Judul: Tumpang Tindih Habitat antara Presbytis sumatrana dan Trachypithecus cristatus di Area Tambang Emas Martabe, Tapanuli Selatan
Abstrak: Habitat overlap frequently occurs when species share resource preferences. This overlap can lead to interspecific competition, particularly when resources are limited. In this study, we examined the habitat overlap and potential competition between the endangered and endemic Presbytis sumatrana and the vulnerable Trachypithecus cristatus, both of which inhabit the same forest areas adjacent to industrial zones in Batang Toru, South Tapanuli. Data collection was conducted using scan and ad libitum sampling methods. The results indicated that the home ranges of both species overlapped by more than 50%. Both P. sumatrana and T. cristatus predominantly occupied the same canopy height of 6-20 m, classified as stratum A. Although 15 of the 31 food species were utilised by both species, the Pianka index, which measures dietary overlap, did not indicate a significant overlap. P. sumatrana primarily consumed fruits, whereas T. cristatus mostly fed on leaves. While clear evidence of interspecific competition was not observed, langur species frequently used man-made substrates and exhibited a high tolerance for human presence. These results suggest that forest enrichment is critical for future conservation efforts to ensure continued coexistence., Tumpang-tindih habitat terjadi ketika spesies memiliki kemiripan preferensi sumberdaya sehingga menempati area yang sama. Hal ini dapat berujung pada kompetisi antarspesies, terutama ketika sumberdaya tersebut terbatas. Pada studi ini, tumpang-tindih habitat dan potensi terjadinya kompetisi diukur pada Presbytis sumatrana, salah satu spesies endemik terancam punah, dan Trachypithecus cristatus, spesies terkategori rawan, yang menempati satu hutan yang juga bersisian dengan area industri di Batang Toru, Tapanuli Selatan. Pengambilan data menggunakan metode scan sampling dan ad libitum. Hasil penelitian menunjukkan bahwa lebih dari 50% habitat kedua spesies tumpang-tindih. P. sumatrana dan T. cristatus juga menggunakan ketinggian kanopi pohon 6-20 m, yang terkategori sebagai strata A. Meskipun 15 dari 31 spesies pohon pakan digunakan keduanya, Pianka indeks, yang mengukur tumpang-tindih pakan, tidak menunjukkan nilai signifikan. P. sumatrana paling sering mengonsumsi buah sedangkan T. cristatus paling sering memakan daun. Meskipun kompetisi antarspesies tidak teramati, kedua spesies seringkali menggunakan substrat buatan dan menoleransi kehadiran manusia. Hasil-hasil ini mengindikasikan pentingnya pengayaan hutan untuk konservasi di masa depan demi koeksistensi berkelanjutan mereka.
Keyword: Batang Toru, Diet, Industrial Area, Interspecific competition, Niche

Judul: Perilaku Harian dan Jenis Pakan Presbytis sumatrana di Area Tambang Emas Martabe, Tapanuli Selatan, Sumatra Utara
Abstrak: Presbytis sumatrana (cek-cek) merupakan primata endemik dari Sumatera Utara dan tergolong terancam punah (endangered). P. sumatrana termasuk pemakan daun (folivore) namun juga memakan biji dan buah. Sampai saat ini belum ada penelitian mengenai P. sumatrana. Oleh karena itu, penelitian terkait perilaku harian dan jenis pakan sangat penting untuk dilakukan. Pengambilan data menggunakan metode scan sampling. Hasil penelitian menunjukkan perilaku istirahat, bergerak, dan makan sebagai tiga perilaku utama P. sumatrana. Perilaku-perilaku ini berbeda nyata (p < 0,05) pada setiap waktu pengamatan. Jenis kelamin tidak memengaruhi perilaku istirahat dan pakan antropogenik sedangkan strata usia tidak berpengaruh hanya pada perilaku pakan antropogenik. Meskipun banyak memakan buah Artocarpus elasticus (Terap), P. sumatrana juga mengonsumsi nasi sisa manusia. Perilaku P. sumatrana ini harus menjadi perhatian pihak manajemen perusahaan, agar akses mereka ke area tempat sampah dapat dibatasi, sehingga kawasan konservasi yang ada di area tambang dapat terjaga kealamiannya., Presbytis sumatrana (local: cek-cek) is an endemic primate from North Sumatra, classified as endangered. This folivorous primate also consumes seeds and fruits. To date, no research has been conducted on P. sumatrana, making studies on its daily behaviour and diet critically important. Behavioural observations were carried out using the scan sampling. The diet was concurrently documented. Three primary behaviours were noted: resting, moving, and eating. Significant behavioural variations (p < 0.05) were observed across different times. While significant differences were found in most behaviours when stratified by gender, no such differences were noted in resting behaviour and anthropogenic feeding. Anthropogenic feed is also not affected by age. Although the natural diet predominantly included the fruit of Artocarpus elasticus (Terap), rice being the most consumed anthropogenic food. The primate's consumption of human food waste is concerning; thus, it is recommended that company management restricts their access to waste areas to preserve the natural conservation area within the mining site.
Keyword: Cek-cek, daily feeding behaviour, leaf-eater monkey, Sumatra's endemic primates

Judul: Hamiltonian formulation for linear waves motion in two layers fluid
Abstrak: The surface wave could be considered as a wave that separates two fluids, namely water and air. Based on this assumption, it is introduced the interfacial wave, a wave between two layers of fluid with different density. The formulation of interfacial waves motion begins with deriving the base equation of irrotational ideal fluid. Furthermore, according to irrotational fluid assumption, the base equation can be stated in velocity potential. In this derivation, the fluids domain is assumed to be restricted by rigid lid boundary conditions, both at the upper and lower limit. Therefore, the interfacial waves motion can be explained in a hamiltonian formulation. In the hamiltonian formulation, total energy is defined as the sum of kinetic and potential energy. The hamiltonian system is obtained from reduction of kinetic energy by using the Dirichlet Neumann Operator. The resulted kinetic energy equation is nonlinear. Therefore, this form is linearized by first part of the Taylor expansion. This linearization gives a dispersion relation of linear wave. Based on this dispersion relation, the phase speed of the linear wave depends on the density ratio of the two layers fluid.
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