Rectal temperature is the best but for safety issues oto-thermometer is the best alternative to check core body temperature in neonates. please refer to the article below it is a wonderful article entitled "Thermometry in paediatric practice" Arch Dis Child 2006; 91:351–356. doi: 10.1136/adc.2005.08883
A remarkable result reported by this study is that camels maintain a near constant body temperature during a six day period. The core body temperature followed the same sinusoidal diurnal pattern of the air temperature but with lower amplitude. See page 334 of the cited paper.