Eating for Two

Eating for Two

Pregnant and breast-feeding human mothers may like to think that they can indulge themselves with extra treats because they are eating for two, but for most nonhuman animals, extra nutrition is not an indulgence. When resources are limited, there are trade-offs between survival, growth and reproduction, and between foraging and vigilance. Pregnancy and lactation for mammals are energetically expensive and mothers usually increase feeding. To meet these greater energy demands, they may forage for longer, even in the hotter parts of the day; they may forage faster, leaving time to maintain vigilance; and they may choose better quality food possibly in habitats that expose them to more dangers.

In this issue, Uriel Gélin and Marco Festa-Bianchet from Université de Sherbrooke, Canada, and Michelle Wilson and Graeme Coulson from the University of Melbourne, Australia, examined how current and previous reproduction affects foraging behaviour. Most previous studies on mammalian herbivores have compared reproducing and nonreproducing females, which potentially confounds individual differences in reproductive potential. By contrast, this study used two populations of eastern grey kangaroos (Fig. 1) in which some females had been treated with contraceptives. Thus the behaviour of lactating, treated and naturally nonreproductive females could be compared.
Figure 1. A mob of eastern grey kangaroo females at Wilsons Promontory National Park, Victoria, Australia. Photo: Antoine Wystrach.

Eastern greys are gregarious herbivores that tend to graze in the open during the cooler parts of the day. These two populations were semihabituated to observers. Kangaroo reproduction is a model of efficiency: females can be caring for young at two different developmental stages. When the oldest reduces suckling and leaves the pouch, a newborn arrives in the pouch and attaches to a different teat. Remarkably the mother supplies milk of two very different compositions to her two suckling youngsters (Griffiths et al. 1972). In the populations studied by Gélin et al., females tended to have only one young per year. Pregnancy is short (36 days) but lactation is long (up to 18 months). The authors compared foraging behaviour of females with small, medium and large pouch-young and young-at-foot. They tested three predictions: (1) foraging would increase with the presence of pouch-young and as the young grew; (2) because males grow faster than females, mothers of sons would forage more than mothers of daughters; and (3) foraging would be affected by past reproductive effort.


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