animsmall_mnmnvr_090819_1618D8

Wobbling

When the female is not moving too much the hovering male is performing a rolling oscillation about the body axis combined with a horizontal oscillation perpendicular to the body axis. This movement will be called wobbling. The frequency of this oscillation is  between 5 Hz  and 9 Hz. Sometimes two males are wobbling above one female. See animation. The animation is 6X retarded.

 

For Eristalis nemorum males the frequency is largely held constant during a film. The amplitude of the horizontal movement is approximately 0.1 cm perpendicular to the body axis. Wobbling may be interpreted as a kind of peering behaviour (Srinivasan et al, 1999) and may function in estimating the vertical distance above the female.  

Wobbling with two males

When two males are hovering above one female, Male2 above Male1 and Male1 above the Female, Male2 may follow Male1. It is also possible that  Male1 is following Male2. An other possibility is that Male1 and Male2 are more or less independently following the Female. Information about who is following who may be obtained from the wobbling movement of Male1 and Male2. On a few occasions the wobbling of Male1 is correlated with the wobbling of Male2. An example from Film 090819_1618 .is given in Fig 1. In Fig 1 the x-position of Male1 is approximately in antiphase with the x-position of Male2.

Wobbling_Fig1

Figure 1: Part of signals from Track 090819_1618_D.  Male2 is moving in antiphase with Male1.

The relation between Male1 and Male2 as observed in the film of Fig 1 has been further investigated. A track of 18 seconds of this film has been divided in 8 subtracks. For each subtrack the maximum crosscorrelation has been determined along with the height of the first peak of the autocorrelation function of both Males. The height of the first peak is a measure of the amplitude of the  harmonic oscillation, to be called “sinepart” here. The maximum crosscorrelation as a function of time (subtrack) is given in Fig  2 along with the sineparts of Male1 and Male2. Male2 is wobbling from the first moment of the film, while Male1 does not show an oscillatory motion at first. Therefore Male1 is probably following the wobbling movement of Male2 until a high (normalized) crosscorrelation has been reached.

 Wobbling_Fig2

 

Figure   2 : Two Males and one Female Film 090819_1618

From Fig_090819_1618_T_mn2_mn1 with 8 parts of 2.25 s.

Maximum Crosscorrelation as a function of time (Partnumber).

 

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