Female Moaning: How Female Moose Control Mating

One of the key questions in sexual selection theory is how do females choose the best mate? Research has generally focused on the role of male-male competition for access to receptive females, with less attention paid to female mate choice. Theory predicts that females should choose to mate with the most fit male available. In sexually dimorphic mammals, females may use body size as a surrogate for fitness, and choose to mate with the largest or most dominant males.

Alaskan moose (
Alces alces) exhibit a polygynous mating system, where dominant (larger) males called harem masters defend small groups of females from other males (Figure 1). Terry Bowyer at Idaho State university and his colleagues studied moose populations in Danali National Park, Alaska for many years. In a recent paper, they report how females manipulate male behavior in order to ensure they mate with the larger, more dominant males.

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Figure 1. A male moose (Alces alces) during the fall mating season in Jackson Hole, Wyoming (From Flickr/Fisherga)

Over the course of three years, the researchers studied group of moose, focusing on the behavior of females. They reveal that female Alaskan moose moan only during courtship attempts. The researchers hypothesized that these protest moans are attempts by females to prevent courtship attempts by subordinate males. They predicted that females should give protest moans more often when courted or harassed by smaller males than when courted by larger males. Furthermore, they predicted that the rate of male-male conflict would increase after protest moans as the harem master arrived and attempted to drive off the other male.

Female protest moans increased during the primary rut, which takes place in October in Danali. A second spike in protest moans occurred in early November during the secondary rut (Figure 2). That protest moans coincided with other rutting behaviors is strong evidence that these moans are rut-related.

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Figure 2. A graph of the rate of protest moans given by female Alaskan moose from August to November. (From Bowyer et al., 2011)

As predicted females gave fewer protest moans when courted by larger males. In addition, fighting between males increased significantly after female protest moans (Figure 3).

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Figure 3. The percentage of male aggressive and non-aggressive behaviors during 15-minute periods with or without female protest moans. (From Bowyer et al., 2011)

These results indicate that female moose use protest moans to reduce courtship by smaller males. Moaning alerts the harem master, who drives off the smaller male and mates with the female, thereby assuring that the females mates with a dominant male. Such indirect mate choice, where females manipulate males into conflict is rarely reported in mammals, but may be more common than previously believed.


References

Bowyer, R., Rachlow, J., Stewart, K., & Ballenberghe, V. (2011). Vocalizations by Alaskan moose: female incitation of male aggression Behavioral Ecology and Sociobiology, 65 (12), 2251-2260 DOI: 10.1007/s00265-011-1234-y