Final answer:
The provided references do not adequately address the question of which species' only problem with aging is getting fatter, making it difficult to provide a definitive answer.
Step-by-step explanation:
The question pertains to which species' only problem with aging is getting fatter. However, there seems to be a disconnect between the provided reference material and the question being asked. The references provided discuss the origin of certain species, including llamas, cattle, and pigs, and how they become infected by certain parasites, but it does not address the issue of aging and weight gain specifically to the options given such as cattle, sheep, poultry, or swine. In biology and veterinary studies, it's known that various species can experience different health issues with aging, including but not limited to weight gain. Therefore, without the proper context or additional references that specifically address the direct correlation to weight gain as the sole problem with aging, it is not advisable to select a certain option as a definitive answer.