Adult Male Aggression

Multiple male seal aggression.
Multiple male seal aggression.
Injury from multiple male aggressions.
Injury from multiple male aggressions.

Male aggression is typically categorized as either involving multiple or single males. These appear to be two distinct phenomena in terms of the male behavior and the age/sex of the subject seals. Multiple male aggression involves several adult males simultaneously competing to mate with a single seal, typically an adult female (although younger animals and males can be targeted in some cases). This can lead to severe traumatic wounds and death. Single male aggression involves one male harassing, mounting and biting a young seal, typically a recently weaned pup of either sex. In the process, the young seal may be drowned or sustain bite wounds that subsequently become infected and lead to debilitation or death.

Aggressive interactions between male seals and smaller animals (of a variety of species, not just monk seals) are normal, and often lead to scratches and relatively minor bite wounds. However, single male aggression in seals becomes a particular concern when an adult male displays an aberrant focus on young animals, with frequent, repeated and severe aggressive behavior that threatens their survival. This is highly unusual behavior amongst monk seals in Hawaii and cannot be well explained, but previous experience shows that the impact of such aggression on smaller seals can be considerable.

Events involving multiple and single adult male Hawaiian monk seals exhibiting aggression towards adult females and immature seals has led to a significant number of severe injuries and deaths. Unfortunately, the loss of even a single female, and the loss of her lifetime reproductive potential, represents a significant setback to population recovery of this endangered species.

The mechanism that triggers male aggression in monk seals is not clear. It is hypothesized that a major contributing factor is an unbalanced sex ratio that is skewed towards males. At islands with the greatest sex ratio imbalance, multiple male aggression is most severe. Although multiple male aggression has been observed at most of the primary breeding sites, this behavior is most notable at Laysan and Lisianski Islands.

Over an 11 year period from 1984-1994, 37 male seals were selectively removed from Laysan Island to restore a balanced sex ratio. These seals were translocated to Johnston Atoll (n=9) or the main Hawaiian Islands (n=21), placed into captivity (n=5), or died (n=2). Following removal, instances of injury or death from multiple male aggression events drastically declined. The removal of these males from the Laysan Island population has contributed to the restoration of a balanced sex ratio and has proven a valuable mitigation strategy.

Single male aggression events have most notably occurred at French Frigate Shoals and more recently at Kure Atoll. Intervention efforts include hazing of identified aggressors, translocating pups from areas where aggressive males frequent, treating injured seals when appropriate and removal of the adult male. The 3 adult males at French Frigate Shoals observed to repeatedly target pups, were translocated to Johnston Atoll (n=2 in 1998) or euthanized (n=1 in 1991). One adult male was brought into permanent captivity in 2013 after he had been observed injuring pups at Kure Atoll. This mitigation strategy effectively reduced pup deaths as a result of adult male aggression at this site.

A possible tool to reduce aggressive behavior, may include the administration of a testosterone suppressant drug to individual aggressive males. However, this technique has not been evaluated in affecting aggressive behavior and is not available at this time.

Further Reading:

Atkinson S, Ragen TJ, Gilmartin WG, Becker BL, Johanos TC
1998. Use of a GnRH agonist to suppress testosterone in wild male Hawaiian monk seals (Monachus schauinslandi). General Comparative Endocrinology 112: 178-182.
Hiruki LM, Stirling I, Gilmartin WG, Johanos TC, Becker BL
1993. Significance of wounding to female reproductive success in Hawaiian monk seals (Monachus schauinslandi) at Laysan Island. Canadian Journal of Zoology 71: 469-47
Johanos TC, Becker BL, Baker JD, Ragen TJ, Gilmartin WG, Gerrodette T
2010. Impacts of sex ratio reduction on male aggression in the Critically Endangered Hawaiian monk seal Monachus schauinslandi. Endangered Species Research [Endang. Species Res.] 11: 123-132. DOI: 10.3354/esr00259