Good dog / bad dog. Are Shetland otter cubs at risk from male otters?

When male Eurasian otters feature in wildlife documentaries and nature writing, they are often portrayed as an ominous presence, a threat to both mum and cubs. But are they really deserving of all the bad press?

An adult female otter and her two cubs seek the safety and comfort of each other in the presence of a much larger male. In this instance there was little in the way of aggression, in fact both the male and the female with cubs slept for short period with only a few meters separating them.

What the males are really after.

If you have the opportunity to observe a male otter with a group of otters (a mother and her cubs), you will likely notice that his attention is directed towards the adult female for mating purposes. The male's goal is to determine her readiness for mating (by sniffing), which is only feasible if she has ceased lactating. Mating then triggers ovulation, and repeated attempts over several weeks ensure fertilisation of the egg. Eurasian otters are polyesterous, meaning they don't have a fixed breeding season and can mate successfully as long as they aren't already pregnant or nursing. This brings us to another intriguing aspect of otter reproduction: females can delay the development of an early-stage embryo. Instead of immediately attaching to the uterus, the dormant embryo floats within and waits. This phenomenon is known as embryonic diapause. In many mammal species, this occurs automatically during every pregnancy. For Eurasian otters, the timing of implantation is influenced by the female's health and condition, with Shetland otters requiring the first half of summer to gain sufficient body weight for a 13 week gestation to commence.

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Across several encounters, this male appeared to be mate guarding. A strategy to ensure that a receptive female isn't mated by another male. There were several mating attempts witnessed too, always in the water (see the following clip). During this period, the male shadowed the female, even though she had cubs in tow. It is likely she was no longer lactating at this point. Interestingly it is the cubs which see the male off here, the adult female (far left) seems less concerned at his presence.

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Whilst I have seen Eurasian otters mating on land, for the most part it happens in water which helps to support the weight of the otters as they wriggle and writhe, over a period sometimes lasting for an hour or more. At first glance it looks, and indeed sounds like aggression. In this clip you can hear the cubs squeaking from the shore. The male has no interest in the sub-adult cubs so pays them little attention; perhaps unsurprisingly the cubs do however find the presence of the males quite stressful.

I photographed this Eurasian otter mating sequence in Shetland on December 12, 2021. Typically, you hear mating otters before you see them! There was a lot of fighting and thrashing in the water, with the male holding the female's neck in his jaws. These prolonged mating sessions can last up to an hour and may lead to injuries for both the male and female otters. This particular encounter was unusual because most of the copulation seemed to occur on land.

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When the threat to cubs is real.

I'm going to get straight to the point here. Males are more often than not a benign presence, posing little or no risk to the females and cubs that share their territory. The risk, however, is a very real one when the male in question is an interloper, that is to say, new to the territory because he has either ousted the previous male or is chancing his luck at a successful mating on someone else's patch. Female otters adopt a series of behavioral changes which have evolved to minimize the risk of their cubs being found by a patrolling male; these are outlined below.

• Natal holt location.

  Otter holts are often close to the shore; however, an experienced female may well choose a holt to give birth and rear her cubs in, well away from the shore side. Even distances over 80 meters are not unusual. This keeps them well back from the likely path of a roaming interloper.

  
  

• A shift in sprainting habits.

  Eurasian otters will ordinarily deposit a considerable amount of spraint outside the holts they use, in addition to sprainting at freshwater sites and prominent, raised areas used by other otters which use the same area. However, a nursing female will change her scent marking habits, preferring to defecate in the water, making both the presence of her and her cubs far less conspicuous.

  
  

• Nocturnal / diurnal activity shift.

  There also appears to be a change in activity period whilst a female has very young cubs. Ordinarily, coastal otters are active during daylight hours, often associating with a state of low tide. In contrast to this, a nursing female is likely to be more active around dawn or dusk, another measure to help keep both her and the presence of her vulnerable cubs under the radar of male otters.

These measures might appear somewhat extreme, particularly since we've observed how little interest a male typically shows in cubs. However, we've only considered scenarios where the male is already established, with the females' territories located within his usual range. When a male is new to the area, his objectives and behavior change significantly. Upon finding a natal holt, there's a genuine risk he will kill and consume the cubs inside (as it would be practical not to waste the calories). A female will certainly attempt to intervene, but males have a substantial weight advantage, making resistance likely futile.

This behavior is known as infanticide and is observed in many other mammal species, from lions to mice. To thoroughly discuss this intriguing aspect of Eurasian otter behavior, I have included my original account of an infanticide case witnessed in Shetland and photographed by Jennie Lee-Gardener, one of my clients that day.

In conclusion.

Are Shetland otter cubs at risk from male otters? In summary, male otters pose a risk primarily if they are new to the territory where a young family resides. Even then, they are mainly a threat to very young, dependent cubs that have not yet left the natal holt. This behavior is common among many mammalian carnivores; it may be harder to accept because of the "cute" image often associated with otters. However, they are just as competitive and voracious as lions or bears. It might be our perception of carnivores and our inclination to label animals as good or bad that needs to change; instead, we should value them as the complex, highly adaptive organisms they truly are.