Disputas: Martin Mayer

Graden avlegges ved Høgskolen i Sørøst-Norge (HSN), Fakultet for teknologi, naturvitenskap og maritime fag (TNM).

Mayers avhandling er skrevet på engelsk. Her er et sammendrag av den: 

In order to increase their chances of survival and reproduction, animals follow different life history strategies. I investigated the mechanisms and trade-offs affecting the onset of natal dispersal, mate change, spatial movement patterns, and the duration of territory occupancy in the Eurasian beaver (Castor fiber).

Beavers are monogamous (they remain with a partner for long periods of time), and live in family groups in fixed territories. In order to establish a family group of their own, subordinate beavers have to disperse away from the family group they were born in. The mean age at dispersal in our study area was 3.5 years, with some individuals remaining within their natal family group until age 7. Subordinates delay dispersal with increasing age of the same-sex parent. This suggests that either older parents are more tolerant towards their offspring, or that subordinates can perceive senescence and thus ”queue” in the natal territory to take it over after the death of the parents. Further, individuals were more likely to disperse with increasing age due to an increased competitive ability, and at lower population densities. Subordinates possibly can perceive changes in population density and the availability of territories before dispersal via extra-territorial movements. After leaving their natal family group, individuals have to establish a territory and find a mate. Dispersers most likely acquire a territory by intruding into an existing territory replacing the same-sex territory holder, or they establish in a territory where the same-sex territory holder disappeared due to human-caused mortality.

Once established, a beaver has to make the best out of the available resources. I found that individuals in smaller territories had reduced costs of patrolling (they travelled at lower speed) compared to larger territories, but stayed further from the shore when foraging, possibly due to resource depletion. Beavers in smaller territories also conducted more extra-territorial movements, likely to assess possibilities for territory expansion. Additionally, older beavers spent more time on land and close to territory borders suggesting a behavioral change with age due to senescence or experience. The duration of territory occupancy ranged between 1 and 11 years (on average 6 years), and was a predictor for the lifetime reproductive success of an individual. Beavers that delayed dispersal and established in intermediate-sized territories occupied them for longer compared to younger dispersers and individuals establishing in smaller or larger territories. This suggests that an individual should await physical and behavioral maturity before the acquisition of a territory, and demonstrates that intermediate-sized territories ensure sufficient resource availability and decreased costs of territorial defense at the same time.

The high population density in our study area is likely a major factor affecting many of the observed patterns, leading to an intense competition for territories, in effect causing delayed dispersal, non-adaptive mate change, and is driving spatial movement patterns related to patrolling and resource availability.

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