'The presence of females in high densities . . . adversely affects a male's health and reproductive success.'
Those conclusions, from recent research on voles and sticklebacks respectively, reflect a growing tendency to wonder whether females are really good for males. In Femme Fatale - the Case of the Threespine Stickleback, by Michelle de Fraipont, Gerard J Fitzgerald and Helga Guderley, (Ethology, vol 91, 1992), they begin by pointing out that courting can be hazardous for a male praying mantis or certain species of spider. They also mention that virgin female fruitflies live considerably longer than non-virgins. But these remarks pale into insignificance when compared with the sexual harassment suffered by male sticklebacks.
'In addition to courting females,' they write, 'male threespine sticklebacks must guard their nests from attacks on the eggs by female cannibals.' They noticed that when sticklebacks were courting, the males had no trouble in accepting another chap in the vicinity, but they were extremely aggressive to females other than the chosen mate. To assess the effects of this potentially stressful situation, they compared males housed alone with their mates with those kept in a tank with the threat of intervention by an addition female of the same species.
For the purpose of the experiments, each male was kept in a Plexiglas tank and given suitable algae as nesting material. When he had built a nest, he was 'provided with a gravid female and allowed to mate,' which he usually did within an hour. After spawning, the females were removed and the males left to hatch the eggs. As soon as hatching began, the male was given another female and allowed to get on with a new breeding cycle. This continued for 80 days, with water changes and fresh algae every two breeding cycles.
While all this was happening, a variety of things could be going on in an adjacent tank: it could be empty, or house another female or male stickleback, or a female of another species. And the tanks could be arranged to allow communication by sight, smell or sound (though, as the researchers mention, it is not known whether sticklebacks communicate by sound). The behaviour and health of the experimental males were carefully monitored over the breeding period to see what differences were made by the inhabitants of the next room.
The results were unambiguous: 'Males kept with conspecific females lost mass faster, completed fewer breeding cycles and died sooner than the other males. They also courted less . . . and took more time to build nests.' They conclude: 'Our results clearly indicate that the presence of conspecific females decreased male fitness.' Not only that, but 'male and female sticklebacks do not need to have physical contact for females to decrease the longevity of the males', although visual cues are not enough on their own to induce premature death. The male must also smell or hear a female if he is to die young.
Neither is it much better for male voles, as Lowell L Getz and Betty McGuire report in A Comparison of Living Singly and in Male-Female Pairs in the Prairie Vole, Microtus ochrogaster (Ethology, vol 94, 1993). Nearly six years of vole-watching enabled the researchers to collect data on 294 happily-married vole couples and 194 single- parent vole mothers.
While there was some difficulty in following up the families of itinerant males (it was a field study rather than a laboratory experiment), the offspring of each female was carefully noted to see whether early home-life was a predictor of either longevity or reproductive success.
The results were interesting. There was no significant difference detected in survival of offspring from one- or two-parent families, but paired females lived on average eight days longer than single mothers. Wandering males, however, lived on average 16 days longer than paired males. Despite these figures, they report: 'Results of the present study do not support the hypothesis that females gain from being paired with a male'.
They conclude that monogamy in prairie voles may have had evolutionary significance in low food habitats. 'It is possible that male-female pairing evolved in the low-food tallgrass habitat and represents a basic behavioural trait that is retained regardless of food resources in the habitats now occupied by the species.' In other words, when there's a lot of alfalfa around, there's nothing wrong with single parent families. 'Back to Basics' is not necessarily good for voles.