Two social groups of rats (monogamous and polyandrous) were studied in a sixteen-week long experiment to evaluate their effects on reproductive and behavioral responses in the Wistar strain of rats. Thirty albino Wistar rats (20 males 10 females) with an average initial weight of 60.22 ± 4.8g were used for the study. The rats were assigned to two social groups comprising 5 males and 5 females for the monogamous group and 15 males and 5 females for the polyandrous group. The result showed that polyandry promotes a faster and statistically significant (p<0.05) conception and parturition rates, higher litter size (5.67 versus 4.40), higher number of pups born per female (9.40 versus 4.40) and total number of pups born (47 versus 22). The two social groups recorded reproductive success but parental investment to the survival of the neonates was better in the monogamous group. The monogamous group also had a better inclusive fitness as inferred from a greater number of offspring that survived to the weaning age and a significantly better weaning weight (94.44% versus 37.95% and 36.68g versus 31.43g), respectively. It is concluded that though polyandry results in better short-term reproductive optimality as depicted by shorter length of periods to first conception and parturition and significantly higher number of pups per females, these merits were obviated by a correspondingly poorer weaning weight, lower pre-weaning survival rate and an equally smaller number of pups weaned per female rat in this social group. Polyandrous grouping probably resulted in a faster rate of physiological conditioning of the female rats towards sexual maturity. In addition, social stress as encountered in the polyandrous group could significantly lower reproductive success of rats and probably of any mammalian species.

Key words: Inclusive fitness, monogamy, parental investment, polyandry, reproductive success

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