Gonçalves-de-Freitas Eliane, Carvalho Thaís Billalba, Oliveira Rui F
Universidade Estadual Paulista e Centro de Aquicultura da UNESP, R. Cristóvão Colombo 2265, 15054-000 São José do Rio Preto, SP, Brazil.
Universidade Federal do Amazonas, Av. Gal. Rodrigo Octávio Jordão Ramos 3000, Manaus, AM, Brazil.
Gen Comp Endocrinol. 2014 Oct 1;207:41-9. doi: 10.1016/j.ygcen.2014.07.024. Epub 2014 Aug 4.
Photoperiod is a major environmental cue that signals breeding conditions in animals living in temperate climates. Therefore, the activity of the reproductive (i.e. hypothalamic-pituitary-gonadal, HPG) axis and of the expression of reproductive behaviors, including territoriality, is responsive to changes in day length. However, at low latitudes the seasonal variation in day length decreases dramatically and photoperiod becomes less reliable as a breeding entraining cue in tropical species. In spite of this, some tropical mammals and birds have been found to still respond to small amplitude changes in photoperiod (e.g. 17min). Here we tested the effect of 2 photoperiod regimes, referred to as long-day (LD: 16L:08D) and short-day (SD: 08L:16D), on the activity of the HPG axis, on aggressive behavior and in the androgen response to social challenges in males of the tropical cichlid fish Tilapia rendalli. For each treatment, fish were transferred from a pre-treatment photoperiod of 12L:12D to their treatment photoperiod (either LD or SD) in which they were kept for 20days on stock tanks. Afterwards, males were isolated for 4days in glass aquaria in order to establish territories and initial androgen levels (testosterone, T; 11-ketotestosterone, KT) were assessed. On the 4th day, territorial intrusions were promoted such that 1/3 of the isolated males acted as residents and another 1/3 as intruders. Territorial intrusions lasted for 1h to test the effects of a social challenge under different photoperiod regimes. Photoperiod treatment (either SD or LD) failed to induce significant changes in the HPG activity, as measured by androgen levels and gonadosomatic index. However, SD increased the intensity of aggressive behaviors and shortened the time to settle a dominance hierarchy in an androgen-independent manner. The androgen responsiveness to the simulated territorial intrusion was only present in KT but not for T. The percent change in KT levels in response to the social challenge was different between treatments (SD>LD) and between male types (resident>intruder). The higher androgen response to a social challenge in residents under SD may be explained by the time course of the androgen response that due to the long time it takes to fight resolution under LD, might have been delayed. This result illustrates the importance of incorporating time response data in social endocrinology studies.
光周期是一种主要的环境信号，它为生活在温带气候中的动物预示繁殖条件。因此，生殖轴（即下丘脑-垂体-性腺轴，HPG）的活动以及包括领地行为在内的生殖行为的表达，对日照长度的变化有反应。然而，在低纬度地区，日照长度的季节性变化急剧减小，在热带物种中，光周期作为一种繁殖诱导信号变得不那么可靠。尽管如此，人们发现一些热带哺乳动物和鸟类仍然会对光周期的微小变化（例如17分钟）做出反应。在此，我们测试了两种光周期模式，即长日照（LD：16小时光照：8小时黑暗）和短日照（SD：8小时光照：16小时黑暗），对热带丽鱼科鱼类伦氏罗非鱼雄性个体的HPG轴活动、攻击行为以及对社会挑战的雄激素反应的影响。对于每种处理，将鱼从预处理光周期12小时光照：12小时黑暗转移到它们的处理光周期（LD或SD），并在蓄养池中保持20天。之后，将雄性个体隔离在玻璃水族箱中4天以建立领地，并评估初始雄激素水平（睾酮，T；11-酮睾酮，KT）。在第4天，促进领地入侵，使得1/3的隔离雄性个体作为领地所有者，另外1/3作为入侵者。领地入侵持续1小时，以测试不同光周期模式下社会挑战的影响。光周期处理（SD或LD）未能通过雄激素水平和性腺体指数测量诱导HPG活动发生显著变化。然而，SD以雄激素非依赖的方式增加了攻击行为的强度，并缩短了建立优势等级的时间。对模拟领地入侵的雄激素反应仅在KT中出现，而T中未出现。处理之间（SD>LD）以及雄性类型之间（领地所有者>入侵者），KT水平对社会挑战的百分比变化不同。在SD条件下，领地所有者对社会挑战的雄激素反应较高，这可能是由于在LD条件下解决争斗所需时间较长，雄激素反应的时间进程可能被延迟。这一结果说明了在社会内分泌学研究中纳入时间反应数据的重要性。
