If convergent evolution was the universal norm, we might expect to travel in space and find another planet with a complex ecosystem and species that mirror those found on earth. Perhaps, if the planet had solid land, there would be plant life with similar functions and ecological roles as on earth, and likewise, similar communities of animal life. There would not be elephants exactly, but there would be something large that serves a similar ecological role. Based on some of the convergent examples already mentioned in this blog, this elephant-like beast might even have a similar appearance and similar life history characteristics. If the planet were water, we might find similarities to earth as well—perhaps some schooling fish-like species, larger predators, bottom-dwellers, broadcast spawners, etc.
Because earth has both land and sea, we can look for parallels in the communities of species that live in each of them, almost like looking at two different planets. In doing so, we find remarkable parallels between two of the very largest species that inhabit these worlds: elephants and sperm whales.
- They are both very large, among the largest land and sea animals.
- They each have the largest brain of any land or sea animal.
- They both range widely, eat a varied diet, and consume so much food that they actually impact the habitat around them. Elephants are said to impact their landscape more than any other land animal except humans; sperm whales consume 100 million metric tons of food each year (equal to all marine fish consumption by humans), despite their reduced numbers from historic whaling.
- Both species are long-lived and slow reproducing. They both:
- mature in their teens
- live about 60 years
- almost always give birth to single calves at about 5-yr intervals
- see female productivity decline after age 40; after that, females serve the role of clan leaders in highly structured female societies.
- Both elephants and sperm whales organize socially around a matrilineal family of about ten animals, all related females and their offspring, headed by a matriarch.
- Females remain in their family groups their entire lives, during which they move, feed, drink, and rest in unison together. The call in synchrony and stop to listen together. They often rub against each other, or caress each other with their trunks, jaws, or flippers (as the case may be).
- Both species engage in communal calf care. Calves nurse from other females in addition to their mother, although they may not actually obtain milk from them. The family groups create defensive formations to protect calves from threats. They come to the aid of injured family members.
- In both species, family groups spend some of their time associating with one to three other family groups.
- Both elephants and sperm whales communicate at frequencies below the range of human hearing, across distances of 4 km for elephants, 8 km for sperm whales (taking advantage of the way sound waves move in water).
- In both species, the adult males rove independently, seeking out females only when breeding. They form loose bachelor groups when not breeding. Males of both species are capable of breeding in their teens, but generally do not do so until their mid-twenties. Non-breeding adult males tend to live distant from the female family groups. (In sperm whales, the family groups congregate in the tropics, while the males may move toward the poles.)
- In the paper describing the similarities between elephants and sperm whales, Weilgart et al. note that “elephants and sperm whales resemble each other more than they do mammals that inhabit the same environment” (such as the rhinoceros or other whales).
- Finally, both species have been subject to human depredation, and both populations recover slowly due to their very low reproductive rates. For both species, it was assumed that there were many “surplus males” in their population, and that these could be harvested without impacting the population’s annual reproduction. In both cases, however, this has been shown to be wrong; the number of newborn calves is closely tied to the number of males in the population.
For the space traveler coming to earth, they would undoubtedly notice a third dominant species that is impacting the planet (including elephants and sperm whales) and shares many of these same demographic and life history characteristics: humans. (Note that matrilineal societies used to predominate among many ethnic groups in the Americans, Asia, and Africa.)