This implies microbial digestion plays only a minor role in their digestive strategy. To survive on this poor-quality diet, they have to eat the high-quality sections of the bamboo plant, such as the tender leaves and shoots, in large quantities, over of fresh leaves and of fresh shoots daily. This food passes through the digestive tract fairly rapidly (about 2–4 hr) so as to maximize daily nutrient intake. Red pandas can taste artificial sweeteners, such as aspartame, and are the only nonprimates known to be able to do so. Red pandas are able to reproduce at around 18 months of age, and are fully mature at two to three years. Adults rarely interact in the wild except to mate. Both sexes may mate with more than one partner during the mating season from mid-January to early March. A few days before birth, females begin to collect material, such as brushwood, grass, and leaves; to build a nest, which is normally located in a hollow tree or a rock crevice. After a gestation period of 112 to 158 days, the female gives birth in mid-June to late July to one to four (usually 1–2) blind and deaf cubs weighing each. After birth, the mother cleans the cubs, and can then recognize each by their smell. At first, she spends 60% to 90% of her time with the cubs. After the first week, the mother starts spending more time outside the nest, returning every few hours to nurse and groom the cubs. She moves the young frequently among several nests, all of which she keeps clean. The cubs start to open their eyes at about 18 days of age. By about 90 days, they achieve full adult fur and coloring, and begin to venture out of the nest. They also start eating solid foods at this point, weaning at around six to eight months of age. The cubs stay with their mother until the next litter is born in the following summer. Males rarely help raise the young, and only if they live in pairs or in small groups. A red panda's lifespan ranges between eight and 10 years, but individuals have been known to reach 15 years. The primary threats to red pandas are direct harvest from the wild, live or dead, competition with domestic livestock resulting in habitat degradation, and deforestation resulting in habitat loss or fragmentation. The relative importance of these factors is different in each region, and is not well understood. For instance, in India, the biggest threat seems to be habitat loss followed by poaching, while in China, the biggest threat seems to be hunting and poaching.