Pups and weanlings: parental investment

Weanlings are the most important component of the population because these unbelievable fatty weaned pups are the final result of a very intensive cycle of breeding activities and they will determine, in the end, the future fate of the population. Weanling are very easy to manipulate: it is easy to sex them by just checking the presence/absence of a penile opening while they sleep lying on their back, to mark them by tagging, to measure their length while they sleep, and to weight them by using a simple weighing bag and a small crane. Data on long term breeding strategies of individuals and knowledge of details of life history of a long living species could be achieved just by detailed tracking of individuals of known age: hence we are putting a vast effort in marking all the pups produced during each breeding season.

To study maternal investment, we carried out intensive weighing of pups. By measuring weight at birth, we are able to estimate increase in weight during lactation, growth curves, variance in parental investment among mothers, and difference due to sex of the pup.

Weighing of a pup

A pup is about to be taken out from the weighing bag

Weighing of pups is a pretty complicated job, because of the potential risks to the pup, the mother, and the researchers. Pups need to be taken from their mothers, sometimes well inside harems, they need to be carried to the weighing apparatus, and they need to be weighed and measured fast to avoid any risk of long mother-pup separation and abandonment. Moreover, the reaction of the mother is unpredictable, and some females are very aggressive and actively try to get back their pup. The solution of all these problems is a fast, well trained team. Understanding the ethical problems of pup weighing, we developed a weighing protocol that permitted us to weight hundreds of pups with a mean handling time per pup lower than five minutes, and with no case of pup damage or abandonment.

Weighing of a weanling

The apparatus used to weigh the weaned pups