Research ArticleZOOLOGY

A shortage of males causes female reproductive failure in yellow ground squirrels

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Science Advances  02 Oct 2015:
Vol. 1, no. 9, e1500401
DOI: 10.1126/sciadv.1500401
  • Fig. 1 Seasonal dynamics of emergence and adult sex ratio in a wild colony of yellow sousliks.

    Cumulative portions of emerged squirrels by sex-age categories are shown (N = 244, nmales = 54, nadult females = 120, nyearling females = 70; a pooled data sample for 2004–2007).

  • Fig. 2 Seasonal dynamics of OSR and female breeding success.

    Breeding success is measured as a portion of subsequent breeders from all females emerged at 5-day intervals. Numbers in boxes are sample sizes of females with determined breeding status. OSR is presented as mean ± SE at 5-day intervals (a pooled data sample for 2004–2007). The mating season is subdivided into early season (<5 days after the beginning), middle season (5 to 20 days), and late season (>20 days).

  • Fig. 3 Effects of OSR at emergence on female breeding success across the years.

    The effects were highly significant in GLMM (P = 0.0008). The sample sizes are 25, 33, 37, and 31 females for 2004–2007, respectively. Error bars represent mean ± 95% confidence interval.

  • Table 1 Effects of local male density, local female density, and standardized OSR at spring emergence on female reproduction in GLMM.

    Female identity was fitted as a random effect in all models. B and SE correspond to model-averaged parameter estimates and SEs in GLMM, whereas χ2 corresponds to likelihood ratio test. All interactions between predictors were insignificant (P > 0.1).

    PredictorsParameters of female reproduction
    Breeding success (breeder/nonbreeder), n = 98Litter size, n = 65
    Local male density (PC1)B = 0.81, SE = 0.28, χ2 = 8.6, P = 0.003B = 0.20, SE = 0.23, χ2 = 0.8, P = 0.4
    Local female density (PC2)B = −0.01, SE = 0.33, χ2 = 0.0, P = 1.0B = −0.20, SE = 0.26, χ2 = 0.6, P = 0.4
    OSR (female/male)B = −0.82, SE = 0.28, χ2 = 10.1, P = 0.0015B = −0.24, SE = 0.23, χ2 = 1.1, P = 0.3
    Female ageB = −0.65, SE = 0.55, χ2 = 1.4, P = 0.2B = 0.39, SE = 0.47, χ2 = 0.7, P = 0.4
  • Table 2 AICc values for candidate models describing the effects of local male density (Males), local female density (Females), OSR, and female age (Age) on female breeding status in yellow ground squirrels.

    Female identity was fitted as a random effect in all models (n = 98). k is the number of parameters estimated by the model, ΔAICc is the difference between the AICc score of the given model and AICc score of the best model (the lowest AICc score was 108.0), and AICc weight reflects relative support for each model. Models with ΔAICc > 2.0 are in boldface.

    ModelkΔAICcAICc weight
    OSR + Males400.44
    OSR + Males + Age50.860.28
    OSR + Males + Females52.20.14
    OSR + Males + Females + Age63.10.09
    OSR + Age47.30.01
    Males38.00.01
    OSR38.40.01
    Males + Age49.30.00
    Males + Females49.40.00
    OSR + Females + Age59.40.00
    OSR + Females410.60.00
    Males + Females + Age510.90.00
    Age315.10.00
    Intercept215.20.00
    Females316.80.00
    Females + Age417.00.00
  • Table 3 OSR and parameters of male and female distribution at female vernal emergence for subsequent breeders and nonbreeders.

    Data are presented as mean ± SD [range] (n) (see statistics in Table 1).

    ParametersBreedersNonbreeders
    YearlingsFemales aged ≥2 yearsYearlingsFemales aged ≥2 years
    Mean distance to the five
    nearest male burrows, m
    95 ± 54 [26–236] (18)106 ± 57 [42–251] (38)152 ± 60 [29–230] (12)105 ± 34 [59–251] (16)
    Number of male* burrows within 100 m
    of the female’s burrow
    3.2 ± 2.3 [0–6] (18)3.1 ± 1.7 [0–6] (38)1.7 ± 1.6 [0–6] (12)2.4 ± 1.4 [1–5] (16)
    Mean distance to the five nearest
    female burrows, m
    50 ± 29 [26–128] (18)66 ± 61 [18–352] (38)50 ± 26 [23–95] (12)59 ± 30 [18–109] (16)
    Number of female burrows within 70 m
    of the female’s burrow
    3.4 ± 3.1 [0–9] (18)3.7 ± 3.1 [0–12] (38)2.5 ± 2.2 [0–8] (12)2.9 ± 2.3 [0–9] (16)
    OSR (female/male)0.6 ± 0.3 [0.2–1.1] (27)0.7 ± 0.4 [0.08–1.3] (47)0.9 ± 0.3 [0.6–1.2] (12)1.0 ± 0.3 [0.2–1.3] (19)

    *Only ≥2-year-old males that emerged no later than the fifth day after the date of emergence of the focal female.

    †Only females that emerged within ±5 days from the date of emergence of the focal female.

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/9/e1500401/DC1

      Fig. S1. Seasonal dynamics of OSR (daily ratio of receptive females to adult males).

      Table S1. PCA of four variables describing the spatiotemporal distribution of females and males around receptive females.

      Table S2. AICc values for GLMM explaining female breeding status in yellow ground squirrels, with study year as additional predictor.

      Table S3. AICc values for generalized linear models explaining female breeding status in yellow ground squirrels.

    • Supplementary Materials

      This PDF file includes:

      • Fig. S1. Seasonal dynamics of OSR (daily ratio of receptive females to adult males).
      • Table S1. PCA of four variables describing the spatiotemporal distribution of females and males around receptive females.
      • Table S2. AICc values for GLMM explaining female breeding status in yellow ground squirrels, with study year as additional predictor.
      • Table S3. AICc values for generalized linear models explaining female breeding status in yellow ground squirrels.

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