Seasonal and social factors associated with spacing in a wild territorial electric fish

Abstract

In this study, we focused on the seasonal variation of the determinants of territory size in the weakly electric fish Gymnotus omarorum. This species is a seasonal breeder that displays year-round territorial aggression. Female and male dyads exhibit indistinguishable non-breeding territorial agonistic behavior and body size is the only significant predictor of contest outcome. We conducted field surveys across seasons that included the identification of individual location, measurements of water physico-chemical variables, characterization of individual morphometric and physiological traits, and their correlation to spatial distribution. G. omarorum tolerates a wide range of dissolved oxygen concentration, and territory size correlated positively with dissolved oxygen in both seasons. In the non-breeding season, territory size was sexually monomorphic and correlated only with body size. In the breeding season, territory size no longer correlated with body size but differed between sexes: (i) the overall spatial arrangement was sexually biased, (ii) territory size depended on gonadal hormones in both sexes, which was expected for males, but not previously reported in females, (iii) female territory size showed a positive relationship with gonadal size, and (iv) females showed relatively larger territories than males. This study demonstrates seasonal changes in the determinants of territory size and thus contributes to the understanding of the mechanisms underlying the behavioral plasticity natural territorial behavior.

Fig 1. Study site and sampling method.

A. The study site is located in Maldonado, Uruguay, in Laguna de los Cisnes. B. The shores of the lake have water hyacinths creating extensive floating mats that constitute the sampling area. C. Census unit illustrating individual spots. Fish location in individual spots was achieved by carrying out an electric census (Survey 1). Once a fish was located, water dissolved O₂ concentration, temperature, and fish EOD rate were measured in each spot. In Survey 2, individual spots were revisited in sequence, fish were collected, and individual traits measured.

Fig 2. Fish spatial distribution based on environmental variables (Survey 1).

The breeding season is represented in green and the non-breeding season in gray; dark green sections implies overlap of both seasons. A. Frequency distribution versus DNN(in cm). B. Frequency distribution versus O₂ concentration (in mg/l) measured at 30 cm from the surface in each individual spot. C. Linear regression of DNN and O₂ concentration in individual spots (Log transformed). Breeding season: R² = 0.44, p = 1 exp -4, N = 35; non-breeding season: R² = 0.21, p = 1 exp-4, N = 50. D. Linear regression of EOD rate and water oxygen concentration in individual spots. Breeding season: R² = 0.47, p = 5 exp-4, N = 22; non-breeding season: R² = 1 exp-3, p = 0.54, N = 39.

Fig 3. Emergence of sex dimorphism during the breeding season.

The plots show values for females (left panels, represented as squares) and males (right panels, represented as circles), from data obtained in Survey 2 (Step 3 of analysis). A- Relative DNN (cm / cm body length). Dots represent individual values, and in A and B horizontal lines represent mean values, and error bars represent SEM. For each sex, breeding values are shown in the left (green) and non-breeding values in the right (grey). * indicate statistically significance (p < 0.05) t-test. B- Linear regression between body size (cm) and DNN (cm) in the breeding season. Females: p = 0.8, R² = 7 exp-3, N = 13; males p = 0.7, R² = 0.01, N = 15. C- Linear regression between GSI and DNN (cm) in the breeding season. Females: p = 1 exp-3, R² = 0.8, N = 9; males: p = 0.14, R² = 0.25, N = 10. D- Linear regression between body weight–gonad weight (g) and DNN (cm) in the breeding season. Females: p = 0.7, R2 = 0.02, N = 9; males: p = 0.6, R² = 0.03, N = 10. E- Linear regression between water O₂ concentration (mg/l) and E₂ (pg/ml) in breeding females: p = 0.03, R² = 0.41, N = 11. The same relation is shown for 11-KT in breeding males with no statistical test due to small sample size (N = 5).

Fig 4. Seasonality in the spatial distribution of sexes (from data obtained in Survey 2).

Sex of the nearest neighbor (expressed in percentage) when the focal fish is a female or a male, both in the breeding season (A, top), and in the non-breeding season (B, bottom). Dashed line represents the expected percentage to have a female as nearest neighbor for a random distribution, according to the empirical sex ratio (41% in the breeding season, and 50.9% in the non-breeding season). * indicates statistical significance (p < 0.05) according to the binomial exact test.