Female-biased sex allocation of offspring by an Apodemus mouse in an unstable environment

Small-mammal characteristics affect tick communities in southwestern Tennessee (USA)

Life histories can influence the degree of parasite infestations on a host. Pressures exerted on hosts based on age and sex convey varying degrees of parasite prevalence due to differences in host lifestyles, but it is not known how interactions between different host traits affect tick numbers. The objective of this study was to determine if host characteristics (e.g., age, sex, weight, and their interactions) affect the mean number of ticks found on small mammals regardless of host species or habitat. Sherman live traps were placed in forest and grass/forb habitats representative of the southeastern United States. After capture, host characteristics were recorded, and hosts were then searched for ticks. A total of 281 small mammals (148 Peromyscus leucopus, 34 P. maniculatus, 76 Sigmodon hispidus, 16 Microtus pinetorum, and 7 Ochrotomys nuttalli) and 610 ticks (488 Dermacentor variabilis, 114 Ixodes scapularis, 1 Amblyomma americanum, and 7 A. maculatum) were collected in this study. Host's age, sex, and weight affected the number of ticks collected from small mammals and significant interaction effects between host traits occurred (weight by sex, weight by age, and sex by age). For instance, female subadult rodents had significantly more ticks compared to female adults, male subadults had significantly fewer ticks compared to male adults, and the number of ticks on a host increased as host body mass increased. These results support the hypothesis that the number of ticks vary on rodent hosts based on life histories and trait interactions. Therefore, understanding the behavioral mechanisms of a host can aid in the management of parasites in the environment.

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Host life histories predict the number of ticks present on a rodent.

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Male and subadult rodents are infested with an overall greater number of ticks than females and adults.

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Rodents have more ticks as their weight increases.

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Interactions between sex and age predict the number of ticks present on a rodent.

Sex allocation and secondary sex ratio in Cuban boa (Chilabothrus angulifer): mother's body size affects the ratio between sons and daughters

Secondary sex ratios of animals with genetically determined sex may considerably deviate from equality. These deviations may be attributed to several proximate and ultimate factors. Sex ratio theory explains some of them as strategic decisions of mothers improving their fitness by selective investment in sons or daughters, e.g. local resource competition hypothesis (LRC) suggests that philopatric females tend to produce litters with male-biased sex ratios to avoid future competition with their daughters. Until now, only little attention has been paid to examine predictions of sex ratio theory in snakes possessing genetic sex determination and exhibiting large variance in allocation of maternal investment. Cuban boa is an endemic viviparous snake producing large-bodied newborns (∼200 g). Extremely high maternal investment in each offspring increases importance of sex allocation. In a captive colony, we collected breeding records of 42 mothers, 62 litters and 306 newborns and examined secondary sex ratios (SR) and sexual size dimorphism (SSD) of newborns. None of the examined morphometric traits of neonates appeared sexually dimorphic. The sex ratio was slightly male biased (174 males versus 132 females) and litter sex ratio significantly decreased with female snout-vent length. We interpret this relationship as an additional support for LRC as competition between mothers and daughters increases with similarity of body sizes between competing snakes.

Offspring of primiparous mothers do not experience greater mortality or poorer growth: Revisiting the conventional wisdom with archival records of Rhesus Macaques

Female mammals often begin to reproduce before achieving somatic maturity and therefore face tradeoffs between allocating energy to reproduction or their own continued development. Constraints on primiparous females are associated with greater reproductive failure, and first-born infants often have slower growth and greater mortality and morbidity than infants born to multiparous females. Effects of early life investment may persist even after weaning when juveniles are no longer dependent on maternal care and mother's milk. We investigated the long-term consequences of birth order in a large sample of rhesus macaques, Macaca mulatta, assigned to the outdoor breeding colony at the California National Primate Research Center (n = 2,724). A joint model for growth and mortality over the first three years of life allowed us to explicitly connect growth rates to survival. As expected, males are born heavier and grow faster relative to females. However, contrary to expectations, later-born males face substantially lower survival probability during their first three years, whereas first-born males survive at greater rates similar to both first-born and later-born females. Primiparous mothers are less likely to conceive during the subsequent breeding season, suggesting that their reproductive costs are greater than those of multiparous mothers. We speculate that compensatory tactics, both behavioral and physiological, of first-born offspring and their mothers, as well as the novel ecology of the captive environment, underlie these findings. The results presented here provide new insights into how maternal and infant life history tradeoffs may influence developmental trajectories even after the period of maternal dependence. Am. J. Primatol. 77:963-973, 2015. © 2015 Wiley Periodicals, Inc.

Holsteins favor heifers, not bulls: biased milk production programmed during pregnancy as a function of fetal sex

Mammalian females pay high energetic costs for reproduction, the greatest of which is imposed by lactation. The synthesis of milk requires, in part, the mobilization of bodily reserves to nourish developing young. Numerous hypotheses have been advanced to predict how mothers will differentially invest in sons and daughters, however few studies have addressed sex-biased milk synthesis. Here we leverage the dairy cow model to investigate such phenomena. Using 2.39 million lactation records from 1.49 million dairy cows, we demonstrate that the sex of the fetus influences the capacity of the mammary gland to synthesize milk during lactation. Cows favor daughters, producing significantly more milk for daughters than for sons across lactation. Using a sub-sample of this dataset (N = 113,750 subjects) we further demonstrate that the effects of fetal sex interact dynamically across parities, whereby the sex of the fetus being gestated can enhance or diminish the production of milk during an established lactation. Moreover the sex of the fetus gestated on the first parity has persistent consequences for milk synthesis on the subsequent parity. Specifically, gestation of a daughter on the first parity increases milk production by ∼ 445 kg over the first two lactations. Our results identify a dramatic and sustained programming of mammary function by offspring in utero. Nutritional and endocrine conditions in utero are known to have pronounced and long-term effects on progeny, but the ways in which the progeny has sustained physiological effects on the dam have received little attention to date.