And we always thought humans were picky when it came to selecting potential mates, well guess not.

Darwin’s theory of sexual selection depends not on the struggle for existence, but rather  on the struggle between males for females. It involves male–male combat (intrasexual) or a female’s choice of attractive males (intersexual). It is mostly seen that females show a preference for males of larger body size or enlarged secondary sex traits, leading to those males leaving a greater number of offsprings (natural selection). However, the means by which the females make such decisions is still unknown for quite a number of species.

This study focused on the mechanisms underlying female preference for male body and sword length in X. helleri (green swordtail fish). It investigated whether females responded differently to concurrently presented males by varying their distances and sizes.

The fishes for this study were collected from Mexico. The female X. helleri fishes were separated from the males for a minimum of 30 days to decrease the chances of pregnancy. The 22 females that were going to be used for testing were placed in the center aquarium and were fed 15 minutes before each test. Four dummy males were used as a control allowing for changes in body size and sword length while keeping other variables constant. They were positioned at distances between 34 – 68 cm from the subjects using a motor and pulley system.

Each female was presented with ten treatments (Table 1) in total over a period of three days, four were presented the first day & then three per day for the next two days (completely random). Apparent sizes of dummy males were calculated by dividing their body lengths by their distances from the female subjects.

In order to gather statistical analyses, preference zones were marked for each dummy male and the total time that each female spent in that preference zone was calculated by looking at the 6 minute videos recorded for each trial.

 

Experiment Description of relative dummy size & viewing distance Treatments
  • Actual length control
  • Single treatment
  • Male actual size and sword lengths held constant
  • Viewing distances from the dummy were varied
Dummy B at 34 cm vs. 68 cm from the female subject
  • Sword Compensation
  • Single treatment
  • Male actual lengths and sword lengths were varied
  • Viewing distances and apparent lengths were held constant (60 and 34 cm)
Dummy B at 34 cm (smaller body length but a 20 mm sword) vs. Dummy D at 34 cm (no sword projection)
  • Body Length Preference
  • Male distance from female was held constant
  • Varying body length (pairing large and small swordless males)
Dummy C at 34 cm vs. Dummy D at 34 cm
  • Apparent sword length preference
  • Two treatments
    • A small sworded (B) paired with a large one (A)
    • Dummy B paired with a swordless male C
  • Distance was held constant but sword lengths were varied
Dummy A at  51 cm vs. Dummy B at 51 cm

Dummy B at 34 cm vs. Dummy C at 34 cm

  • Apparent length controls
  • Four treatments
  • Body length, sword length and viewing distances were varied
  • Apparent size was held constant
Dummy A at 68 vs. C at 34cm

Dummy B at 51 vs. A at 68cm

Dummy B at 51 vs. C at 34 cm

Dummy C at 34 vs. D 51 cm

  • Experimental treatment
  • Single treatment
  • Male B paired with A making B appear to have a longer body
Dummy B at 34 vs. A at 68 cm

 

The results gathered from the study are summarised in the figure below (Table 2).

 

Experiment Results
  • Actual length control
Females spent more time around Dummy B at 34 cm
  • Sword Compensation
No preference was exhibited for either of the males
  • Body Length Preference
Spent more time towards the male with a larger body size
  • Apparent sword length preference
Preferred males with a longer sword
  • Apparent length controls
No significant preference was observed
  • Experimental treatment
Females were seen spending more time with the apparently larger male (B) than the one that was actually large

 

These results suggested that female preferences in this study were not based on the actual body length (sword + body) of the males but rather their apparent lengths. This may be due to them providing the greatest visual simulation for females. These results support the idea that swords serve as a representative for increased body growth, indicate that the females used in this study lack depth-perceptive abilities and are unable to evaluate sword length in prospective mates. However, based only on these results we can not eliminate the possibility of X. helleri female fishes possessing depth perceptive abilities since it is possible that these females do not make use of such abilities when assessing male body size.

In X. helleri, the female preference (mate choice) for apparent body size places strong directional pressure on males leading to the evolution of morphological and secondary sex traits associated with courtship. In conclusion, it can be said that a female preference mechanism plays a huge role in driving the evolution among male fishes, when only taking in consideration apparent body size and visual stimulation.

 

References

  • Maclaren, R. D. (2016). Effects of male apparent length on female preference for absolute body size in Xiphophorus helleri. Acta Ethologica, 20(1), 27-36. doi:10.1007/s10211-016-0245-0