| Research Abstract |
During the three years of this project, we have consistently conducted laboratory experiments using the cooperatively polyandrous cichlid, Julidochromis transcriptus. Main studies of this projects are three : (1) The effect of body size on mating systems and parental roles in J. transcriptus, (2) Sperm competition between alpha and beta males in a cooperative polyandrous trio in J. transcriptus and (3) Female control of paternity in cooperative polyandrous trios in J. transcriptus.
In he last year, we conducted laboratory experiment on the second and third themes. The results of these experiment may have a strong impact. To test the female control hypothesis, we conducted aquarium experiments. Both alpha and beta males of this cichlid jointly take care of broods. Alpha males are aggressive against beta males during spawning, where they are in strong sperm competition. We created breeding pairs (female with alpha or beta male) and trios (female with both males) in various experiments. In
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the first experiment we tested whether females in pairs or in trios prefer wedge-shaped sites for spawning (experiment la, Fig. la). The hypothesis tested is that females only prefer wedge-shaped nests in trios (i. e. to allow beta males access to spawning and induce beta male care). Then, to verify the significance of using wedge-shaped sites, only an open (non-wedge) nest was given to trios and their nest usage was observed. In the second experiment we examined the reproductive behavior of females in trios, and compared them to females paired only with an alpha male or a beta male.
As expected, in experiment la, our first experiments show that females of J. transcriptus in trios prefer to use the wedge-shaped sites, which allow both beta and alpha males to enter the nest site and participate in spawning. The second prediction is that wedge-shaped nests allowed females to divide the paternity amongst multiple males. Beta males are expected to fertilise the eggs laid in the inner-part of the wedge, alpha males in the outer-part of wedge, and thus the paternity distribution will change according to the egg and clutch position in the wedge-shaped sites. As predicted, eggs in the frontal, wider part of the clutches were predominantly sired by the alpha males, whereas the beta males sired more offspring in the back, narrower part of the clutches. Similarly, the proportion of offspring sired by beta males increased when eggs were deposited at the more inner part of the wedge. Females appear to manipulate the likelihood of both males gaining paternity by choosing the position of the clutch inside the wedge carefully. The third prediction is that shared paternity increases the brood care performed by the males, and it reduces female brood care. Neither alpha nor beta male showed an increase in brood care behavior with siring success. However, females had a lot to gain from inducing beta male brood care: (1) in trios, beta males performed more brood care than alpha males. (2) Beta males in pairs (where they have full paternity) provided more brood care than beta males in trios. (3) Alpha males in pairs tended to provide less brood care compared to alpha males in trios. Thus, total male brood care in trios was much larger than brood care by alpha males in pairs, but was not different from brood care by beta males in pairs. Consequently, females in trios provided less brood care than females paired with only alpha males, but was similar to females paired only to a beta male. We conclude that females may reduce their share in brood care by attracting a beta male care-giver to the brood, which will be more likely to succeed when she selects wedge-shaped nest sites. This strongly suggests that female nest choice in J. transcriptus is strategic and relates directly to attracting both potential care-givers to her nest. We have shown that experimental studies of female control of group membership and paternity are needed to gain a deeper understanding of vertebrate cooperative breeding systems, as it may affect e. g. reproductive skew, sexual conflicts and mating systems. Less
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