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Cameron, E. Z., Linklater, W. L., Stafford, K. J., & Minot, E. O. (2000). Aging and improving reproductive success in horses: declining residual reproductive value or just older and wiser? Behav. Ecol. Sociobiol., 47(4), 243–249.
Abstract: In many mammalian species, female success in raising offspring improves as they age. The residual reproductive value hypothesis predicts that each individual offspring will be more valuable to the mother as she ages because there is less conflict between the current and potential future offspring. Therefore, as mothers age, their investment into individual offspring should increase. Empirical evidence for an influence of declining residual reproductive value on maternal investment is unconvincing. Older mothers may not invest more, but may be more successful due to greater experience, allowing them to target their investment more appropriately (targeted reproductive effort hypothesis). Most studies do not preclude either hypothesis. Mare age significantly influenced maternal investment in feral horses living on the North Island of New Zealand. Older mares, that were more successful at raising foals, were more protective for the first 20 days of life, but less diligent thereafter. Total maternal input by older mothers did not seem to be any greater, but was better targeted at the most critical period for foal survival and a similar pattern was observed in mares that had lost a foal in the previous year. In addition, older mothers were more likely to foal in consecutive years, supporting the hypothesis that they are investing less than younger mares in individual offspring. Therefore, older mothers seem to become more successful by targeting their investment better due to experience, not by investing more in their offspring.
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Cameron, E. Z., Linklater, W. L., Stafford, K. J., & Minot, E. O. (2008). Maternal investment results in better foal condition through increased play behaviour in horses. Anim. Behav., 76(5), 1511–1518.
Abstract: Play behaviour is widespread in mammals, but benefits to play have been difficult to demonstrate. Physical training is one of the many proposed hypotheses, suggesting that males and females should play differently, that increased maternal investment should lead to increases in play, and that increases in play should result in physical advantages. In a population of feral horses, Equus caballus, males and females did not differ in their play behaviour except that males initiated more of their play bouts than females. Maternal condition influenced play behaviour only in males, with sons of mothers in good condition playing more. However, when we controlled for maternal effects by comparing a son and a daughter of the same mother, daughters played more when their mother was in poor condition and sons played more when their mother was in good condition. Mothers of foals that played more lost more condition. Therefore, the difference in play behaviour could not be explained by offspring sex or maternal condition alone, but play behaviour mirrored variation in maternal investment. In addition, those individuals that played more survived better and had better body condition as yearlings despite weaning earlier. Since increased activity has been linked to enhanced musculoskeletal development in domestic horses, we suggest that play provides a link between increased maternal investment, increased body condition and future reproductive success in feral horses, and probably in other species.
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Stanley, C., & Shultz, S. (2012). Mummy’s Boys: Sex Differential Maternal Offspring Relationships in Semi-feral Horses. In K. Krueger (Ed.), Proceedings of the 2. International Equine Science Meeting (Vol. in press). Wald: Xenophon Publishing.
Abstract: In polygynous species with high reproductive skew in males, mothers often show differential investment between sons and daughters; however consistent sex differential investment has not been found by previous studies in horses. We investigated sex differences in mother-offspring relationships in nutritionally independent sub-adult semi-feral Carneddau Welsh mountain ponies Equus ferus caballus. Mothers and their sub-adult sons had consistently closer relationships than mothers and daughters. Stronger affiliative bonds between mothers and sons were quantified by their maintenance of closer proximity, higher rates of affiliative interactions and more frequent suckling bouts. These measures of affiliation were temporally associated with higher aggression levels directed towards sub-adults by other band members, particularly stallions. We suggest that aggression may serve as the proximate mechanism driving male dispersal in feral horses and that the stronger mother-son bond signals an attempt to delay their dispersal, highlighting conflict within the band concerning dispersal timing. Since males become increasingly central within the band over time, with mature stallions requiring excellent social skills in order to both acquire and keep a band of mares, we propose that delaying colts' dispersal allows for further development of these skills in a relatively safe environment. This additional investment is expected to maximise their reproductive success. This study illustrates how social network theory can be used to quantify individuals' social experiences, contributing to a greater understanding of the evolution of group living. It also gives us further insight into the mechanisms underlying dispersal in wild and semi-wild horse populations and how conflict often arises when individual needs differ.
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