Seasonal variation in female mate choice and operational sex ratio in wild populations of an annual fish, Austrolebias reicherti

Abstract

The intensity of mating competition and the potential benefits for female of mating with certain males can be influenced by several extrinsic factors, such that behavioral decisions can be highly context-dependent. Short-lived species with a single reproductive season are a unique model to study context-sensitive mating decisions. Through exhaustive sampling in the field and simultaneous choice tests in the laboratory, we evaluated operational sex ratio (OSR) and female mate choice at the beginning and end of the reproductive season in the annual killifish Austrolebias reicherti. We found seasonal change in both OSR and female mate choice. At the start of the reproductive season the OSR did not deviate from parity, and females preferred larger males. Later in the reproductive season, while the proportion of males in the ponds decreased, females became unselective with respect to male size. The particular biological cycle of annual killifish, where both life expectancy and mating opportunities decline sharply over a short timescale, could account for the seasonal change in female choice. Reduction in choosiness could arise from diminished reproductive prospects due to a decline in male availability. Moreover, as the end of the season approaches, any benefits of choosiness are presumably reduced: a female's fitness will be higher if she mates with any male than if she forgoes reproduction and dies. Future work will disentangle the mechanisms underlying seasonal changes in mating preferences, notably direct responses to demographic factors, environmental cues, or intrinsic changes during development.

Figure 1. Sex ratio of Austrolebias reicherti.

(a) Locations of temporary ponds sampled in eastern Uruguay. CR = Cebollatí River; YR = Yaguarón River; TR = Tacuarí River; PS = Parao Stream. Numbers refer to the pond numbers mentioned in the text. (b) Sex ratio (proportion male) from temporary ponds early (dark gray) and late (light gray) in the reproductive season. Numbers above the bars denote sample sizes and asterisks denote p<0.05 according to chi-squared tests. Pond codes refer to locations in map above.

Figure 2. Female mate choice and male body size.

Relationship between male size difference (standard length of preferred male − standard length of non-preferred male) and strength of female preference (difference in time spent with preferred and non-preferred male) in the annual killifish A. reicherti at the beginning (black circles, black line) and end (white circles, dashed line) of the reproductive season. Negative scores indicate more time with smaller male, positive scores indicate more time with larger male.