Chi-Hang Yuen

Publications (3)13.95 Total impact

• Article: Hormone levels of male African striped mice change as they switch between alternative reproductive tactics.

  Carsten Schradin, Chi-Hang Yuen
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  ABSTRACT: Alternative reproductive tactics occur when individuals of the same species follow alternative ways to maximize reproductive success. Often younger and smaller males follow tactics that result in lower fitness than that of dominant larger males. The relative plasticity hypothesis predicts that hormone levels change as males change tactics, but direct tests of this hypothesis are missing. It has been demonstrated in a number of studies that males following different tactics also differ in hormone levels (unpaired data), but not that individual males change their hormone levels as they change tactic (paired data). We compared hormone levels in the same individuals before and after they changed their tactic, using field samples collected over a period of 6 years. We studied male striped mice (Rhabdomys pumilio) following three alternative reproductive tactics: 1. alloparental philopatric males; 2. solitary roaming males, and 3. group-living dominant breeders. Testosterone levels increased and corticosterone levels decreased when philopatric males became roamers or breeders. The increase in testosterone levels tended to be higher in philopatric males that became roamers than in philopatric males that became breeders. Testosterone levels decreased when roamers became breeders. Prolactin levels increased when males of any other tactic became breeders. Thus, males significantly changed their hormone profiles as they changed tactics. These results are in agreement with the hypothesis that changes in hormone levels are associated with the switch from one alternative reproductive tactic to another.
  Hormones and Behavior 11/2011; 60(5):676-80. · 3.87 Impact Factor
• Source

  Available from: stripedmouse.com

  Article: Social flexibility and social evolution in mammals: a case study of the African striped mouse (Rhabdomys pumilio).

  Carsten Schradin, Anna K Lindholm, Jes Johannesen, Ivana Schoepf, Chi-Hang Yuen, Barbara König, Neville Pillay
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  ABSTRACT: Environmental change poses challenges to many organisms. The resilience of a species to such change depends on its ability to respond adaptively. Social flexibility is such an adaptive response, whereby individuals of both sexes change their reproductive tactics facultatively in response to fluctuating environmental conditions, leading to changes in the social system. Social flexibility focuses on individual flexibility, and provides a unique opportunity to study both the ultimate and proximate causes of sociality by comparing between solitary and group-living individuals of the same population: why do animals form groups and how is group-living regulated by the environment and the neuro-endocrine system? These key questions have been studied for the past ten years in the striped mouse Rhabdomys pumilio. High population density favours philopatry and group-living, while reproductive competition favours dispersal and solitary-living. Studies of genetic parentage reveal that relative fitness of alternative reproductive tactics depends on the prevailing environment. Tactics have different fitness under constrained ecological conditions, when competitive ability is important. Under conditions with relaxed ecological constraints, alternative tactics can yield equal fitness. Both male and female striped mice display alternative reproductive tactics based on a single strategy, i.e. all individuals follow the same decision rules. These changes are regulated by endocrine mechanisms. Social flexibility is regarded as an adaptation to unpredictably changing environments, selecting for high phenotypic flexibility based on a broad reaction norm, not on genetic polymorphism for specific tactics.
  Molecular Ecology 09/2011; 21(3):541-53. · 5.52 Impact Factor
• Article: Age at puberty in male African striped mice: the impact of food, population density and the presence of the father

  Carsten Schradin, Carola Schneider, Chi Hang Yuen
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  ABSTRACT: Summary1. The time at which animals enter puberty and become sexually mature is a significant life-history trait, influencing lifetime reproductive success. Great variation exists both between and within species.2. The proximate mechanisms regulating the time at which a male enters puberty are not well-understood. Environmental cues are predicted to provide the relevant information on resource availability and opportunities for reproduction. When these are good the onset of puberty begins whereas at other times investment in survival becomes more important.3. Male African striped mice (Rhabdomys pumilio) demonstrate large variation in the age at which they enter puberty, with grassland populations starting at 4 weeks old and semi-desert populations at over 10 weeks old.4. We predicted that differences in the availability of food, social organization and population density could explain these differences.5. Using data on 170 individual males from 4 years of field studies in a semi-desert population, we found that males became scrotal at a younger age when no breeding male was present in their group and when food was abundant, while population density had no effect.6. In laboratory experiments we demonstrated that males fed with poor protein food, that regularly encounter larger unfamiliar males (mimicking high population density), and that live in family groups with their father present, become scrotal at a significantly later age, independent of their growth rate.7. Males housed in family groups have lower testosterone but higher corticosterone levels than singly housed males, indicating they are sexually suppressed. When they become scrotal in their family group, their testes are only half as large as those of their singly housed brothers, and they contained significantly less sperm.8. We conclude that male striped mice have a flexible response to the onset of puberty, and that the onset of sexual maturity is dependent on several environmental cues. Our results indicate that there is no threshold body mass, which, when reached, would automatically trigger puberty in male striped mice.9. Male helpers in some species are reproductively suppressed, but ours is the first study that demonstrated the importance of different ecological factors in the timing of puberty in male helpers in a facultative cooperatively breeding species.
  Functional Ecology 04/2009; 23(5):1004 - 1013. · 4.57 Impact Factor