Fossa

The enigmatic fossa and the evolution of sociality

Madagascar is a land filled with wonderful creatures, from charismatic lemurs to chameleons so small they can stand on the tip of a pencil. It is also home to the fossa, Cryptoprocta ferox, a carnivorous cat-like mammal endemic to the island and thought to be closely related to mongooses (although this is contested). Unusually for a mammal, male fossas sometimes form long-term social groups of two, or occasionally three, individuals that work as a team when hunting allowing them to take down big prey such as some of the larger lemurs. Sociality clearly provides animals with major benefits, not only when hunting but also in terms of decreased predation risk and an improved ability to defend resources such as territory, food supplies or mates. However, there are also significant costs such as increased competition for food when supplies are low or for mates when mating opportunities are limited.  In most male mammals it seems that the costs of sociality outweigh the benefits. Although it is known from a few other species such as cheetahs and kinkajous, for male mammals to form long-lasting social bonds is extremely unusual, especially when the females are solitary, as is the case with the fossa.

Fossa

As a general rule female social groups occur in response to the distribution of food in the environment, while male social groups form in response to the distribution of females. When food sources are clumped females tend to aggregate at feeding areas while males form groups around them. In contrast, when food is evenly distributed both sexes are expected to lead mostly solitary lifestyles coming together only to reproduce. The fossa then is a bit of an oddball. It is the largest carnivore in Madagascar preying on small to medium-sized animals, its food supply is evenly distributed and, as theory predicts, the females live solitary lives. Why then do some males go against theory and form social groups? It may be that more sociable males benefit not only from cooperative hunting but also from increased mating success.

The fossa’s mating system is unique among mammals. During the breeding season large numbers of males gather at traditional mating trees which are occupied by small groups of up to three females. This leads to fierce contest competition among the males for access to mates and reportedly intense sperm competition. Those males that are able to monopolise access to females and gain the longest copulations are likely to reap the benefits of increased reproductive success. It may be that those males that form social groups share access to females and so gain a reproductive advantage over their solitary counterparts. However, intense sperm competition and typically small litter sizes may also impose high fitness costs to individuals belonging to groups of unrelated males thereby constraining the evolution of sociality.

This image from Arkive shows the fossa mating tree.
This image from Arkive shows the fossa mating tree.

The evolution of group-living from solitary ancestors has long been a hot topic in behavioural ecology. The unusual social behaviour of male fossas makes them an ideal study species for testing predictions about why sociality arose in the first place. Answering the question of why some male fossas go it alone while others form associations is the subject of recently published research by Mia-Lana Lührs and her colleagues.

Lührs and her fellow researchers were able to track and gather data on 22 wild male fossas between 2007 and 2010. They began by capturing individuals using live-traps and taking DNA, tissue and hair samples together with data on body mass and length, testis volume, and canine width. Some of these males were also fitted with GPS-accelerometer collars before release.

Since social groups of males were expected to move and hunt in synchrony, accelerometers were used to record when, and how vigorously, individuals were moving. This allowed the researchers to determine when their tracked males were moving and hunting and if they were hunting as a group or alone. In order to determine male mating success seven females were observed during the breeding seasons between 2007 and 2010 and the number and duration of copulations by known males recorded.

The results of this study are fascinating. Male fossas were found to exhibit two distinct phenotypes which differ according to social organisation. Those males that formed associations were found to be 38% heavier and 13% larger than solitary males, most likely due to differences in diet and an increased foraging efficiency. Dietary analyses revealed that the diet of social males contains larger and more agile bodied prey than solitary males and this has been linked to cooperative hunting. Data from associated males fitted with accelerometer tags shows that their movements, including movements typically associated with hunting, were highly synchronised, supporting the view that associated males hunt cooperatively. These results suggest that the benefits of hunting in a group may more than compensate for the costs of increased competition for scarce food resources. The physical superiority of social males may also grant them reproductive advantages as they are able to out-compete solitary males for access to females.

This boxplot shows the proportions of major mammal species in the diet of solitary (grey bars) and social (black bars) males. Prey species are sorted by size from left to right, largest to smallest. You can clearly see that the diets of the two phenotypes differ with social males consuming far more large bodied prey than solitary individuals.
These boxplots shows the proportions of major mammal species in the diet of solitary (grey bars) and social (black bars) males. Prey species are sorted by size from left to right, largest to smallest. You can clearly see that the diets of the two phenotypes differ with social males consuming far more large bodied prey than solitary individuals.

Since males appear to benefit from sociality it is worth considering why females of this species do not also form associations. Lührs and her colleagues propose that the energetic costs of competition for food when communally raising offspring are too high, for female fossas it just does not pay to be social. This may also explain why males usually only associate in pairs and, when groups of three have been observed, one member is usually in very poor condition. There is a fine balance between the benefits of cooperation and the costs of increased competition, two’s company but three’s a crowd.

The increased competition that comes with sociality not only applies to food but also to mating opportunities. This raises the question of how associated males share these limited opportunities to reproduce. A likely answer is that the costs of competition are accounted for by inclusive fitness benefits. This does appear to be the case. In four out of six associations the males were found to be likely litter mates, presumably brothers. Furthermore, the only unrelated male associates in this study were found to share mating opportunities. This suggests that forming associations provides males with significant benefits whether they are related or not.

This raises one final question, if forming associated pairs is so beneficial to males then why are there still any solitary males at all? Why hasn’t natural selection removed solitary behaviour from the population? One possibility is that the solitary males that were observed did not have any litter mates and so are constrained in their opportunities to associate with a familiar male. Put simply, they are solitary because they had no choice. No male littermates were found for any of the solitary males observed in this study, supporting this conclusion. Another possibility is that solitary males represent an alternative life strategy maintained by frequency dependent selection. Lührs and her colleagues suggest that the small size of solitary males may allow them to extract small lemurs from tree-holes or rodents from burrows. This seems unlikely since, when it comes to competition for mates, solitary males are at a significant disadvantage. Interestingly, Lührs and her colleagues note that in some species inferior males invest relatively more in sperm competition by producing more sperm. In fossas however this is not the case and this suggests that, for fossas, going solo is not an alternative strategy. At present the view that solitary males occur due to a lack of littermates with which to associate seems the most likely.

                                                                                                                        

claimtoken-51352dc0d52f9Luhrs M.L., Dammhahn M. & Kappeler P. (2012). Strength in numbers: males in a carnivore grow bigger when they associate and hunt cooperatively, Behavioral Ecology, 24 (1) 21-28. DOI:
(pdf)