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The Influence of Reproduction and Fertility Manipulation on the Social Behavior of Feral Horses (Equus caballus)
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Sex and reproduction influence mating systems and both are important for maintaining long-term associations between males and females. The mating decisions of females can have consequences for their reproductive success as females decide who to mate with and how often to mate. Females can mate with several males, but risk increasing chances of infanticide by having an offspring not sired by the behavioral father. Sexual interactions can also have social consequences, whereby females may engage in consortships that result in protection from harassment by other males. Thus, both sex and reproduction have important functions in the maintenance of male-female bonds in many mammalian species. Feral horses exhibit a rare social structure among ungulates in that they stay in year-round bands with several females and usually one male. Bands function to decrease harassment by other males and females in stable bands have higher reproductive success by forming consortships with harem stallions. Paternity analysis confirmed that most mares remain loyal to the band stallion and these foals have higher survival than those born in a band without their sire, indicating the importance of band fidelity for infanticide risk. I found females that changed bands suffered higher aggression rates, spent more time traveling and avoided the band stallion, further demonstrating the costs of male harassment. The socio-sexual behavior of females has been postulated as the proximate cause for the long term associations between mares and stallions. I found that fertility manipulation with two contraceptive formulations did not change male-female relationships, sexual behaviors or band fidelity. In addition, lack of foal production did not alter band fidelity. The two contraceptive formulations worked for multiple years with one injection, but were not as efficacious as other tested formulations. However, the lower efficacy may be explained by the high intensity of research, resulting in high detection of contraceptive failure. Furthermore, there were individual differences in responsiveness as there was more contraceptive failure when females were in poor body condition. A meta-analysis of other studies on contraceptives confirmed that behavioral side effects occur after contraceptive treatment in a variety of species, suggesting the importance of experimental studies on the behavior and welfare of contracepted individuals.