Ivory Towers: Having a gander at a possible new mate

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The Independent Culture
HOW does a goose choose its mate? A recent two-year study of captive barnacle geese revealed two strategies of mate-selection and identified vigilance in males as a trait that may lead to the acquisition of more than one long-term partner.

The results are reported in 'Mate-selection behaviour and sampling strategies in geese' by Sharmila Choudhury and Jeffrey M Black (Animal Behaviour, 1993, 747- 757). Their observations appear to have potential applications far beyond the realm of geese.

Previous animal research has identified five mate-selection strategies: random mating (settling for the first potential mate that comes along); fixed-threshold strategy (settling for the first one that meets some mimimum standard of acceptability); one-step decision strategy (assessing each potential mate in turn, deciding whether to acept or reject, but never returning to a rejected candidate); sequential-comparison strategy (where each candidate is compared with the previous one); and the best-of-N-mates strategy (where a fixed number of potential partners are assessed, and the best one selected). The size of N depends on the time and energy available to the selector.

The researchers watched 39 male and 39 female barnacle geese going about their mate-selection business.

Geese tended to sample between one and six potential partners, in liaisons lasting from a few days to a few weks, before settling down to a quiet family life. What was most interesting, however, was the emergence of two distinct mate-selection strategies. In 'about 40 per cent' of the cases, geese moved 'step-wise from one trial partner to the next'. Since no goose ever went back to a previous mate, a one-step decision or threshold strategy seems to be indicated.

In the remaining 60 per cent of cases, birds used a 'partner-hold' strategy, still clinging to the old partner while sampling a new one. 'A male using the partner-hold strategy would usually start by alternating between the two females he was courting. He would direct most of his attention at the new female and return regularly to the old one, as if to reassure her of his intentions.' This behaviour is most common in males, while females incline towards the one-step 'love 'em and leave 'em' approach.

The partner-hold strategy, though apparently caddish, does have a clear advantage over other selection techniques: it gives the best chance of ending up with the optimal mate without making any demands on the memory.