:** * **ID**: The individual ID of a
 male. * **Fitness**: The number of pups an individual produced. *
 **relative_fitness**: The number of pups an individual produced divided by
 the average population fitness. This variable is calculated directly using
 the R code. * **adult_males**: The number of adult males present in the
 population as measured in the monitoring. * **active_females**: The number
 of sexually active females (i.e., not pregnant or lactating) present in
 the population as measured in the monitoring. * **pop_size**: The number
 of all individuals (young & adults) in the population as measured in
 the monitoring. * **pop_males**: The number of males (young & adults)
 present in the population as measured in the monitoring. *
 **pop_females**: The number of females (active, pregnant, and lactating)
 present in the population as measured in the monitoring. * **date_mass**:
 The date the monitoring took place. * **generation**: The generation of
 each male. * **room**: Indicates the semi-natural enclosure the male lived
 in. * **MM**: Indicates the month of the life of the male that the
 measurement of its mass took place. * **mass**: Indicates the mass (in
 grams) for the respective month. Whenever there is an empty cell, this
 indicates that the observation for that respective month is missing.  *
 **MS**: Indicates the tactic for the respective month based on the
 location found. Whenever there is an empty cell, this indicates that the
 observation for that respective month is missing.  * **fit_age**: Fitness
 divided by age of the individual. * **reproduced**: Binary showing if a
 male reproduced (1) or not (0). **Specialized formats or other
 abbreviations used:** * **OSR** (=the number of adult males to the number
 of sexually receptive females) is used as a variable but is not included
 in the initial dataset since it is calculated in R directly. * **NTD** =
 For that particular month, the tactic of the individual could not be
 determined (not determined). * **not_territorial** = The male was found
 inside a house but was not the territory holder since it was too young
 (i.e., 1 month old) to have a tactic. \--- ### DATA-SPECIFIC INFORMATION
 FOR: Darmis et al.\_Dissection data **Number of variables:** 7 **Number of
 cases/rows:** 133 **Variable List:** * **ID**: The individual ID of a
 male. * **mass_g**: The mass of the male (in grams). *
 **mass_measurement**: Which month of the male the mass measurement was
 taken. * **animal_size**: The length of the male in cm. *
 **testes_weight_g**: The male's testes weight (in grams). *
 **sperm_counts_av**: The average number of sperm per mL of that male. *
 **testes_bodymass %**: The gonadosomatic index, the testes’ percentage to
 total body mass. \--- ### DATA-SPECIFIC INFORMATION FOR: Darmis et
 al.\_transitions Whenever there is an empty cell in a row where the MS
 column writes "dead" -&gt; this indicates that the observation
 for that month is missing because the animal died within the previous 30
 days. **Number of variables:** 14 **Variable List:** * **ID**: The
 individual ID of a male. * **adult_males**: The number of adult males
 present in the population as measured in the monitoring. *
 **active_females**: The number of sexually active females (i.e., not
 pregnant or lactating) present in the population as measured in the
 monitoring. * **pop_size**: The number of all individuals (young &
 adults) in the population as measured in the monitoring. *
 **mass_measurement**: Which month of the male the mass measurement was
 taken. * **mass**: Indicates the mass (in grams) for the respective month.
 * **MS**: Indicates the tactic for the respective month based on the
 location found. * **MSnumeric**: Tactic during this month (1 =
 territorial, 2 = roamer). * **MM1**: Age in months. * **age**: Age in days
 (calculated approximately using 30-day intervals). * **date_mass**: The
 date the monitoring took place. * **generation**: The generation of each
 male. * **age_correct**: The actual age of the individual.