All mammal mothers (and some fathers) invest in their young to some degree, whether it's humans cooking balanced meals or wildebeest warding of a hyaena attack. The commitment of time and resources by parents is a critical variable in their offspring's survival, but a difficult one to measure. Recent findings from Earthwatch scientists demonstrate the relative value of different measures of parental investment in southern elephant seals, considered an ideal subject of parental investment studies.
In a recent article in Polar Biology, Dr. Filippo Galimberti (Elephant Seals Research Group) and colleagues assessed the parental investment of southern elephant seals in the Falkland Islands. The authors weighed 121 elephant seal pups at birth and at weaning to determine their weight increase, the most reliable measure of parental investment, and compared that to several behavioral measures in the same pups. They found that behavioral observations, such as time spent suckling or numbers of suckling bouts, were not a reliable measure of parental investment.
"At the beginning, we expected to find a good correlation between the time spent suckling and weaning weight," said Galimberti, principal investigator of Earthwatch's Elephant Seals of the Falkland Islands project. "The intensity and continuity of suckling bouts is very variable, so suckling itself is not a good index of the amount of milk that is transferred from the mother to the pup."
Parental investment is a valuable indicator of the health of any population because it stems directly from the condition of breeding females, the availability of food resources, and other environmental conditions. Southern elephant seals and other Antarctic seal species are becoming popular indicators of global environmental and climate changes, because their parental investment is relatively easy to study. Maternal care is concentrated into a short developmental period of about a month, during which the pup is fed extremely rich milk and triples in size, and can easily be measured by weighing pups at birth and at weaning.
Although observing suckling behavior has been used as an index of parental investment for other mammals, especially primates, it has recently been criticized as an inaccurate measure. The results of Galimberti and his colleagues confirm for elephant seals what some other scientists have found for other mammals, that more direct measures of parental investment are preferable to behavioral observations.
"Our results from elephant seals add to previous results in other species, including, for example, wild horses, in which behavioral measures were sloppy indices of investment," said Galimberti. "I expect the same to be true in most mammal species. It's very difficult to accurately observe suckling and it's impossible to evaluate, by observation only, how much milk, and therefore energy, is actually transferred."
Galimberti and his colleagues are now experimenting with three-dimensional photogrammetry, a new technique that will be more accurate than behavioral observations but will be less invasive than physically weighing pups. Earthwatch volunteers on Elephant Seals of the Falkland Islands this fall will be helping to implement this technique, which involves taking four simultaneous digital photos to reconstruct a three-dimensional image. Using photogrammetry, the scientists hope to develop an accurate index of the population's status without resorting to weighing the pups.
For more information, see "Behavioural and morphometric measurements of parental investment in southern elephant seals at the Falkland Islands." F. Galimberti, A. Gallastroni, and S. Sanvito. Polar Biology (2002) 25: 399-403
For information on volunteering on the Elephant Seals of the Falkland Islands project, click here.
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