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Sayyed Hussain

Sayyed Hussain smiling

Galapagos Tortoise and Marine Iguana Expert

The Galapagos Hawk: Reverse polygamy and size matters (not really, but kind of)

Treading on the black volcanic sand of Fernandina Island, in the Galapagos archipelago, we hoped to see some snakes. To our amazement, we saw the Galapagos Hawk instead. The apex predator of the Galapagos.

Galapagos Tortoise Illustration by Sayyed Hussain

The big, dark hawk tore into the flesh of a small marine iguana with its bright yellow beak, as a smaller hawk joined us and watched the scene unfold from a safe distance. Shrouded in what appeared to be a dark brown-black cloak, the Galapagos hawks have secrets of their own that are not obvious upon first glance—not to a few new students of evolutionary biology, anyway. A novice bird enthusiast might think that the smaller hawk was simply a juvenile one, whereas the larger one was the adult. However, that wasn’t the case. The larger hawk that devoured the marine iguana was a female, while the smaller hawk was just one of its several (smaller) male mates.

Most bird species are considered monogamous. One male and one female generally form a “pair bond” that lasts for one or more breeding seasons. Some monogamous birds occasionally mate outside of their pair bond, but almost 90% of all known bird species are primarily monogamous. Less than 3% are thought to be polyandrous [3]. The Galapagos hawks are even more special because they are usually cooperative-polyandrous—less than 1% of all known bird species are cooperative-polyandrous [1]! This means that in the Galapagos hawks, multiple males breed with a single female, usually for their life time, and aid in rearing the chicks [1].

The primary reasons for polyandrous behavior is due to the nature of territory acquisition and the variable climate of the Galapagos [3]. A group of males, in cooperation, is better able to acquire and maintain its hold on a territory for a female to nest in, while solo males are less successful in territorial acquisition. Additionally, multiple males are more capable of raising chicks since they are better able to distribute the cost of raising chicks among themselves and reduce the weight of the load that comes with raising chicks: up to four males cooperate when raising chicks, meaning that each male doesn’t need to forage as much as it would have had there been no help. Consequently, each male in a polyandrous group, can forage more food for itself, and survive longer than a typical male in a (less common) monogamous pair bond. This is especially true during periods of low food availability, which are often periodic, due to the nature of El Nino and La Nina in the Galapagos.

It comes as no surprise that polyandrous groups generally raise more chicks to adulthood than monogamous pairs [2], [3]. Chicks raised by polyandrous groups tend to have a higher chance of reaching adulthood than those raised by monogamous pairs, since the former tend to receive extra feedings. These extra feedings are a direct benefit from additional male parents that contribute their fair share to improve the quality of feedings for the chicks. Furthermore, this difference in the quality of feedings results in a major head start for chicks raised to adulthood by polyandrous groups over those raised by monogamous pairs. This is due to the understanding that better feedings, while in the nest as chicks, result in better developed adult hawks out of the nest. This better physical development of chicks raised by polyandrous groups may allow for quicker territorial acquisition, a longer life span, and more life-time offspring produced in comparison to hawks that were raised by monogamous pairs as chicks.

Occasionally, there is a slight downside for some males in a polyandrous group. Even though more chicks are raised to young adulthood by polyandrous group as compared to monogamous pairs on a per nest basis, each male in a polyandrous group generally produces fewer offspring of its own than a male in a monogamous pair on an annual basis. This is somewhat expected since the number of eggs produced by a female is limited, and some males in a group aren’t always able to secure an offspring of their own. The caveat here is that males in a polyandrous group tend to live longer and breed for a longer duration, which compensates for the smaller annual yield with an overall larger life-time yield. Furthermore, it is suggested that when other males in a polyandrous group die, the female continues to mate with the remaining male. This means that in a polyandrous group, each male generally tends to fare better than a male in a monogamous pair over the course of its lifetime, and the surviving male of a polyandrous group tends to fare far better over the course of its lifetime than all other males [3]. If producing more offspring was the chief goal in the war for ultimate survival, monogamous males may win a few battles, but they lose the war. It is more beneficial to be a male in a polyandrous group than a male in a monogamous pair over the course of a hawk’s lifetime.

