Dan Bagur explores the fascinating mating habits of the pine marten in an extract from his new book

An extract from Pine Marten: The Secret Life of Martes martes (Pelagic Monographs) by Dan Bagur

by Dan Bagur

Pine Martens can be very reluctant to breed in captivity (Sleeman 1989: 98). Compared to other mustelids they have smaller litters at an older age, and they are slow to mature and start breeding. Males need 27–36 months and females 24 months before they are ready (Buskirk 1994: 405/421; Birks 2020: 122).

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Like Red Squirrels, the Pine Marten’s penis is supported by a baculum bone. Growth of this bone is stimulated by testosterone and its size increases significantly during sexual maturity (Buskirk et al. 1994: 419; Bosch and Lurz 2012: 40). Male Pine Martens do not produce sperm between October and December. During this time their testicles have a combined weight of approximately 0.5 g.  It is thought that increasing light levels bring on the hormonal changes responsible for the onset of spermatogenesis, and males are usually fertile at least four weeks before mating occurs. At the height of the breeding season, between May and July, the combined weight of their testicles reaches its peak of 3.2 g, over six times their winter weight.

At the height of the breeding season…the combined weight of their testicles reaches its peak of 3.2 g, over six times their winter weight

They normally produce sperm during the breeding season well into old age (Buskirk et al. 1994: 419–421). Males are only interested in females during the mating season, when they follow their scent to track them down. They show little interest throughout the rest of the year (Zalewski and Jedrzejewski 2006). Plasma testosterone peaks in male Pine Martens in May; these increases in testosterone are thought to cause ‘increased locomotor activity and aggressive behaviour’ (Buskirk et al. 1994: 420). Birks (2020: 123) also observed that ‘males range widely at this time in search of females’.

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This is particularly intriguing, as I had noted that for each year a regular male Pine Marten had suddenly disappeared in May. In 2021 this happened on 10 May, and in 2022 it occurred on 12 May. Both males were never seen again in this territory. At first, I feared that something had happened to them. While that remains possible, it now seems likely that they left the area and travelled many kilometres in search of females and hopefully established new territories in another location. If so, this behaviour occurred on almost the same day two years in a row, in two different males.

I had noted that for each year a regular male Pine Marten had suddenly disappeared in May

Perhaps this is not surprising given the other natural events that are almost perfectly synchronised by seasonality: the spawning of Common Frogs Rana temporaria in my ponds, for instance, on 25 February 2021 and 23 February 2022; the Swallows Hirundo rustica arriving to nest in my outhouse on 21 May 2020 and 20 May 2021. Here again, nature is accurate almost to the day. Research on Stoats suggests that certain males travelled much further beyond the edges of their territories in search of females during the mating season (Sandell 1986; Buskirk et al. 1994: 106). Male Polecats Mustela putorius have also been shown to travel more widely than females and settle in distant territories (MacPherson 2024: 87). It seems that this may also be the case in Pine Martens.

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As male Pine Martens roam they pick up the scent of the females, which leads them to their den. They climb up to the den, uttering their vibrating courtship call as they approach. They can tell with one sniff if there is a female home and, if she is not, they descend again and follow her trail into the woods. If she is in the den then the male pushes his head into the entrance. This can often lead to a hostile encounter. The female and her kits from the previous year’s mating growl and snarl at the intruder.

Female Pine Martens seem reluctant to mate

Female Pine Martens seem reluctant to mate. Possibly they are concerned for the safety of their kits. Eventually the female accepts a male and mating can occur in the den or in the surrounding woods. The mating season of Pine Martens lasts from June to August (Hofmann 1995: 130; Birks 2020: 95). However, females are only receptive for a brief period in June/July (Buskirk et al. 1994: 413/416). The males are ready to mate for a longer period than the females, with spermatogenesis beginning in late April and running until end of August (Danilov and Tumanov 1976). 

The above is an extract taken from Pine Marten: The Secret Life of Martes martes (Pelagic Monographs) by Dan Bagur.

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References:

Sleeman, P. (1989) Stoats & Weasels, Polecats & Martens. London: Whittet Books Ltd.

Buskirk, S.W., Harestad, A.S., Raphael, M.G., Powell, R.A. (eds) (1994) Martens, Sables, and Fishers: Biology and Conservation. New York: Cornell University Press.

Birks, J. (2020) Pine Martens. Essex: Whittet Books Ltd.

Bosch, S., Lurz, P.W.W. (2012) The Eurasian Red Squirrel: Sciurus vulgaris. Hohenwarsleben: Westarp Wissenschaften-Verlagsgesellschaft mbH.

Zalewski, A. Jedrzejewski, W. (2006) Spatial Organisation and Dynamics of the Pine Marten Martes martes Population in Białowiez˙a Forest (E Poland) Compared with other European Woodlands. Ecography 29(1): 31–43. DOI: 10.1111/j.2005.09.06–7590.04313.x

Sandell, M. (1986) Movement Patterns of Male Stoats Mustela erminea during the Mating Season: Differences in Relation to Social Status. Oikos 47(1): 63–70. DOI: 10.2307/3565920

MacPherson, J. (2024) Stoats, Weasels, Martens & Polecats: A Natural History of British & Irish Small Mustelids. London: Harper Collins.

Hofmann, H. (1995) Wild Animals of Britain & Europe. London: Harper Collins Publishers.

Danilov, P.I., Tumanov, I.L. (1976) Mustelids in North-Eastern USSR. Nauka Publishing House, Leningrad, Soviet Union.