Does size dimorphism reduce competition between sexes? The diet of male and female pine martens at local and wider geographical scales

1. What they did

Male pine martens are much bigger than females. One idea is that this size difference lets the sexes eat different food, so they compete less. Another is that the size difference comes from males competing for mates, and any diet differences come from the sexes behaving differently. The author tested these ideas in Białowieża Primeval Forest, eastern Poland, where rodent numbers rise and fall from year to year with tree seed crops.

• Body size. Males and females were measured from live-trapped martens and museum specimens.
• Diet. Scats from radio-tracked martens of known sex were collected from latrines near their tree resting sites (mostly 10 to 25 m up in the trees) and during snow-tracking. Prey were identified from bone fragments, guard hairs and seeds, and diet was compared between the sexes across three seasons and in years of high-moderate and low rodent abundance.
• Foraging in winter. Radio-marked martens were snow-tracked, and everything they did along their trails was recorded, such as walking on or under fallen logs, climbing trees, entering cavities and attacking prey.
• A wider comparison. The author gathered all published winter diet studies that reported males and females separately, and compared six samples from Poland and Russia.

2. What they found

Size

• Males were 41% heavier and 11% longer than females (average 1.36 kg versus 0.96 kg).

Diet in Białowieża

• Small mammals, especially bank voles and yellow-necked mice, were the main food in both good and poor rodent years (in over 49% of scats).
• Birds, invertebrates and plant material were common in summer; ungulate carcasses were eaten mainly in autumn-winter.
• Overall, males ate more amphibians and reptiles, ungulate carcasses and plant material; females ate more squirrels and, in most seasons, more birds.
• When rodents were scarce, both sexes ate larger prey and a wider range of foods.
• Males and females did not differ in average prey size or in how broad their diets were.
• Diet overlap between the sexes was lowest in spring and in years when rodents were scarce.

Foraging in the snow

• Both sexes used fallen logs and root plates of fallen trees much more than standing trees, and in the same way.
• Females climbed standing trees more often than males. They also tended to enter cavities more often and travelled further in the tree canopy, but these differences were not statistically significant.
• Males attacked prey more often (1.00 versus 0.09 attacks per 250 m of trail).

Across Europe

• Diets differed greatly between the six populations.
• In every population, females ate more red squirrels and birds, and males ate more ungulate carcasses.
• Prey size varied between places but not between the sexes.

3. What they concluded

• Contrary to the niche variation idea, the larger males did not consistently eat bigger prey. Diet differences between the sexes are not the main cause of the size difference.
• Diet differences between the sexes change with food supply and place, and come mainly from behaviour: when and where each sex is active, which microhabitats they use, and how far they range.
• Smaller, lighter females may be able to follow squirrels across thin branches and gaps in the canopy, which may explain why they ate more squirrels and birds. Males, with larger home ranges, may be more likely to find patchy food such as carcasses and fruit.
• Overall, the size difference makes only a small contribution to diet separation, mainly by giving females easier access to prey in the trees.

4. How tracking was or was not used

Snow-tracking of radio-marked martens was used in two ways. First, the trails provided scats of known sex. Second, 250 m sections of trail were read to record how each marten foraged: walking along, on or under fallen logs and root plates, climbing standing trees, entering cavities, distance travelled in the tree canopy, and attacks on prey. Scats were also collected at arboreal latrines near resting sites found by radio-tracking. Fallen scats were collected only when fresh, to avoid mistaking fox scats for marten scats.

5. Tracker or observer certification

Not reported. Who did the snow-tracking and how the trail records were read is not described. The acknowledgements thank E. Bujko, A. Szymura and students from Farnborough College of Technology (England), Warsaw University and Jagiellonian University in Kraków (Poland) for help with field work.

6. Other technologies, skills or innovations

• Radio-tracking to find resting sites and to locate martens for snow-tracking.
• Prey identification from guard hairs (cuticle scales and medulla patterns), bone keys and a reference collection.
• Rodent abundance measured by removal trapping three times a year.