Nevertheless, the preferred familial structure and behavior of the Galapagos Hawk is rare among the rare. Polyandrous behavior is very uncommon among birds, but cooperative polyandrous behavior, as demonstrated by the Galapagos Hawk, is exceptionally rare [1], [4]. Very few other bird species demonstrate cooperative polyandrous behavior.

A shortage of breeding opportunities in combination with small territorial availability may create the right conditions for cooperative polyandry to be the most beneficial mating system for parents and offspring alike. These conditions don’t apply to most birds, but they adequately describe the condition of the Galapagos hawks; the Galapagos hawk’s effective population size is extremely small, and the availability of suitable territory for hawks in the Galapagos Islands is very low due to the harsh nature of the environment. Unsurprisingly, the polyandrous mating system has been observed to be widespread in the Galapagos hawks Evolutionarily speaking, the most beneficial behavior will be selected for until it becomes the norm in nature, and that’s what may have happened with polyandry in the Galapagos hawks. Since cooperative polyandry was most beneficial for the Galapagos hawks, they adapted as a species to optimize the cost-benefit mechanism to yield more fit offspring at a relatively lower cost. Of course, this behavior became prevalent after a long time, and it is likely that some of the first Galapagos hawks may have behaved entirely differently than their modern-day counterparts—similar to how early humans may have behaved entirely different from modern-day humans with respect to monogamy.

All this explains why the female Galapagos Hawk may have several male breeding partners, but it doesn’t explain why the female hawks tend to be bigger than their male counter parts So why is it so? The devil is in the details; in this case, some of the missing details hold the necessary clues required to understand the difference in size between the sexes.

Males are smaller than females primarily due to partial role reversals within the species. Females compete to lay eggs in a nest on a territory secured by males; in this competition, larger females tend to be favored over smaller ones due to their ability to invest more in each chick and produce healthier offspring. On the other hand, males generally compete to only acquire the territory, and after both sexes win in their respective competitions, the female tends to incubate the eggs while the males forage for food. Since males constantly expend energy to search for food, they may have evolved to be smaller than females to increase foraging efficacy [5]. Usually, both parents contribute a significant portion of their energy in the upkeep of chicks after they hatch, but this is not the case with the Galapagos Hawk as much of the post-copulatory feeding is done by the male parents. This may explain some of the obvious sexual dimorphism in the Galapagos hawks. Sexual dimorphism is common in many birds, and it often describes the common differences in plumage or variation in physical structures, between male and female birds of the same species, but in the case of the Galapagos hawk, it is size that really matters.

Of course, a novice bird watcher wouldn’t know these things, but congratulations—you are well on your way to becoming a Galapagos hawk specialist! It is not often that one encounters a bird enthusiast, but you are now equipped to chime in about the Galapagos hawk during your next conversation with the bird enthusiast in your life. The Galapagos hawk exhibits fascinating behavior, with interesting differential physical development. Witnessing a pair up close—especially when one of the hawks, the female, was feeding—was an absolute delight. In humans, reverse polygamy (polyandry) is heavily frowned upon, and men are expected to be larger than women in most societies, but in nature (sometimes) these roles are completely reversed. A case in point—the Galapagos hawk.


  1. Ehrlich, Paul R. et al. (1988), “Polyandry” & “Monogamy”- Separate Essays. Stanford.
  2. Faaborg, John, et al. (1980), “Preliminary Observations on the Occurrence and Evolution of Polyandry in the Galapagos Hawk (Buteo Galapagoensis).” The Auk, vol. 97, no. 3, pp. 581–590
  3. Faaborg, John, et al.  (1986), Reproductive success and survivorship of the Galapagos Hawk Buteo galapagoensis: potential costs and benefits of cooperative polyandry. Ibis, 128: 337–347.
  4. Faaborg, John, et al. (1995), “Confirmation of Cooperative Polyandry in the Galapagos Hawk (Buteo Galapagoensis).” Behavioral Ecology and Sociobiology, vol. 36, no. 2, pp. 83–90.
  5. Snyder, N.F. & Wiley, J.W. 1976. Sexual size dimorphism in hawks and owls of North America. Ornith. Monogr. 20. American Ornithologists’ Union