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How Do Great Tit (Parus Major) Pair Mates Cooperate in Brood Defence?

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Using economic theory CHASE (1980) predicted that a member of a group should contribute to the group's common interest, i. e. cooperate, as a function of the other members' contributions. In a group of two, e.g., a pair of parents caring for offspring, there are three possible stable equilibria of genuine cooperation: 1) Solely the male invests; 2) Solely the female invests; 3) Male and female invest into their common interest. It was examined in which of these ways members of great tit (Parus major) pairs cooperate while defending their nestlings and themselves against predatory danger of two sorts (owl (Glaucidium perlatum) + mixed mobbing chorus; the latter only). Employing multiway contingency analysis and information theoretic analysis it was also examined whether pair mates coordinate their defence among themselves. The experiments comprised of 1st and 2nd broods over five seasons in Lower Saxon wood plots. The following results were found: 1. Pair mates cooperate by confronting the danger jointly in 77.2 % of all cases tested (n = 158 pairs). In pairs with only one mate cooperating it is more often the male (15.2%) than the female (3.2%) that contributes to defence. In only 4.4% of all pairs both pair mates display noncooperative behaviour. There is no difference among broods of a season and the stimuli employed. 2. During a defence episode, mates of 1st brood pairs repeatedly approach the owl in succession jointly and withdraw from it likewise rather than alternate direction among each other's successive moves. The spatio-temporal dynamics implied in this synergism cannot be due to pair-specific environmental commonalities (brood, nest environs, etc.) at the time of the test, as is possible for the gross cooperation mentioned (1.); rather pair mates appear to gear their risk-taking to one another. From 5 to 29% of the variance of all moves' directions is thus explained by an influence of the mate's move. Another significant influence lies in a bird's own moves in that, on average, homing in on the danger and retreating from it, tend to alternate. The diminution of synergism between broods in both mates may be due to their becoming more familiar with their nest environs, as indicated both by comparison within the 1st brood and by a sexual difference in the strength of synergism. 3. In moving synergistically (see 2.), the male exerts a stronger facilitatory influence on the female than vice versa. This sex-specific effect abates between broods. The facilitatory effect on the mate tends to disappear after 15 to 30 s, i. e. tends to affect but the mate's next move in time. 4. Coordination of defence is also borne out by both mates homing in on the predatory danger more closely once both are present, as measured by their within-trial average distance from danger. This suggests for the first time that, apart from the accepted determinants of altruism, that the degree of altruism exhibited by a group member feeds back positively on that of another. A short term self-protection hypothesis for this type of synergism is rejected. Like the synergism of moving jointly (2.), this type of synergism is due to mutual positive feedback among the pair mates. 5. Between 1st and 2nd broods, the female's within-trial average distance from danger tends to relax both in the presence and in the absence of the male whereas the male's does not. This sex difference may be due to the male's greater interest in the pair's home range. Pestering a raptor with perseverence at an average distance tends to move it on. By contrast, the 2nd brood male's riskier, yet shorter-term minimum distance in the female's presence has relaxed, thus supporting a female-investment explanation for this measure.

Affiliations: 1: (Arbeitsgruppe für Verhaltensforschung, Abteilung für Biologie, Ruhr-Universität, Postfach 10 21 48, 4630 Bochum, F.R.G.

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